FOUS30 KWBC 132000
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Thu May 13 2021

Day 1
Valid 16Z Thu May 13 2021 - 12Z Fri May 14 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF EXTREME SOUTHEAST FLORIDA...

1600 UTC update

Only some minor tweaks to the previous marginal risk area across
coastal Southeast Florida after viewing the 1200 UTC hi res
guidance. The 1200 UTC HREF neighborhood probabilities have not
changed appreciably, still showing high probabilities of 2 and 3"+
precip totals in the 6 hour period ending 0000 UTC Friday across
Southeast Florida. Concerns continue for isolated urban runoff
issues across the marginal risk area.

Oravec

0900 UTC discussion

An upper trough centered over the eastern U.S. will support an
organizing surface low drifting northeast from the Southeast Coast
further into the western Atlantic. In its wake, a trailing cold
front will sink into South Florida on Thursday. Southerly to
southwesterly winds ahead of the front will support a ribbon of
deepening moisture, with PWs around 1.75 inches, across South
Florida. This moisture interacting with larger-scale ascent
generated in part by a mid-level shortwave and right-entrance
region upper jet forcing is likely to support widespread shower
and thunderstorm development ahead of the front. General
consensus of the 00Z hi-res guidance shows slow-moving storms
developing along the southeast coast by the late afternoon, where sea-breeze/onshore flow is likely to enhance low level
convergence. The 00Z HREF neighborhood (40km) probabilities for
rainfall amounts of 3 inches or greater are at or above 50 percent
within much of the Marginal Risk area. This includes the
urbanized corridor extending from Ft. Lauderdale to Homestead.

Pereira


Day 2
Valid 12Z Fri May 14 2021 - 12Z Sat May 15 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Pagano

Day 3
Valid 12Z Sat May 15 2021 - 12Z Sun May 16 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS AND TEXAS...

...20Z Update...
Only minor changes made to the Marginal Risk areas to account for
the 12Z model guidance.

With respect to the convection across the Central Plains, model
differences still exist based on the stationary boundary placement
and how the mid-level impulses will interact with the instability
gradient in place. However, the Marginal Risk placement within
this region aligns well with the ensemble means and expected
convective development and evolution at this time. Perhaps a
Slight Risk area could be introduced if guidance convergences on
higher QPF amounts across wetter soils in MO. Hourly rain rates
may exceed 1.5 inches/hour, especially if training comes to
fruition. Areal average precipitation may also range from 2-4+
inches, which is dependent upon a location observing multiple
rounds of precipitation through the forecast period.

Farther south, convection is expected to break out within the warm
sector during the afternoon across south TX with another round of
convection just ahead of trough axis across central TX. There is
even more uncertainty within this region among the models likely
due to the weaker dynamical forcing. Areal average precipitation
is a bit lower given the model spread, though hourly rain rates
could easily climb above 2 inches/hour, especially with slow
moving convection.

Pagano

...Previous Discussion...
Low-level flow becomes more southerly with time along the western
Gulf Coast and Southern Plains region as sprawling high pressure
over the central U.S. moves slowly eastward. Most of the deep
moisture and instability has been shunted south over the Gulf in
the past couple of days...but rainfall amounts of an inch or two
are possible from a consensus of numerical guidance as moisture
begins to return northward on the periphery of the high. Model
mass-fields were in general agreement that PW values in excess of
1.75 inches develop and start being drawn northward on a 25-35kt
low level jet (most pronounced after 16/03Z)...which ranged from
1.5 to 2 standard deviations above climatology for both PW and low
level moisture flux. Thus those parameters were used to define the
Marginal Risk boundaries. It was noted that each run of the ECMWF
going back the past few days has shown some grid-scale feedback
bullseyes somewhere in Texas that resulted 4 to 7 inch amounts (in
excess of a foot on the finer grid scale resolution versions).
For this reason, the deterministic ECMWF was given little
consideration...even though the 13/00Z run did not appear to
suffer from such problems.

Moisture continues to get drawn northward across the Southern
Plains where it encounters a quasi-stationary front draped
east-to-west mainly across Kansas...which is where low-amplitude
shortwave energy embedded within the broader mid-level flow will
help support the convection. The Marginal Risk area was confined
to areas where the SREF and GEFS QPFs plots showed the best
clustering of 2 inch contours...although the axis of highest
precipitable water values and moisture transport vectors suggest
locally heavy rainfall and/or downpours may extend west of the
Marginal Risk area into areas of higher flash flood guidance.

In both cases, the axis of heaviest rainfall forecast remained
displaced from the areas soaked over the past few days. As a
result, felt a Marginal Risk is sufficient for the time being.

Bann

$$
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FOUS30 KWBC 150824
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Sat May 15 2021

Day 1
Valid 12Z Sat May 15 2021 - 12Z Sun May 16 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS AND SOUTH-CENTRAL TEXAS...

...Central Plains...
Ongoing convection over Kansas is expected to continue east
through the morning before dissipating over southern Missouri
during the late morning and afternoon hours. Then beginning
during the evening and continuing through the overnight, models
show convection redeveloping along the high terrain and then
extending east along a slow-moving, east-west oriented boundary
across Kansas and Missouri. Guidance shows low level inflow
intensifying and moisture deepening along the boundary ahead of
mid-level shortwave moving into the central High Plains Saturday
evening. There remains a good signal for back-building/training
convection, with at least locally heavy amounts likely across
portions of central and eastern Kansas into western Missouri.
This potential for additional heavy rains, in addition to the
ongoing convection this morning has raised concerns for localized
runoff concerns, prompting the upgrade to a Slight Risk for
portions of the region. However, there still remains a fair
amount of uncertainty as to where the heaviest amounts will occur.
Development will likely be largely dependent on the location of
the synoptic scale and lingering outflow boundaries, with each
model differing on the details. The initial Slight Risk area is
centered along an axis of high neighborhood probabilities (40km)
for accumulations of 2-3 inches as indicated by the 00Z HREF.
Given the lingering uncertainty, future adjustments may be
necessary.

...Southern Texas...
Models show southeasterly flow supporting a deepening plume of
moisture extending north from the lower Texas coast along the Rio
Grande and into South-central Texas. Latest runs of the NAM and
RAP show PWs at or above 1.75 inches extending north from the
Lower Rio Grande through South Texas later today. Several of the
hi-res guidance members indicate slow-moving storms, with heavy
rainfall rates developing over portions of South and South-central
Texas during the afternoon/evening hours, followed by convection
moving east from Mexico into some of the same regions during the
overnight hours. While confidence in the details is limited,
concerns for heavy amounts and potential flash flooding have
increased, with some of the hi-res guidance members indicating
very heavy totals across portions of the region. HREF
neighborhood probabilities (40km) for 2-inches or more are well
above 50 percent within much of the Marginal Risk area. The
Slight Risk area was drawn where the HREF indicated high
neighborhood probabilities for 3-inches or more. This included
the San Antonio Metro.

...Colorado...
Will continue to monitor the need for a potential upgrade across
portions of Colorado where locally heavy rain is expected to
develop later today. A weak mid-level wave interacting with
southeasterly low level inflow and daytime heating is expected to
support convective development across the high terrain. Areas
impacted may include those with observed above normal
precipitation and low flash flood guidance values. However as
noted in previous discussions, this convection is expected to
progress fairly steadily to the east, limiting the threat for
widespread runoff concerns. With the 00Z HREF indicating lower
probabilities for heavier amounts than its previous run, opted to
continue to hold off on a Marginal Risk for now.

Pereira


Day 2
Valid 12Z Sun May 16 2021 - 12Z Mon May 17 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN OKLAHOMA AND NORTH-CENTRAL TEXAS...

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN AND CENTRAL PLAINS AS WELL AS COLORADO...

...Texas/Oklahoma...
The central U.S. will continue to see heavy precipitation amounts
as a frontal boundary slowly pushes eastward ahead of an upper
level trough axis. Model consensus over the past two days has
placed significant precipitation amounts over portions of
north-central TX into southern OK. A few models have 3 to 4+ inch
bullseyes for these areas with the majority showing at least 2 to
3 inches. Confidence for this region has increased overnight as
models continued the wet trend and thus, a Slight Risk area has
been introduced. PWAT values sit around 2 standard deviations
above climatology for this region with areas creeping near 150% of
normal over the past week in precipitation.

...Southern/Central Plains...
Uncertainty continues across the central U.S. as a storm system
develops ahead of a trough axis slowly moving across the
Southwest. Much of the model spread is connected to the mesoscale
convective features that are expected to occur on Day 1, which
will largely influence the placement of surface boundaries and
thus convection on Day 2. The Marginal Risk area remains in place
that was introduced during the previous update.

...Colorado...
The Marginal Risk area for this region was extended a bit further
west as latest model guidance shows a more westward extent for
heaviest rainfall as convection is expected to develop across the
high terrain and slowly shift east through the afternoon/evening
on Sunday. While the trough axis in the Southwest will help to
usher rich moisture north into the region, it is the differential
heating and weaker shear that could result in more slow moving
convection, as compared to the activity expected on Day 1. The
forecasted heavy precipitation on Day 1 could prime the soils
creating more vulnerability to localized flash flooding on Day 2,
especially for burn scars. Areal average precipitation will range
from 0.5-1.5+ inches with locally higher amounts anticipated.


Chiari/Pagano



Day 3
Valid 12Z Mon May 17 2021 - 12Z Tue May 18 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN AND CENTRAL PLAINS AS WELL AS COLORADO...

...Colorado...
The frontal boundary draped across the Central US continues to
bring heavy precipitation for portions of CO during this time.
Models are placing consistent areas of 0.5 to 1.5 inches of
precipitation for this region for Day 3 and with QPF forecasts for
both Day 1 and Day 2 showing signals in roughly the same area,
burn scars and other especially sensitive areas are of continued
concern. Therefore, it was deemed necessary to introduce a
Marginal Risk area to account for this ongoing threat.

...Southern/Central Plains...
This region continues to be of concern as the frontal boundary
brings continued precipitation to areas already seeing 150 to 400%
of normal. Models are struggling to come to agreement on exact
placement of this boundary, but given the history over the past
few days a Marginal Risk area has been introduced to cover pockets
of heaviest precipitation among the models. This area will likely
be adjusted as the event comes closer and models become better
aligned.


Chiari

$$
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FOUS30 KWBC 192003
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Wed May 19 2021

Day 1
Valid 16Z Wed May 19 2021 - 12Z Thu May 20 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ALONG THE TEXAS
COAST THROUGH SOUTHERN LOUISIANA...

...South and southeastern Texas to southern Louisiana...
Ongoing MCS over the lower Rio Grande Valley will progress into
the Gulf/Mexico through 18Z. The track of the left bookend of this MCS/developing MCV along with the plume of moisture/instability
from the Gulf and over LA are the main players for heavy
precipitation for the western Gulf Coast area rest of today. 12Z
guidance varies with the track of the left bookend with the ARWs
turning it inland near Houston this evening while the 12Z 3kmNAM
shifts it inland sooner this afternoon. Meanwhile, the HRRRs have
kept it more offshore with a resultant lower rainfall total on
land. The MODERATE RISK was able to be shrunk toward the coast in
the wake of the MCS and with more confidence on lesser inland rain
threats.

...Northeastern Texas through Arkansas to eastern Kansas and
western Missouri...
Deep, moist, southerly flow with embedded energy aloft will
support south to north training storms and the potential for
locally heavy amounts across this region. Was able to shift the
Slight Risk area farther east into this plume (and away from north
TX due to confidence displayed in 12Z CAM guidance). The one
caveat is the potential for the left bookend MCV to track inland,
which could produce locally heavy/repeating rain.

...Northwestern Texas and Oklahoma...
Daytime heating along with deepening moisture and increasing
ascent ahead of an upper low will continue to support developing
convection across the Texas and Oklahoma panhandles, with some of
the guidance showing south to north training cells producing
locally heavy amounts across the region. Given the recent heavy
rains and relatively low flash flood guidance values, these
additional rains may pose localized runoff concerns and the
Marginal Risk was maintained/shifted a bit east.

Pereira/Jackson


Day 2
Valid 12Z Thu May 20 2021 - 12Z Fri May 21 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER
SOUTHEASTERN TEXAS INTO SOUTHWESTERN LOUISIANA...

Digging upper low in the West and building upper high over the
central Appalachians will maintain a sharp/elongated NNW-SSE
mid-level trough over the NW Gulf into southeast TX/southwest LA
on Thursday. Ample moisture will continue to be in place (PW
values around 1.75-2" or about +2 to +3 sigma) before the trough
weakens and the pattern shifts westward as heights rise from the
east by day 3 and beyond. 850mb winds of 20-35 kts from the SSE to
SE will continue between the upper low and high with long skinny
CAPE profiles through a saturated atmosphere. Rainfall of 1-3"
(local maxes higher) with hourly rates ~0.75-1"/hr will exacerbate
ongoing flooding issues. Multi-cycle trend has been to shift the
rainfall axis a bit farther east near the precipitable water/CAPE
gradient with the flow off the Gulf, partly contingent on this
morning's MCS evolution. Moisture plume will extend northward
through OK/AK into KS/MO with more uncertainty in how strong any
convective elements may become farther north. Maximum 24-hr QPF
values/coverage in the guidance drop off north of the ArkLaTex (<
~1.75") and capped the Slight Risk area in that vicinity.

Per the 12Z CAMs and trends, focused the heaviest QPF over
southeastern TX into southwestern LA where the best consensus was
seen, supporting the Moderate Risk area given the very wet
antecedent conditions. Trimmed back the western edge of the
Marginal and Slight Risk contours over eastern Texas per
coordination with the local offices as the QPF consensus was about
40-100 miles east of the previous ERO centroid axis. Also expanded
the Slight Risk contour a bit eastward across LA as there was an
increased probability of heavier rainfall near BTR given a shift
eastward in the QPF/moisture. The Risk areas will be further
refined in the day 1 period as ongoing/overnight convection over
the Gulf evolves.

Fracasso

Day 3
Valid 12Z Fri May 21 2021 - 12Z Sat May 22 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EXTREME SOUTHEASTERN TEXAS AND SOUTHWESTERN LOUISIANA...

By Friday morning, moisture plume/axis will start to drift
westward, nudged by the upper high building to the northeast over
the Appalachians. This will slowly move the QPF axis westward as
well but with less favorable upper support (stronger 500mb
shortwaves will have lifted north) and some drier air from the
east. However, models indicate the possibility of convection off
the Gulf to move inland near the TX/LA coast on Friday, with the
potential for locally heavy rainfall as precipitable water values
remain near 2" along the coast. Though spread in the guidance both
spatially, temporally, and in principle result in less confidence
overall, best ensemble overlap was again over the TX/LA border
with the potential for at least 1" of rainfall and some embedded
heavier rates. Per coordination with the local offices, introduced
a small Slight Risk area near this best consensus given the heavy
rainfall in recent days coupled with the expectation of more
rainfall in days 1-2. Expanded the larger Marginal Risk contour to
encompass a bit larger area where the ensemble maximum QPF values
were still over 5" per the 12Z global guidance.

Fracasso

$$
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FOUS30 KWBC 201959
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 PM EDT Thu May 20 2021

Day 1
Valid 16Z Thu May 20 2021 - 12Z Fri May 21 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER
SOUTH-CENTRAL LOUISIANA...

...Western Gulf Coast into Eastern Kansas/Western Missouri...
Large scale flow continues to show anomalous ridging over the
eastern U.S. and a low amplitude trough axis over the southern
Plains. A narrow corridor with precipitable water values of 1.5-2"
will continue to be drawn northward from the Gulf of Mexico,
across Louisiana and into the Midwest. Recent surface
observations show a surface low offshore Galveston with what
appears to be a baroclinic zone somewhat offshore the western Gulf
Coast. The magnitude of the low level flow is greater than the
deep-layer mean wind which would allow for training and repeating
of storms from south to north and high rainfall efficiency.

Short term trends in satellite and radar imagery continue to show
the potential for development of an axis of training heavy rain
over southeast Louisiana, but expectations are that a new
thunderstorm band could fill in to its west in an area of
insolation/cloud thinning and sunshine from Marsh Island northwest
across western LA within an area that should be destabilizing.
Hourly rain totals of 2-3" would be possible where training
develops. Southeasterly 850 mb wind speeds of 30-40 kt are
expected to continue through the day today and into Friday
morning, but forecasts of instability show decreasing values into
early Friday morning. Nonetheless, the potential for locally heavy
rain will be in place for the whole 24 hour period, but with
greater probabilities through sunset. The west-central Gulf Coast
has been extremely wet over the past 4, 7, 30, and 60 days which
has left many areas with saturated soils, increasing
susceptibility to flash flooding. The 12Z HREF supports a 50%+
percent chance of 5+ inches of rain in and near the Atchafalaya
ending 12Z Friday.

Farther north, unidirectional southerly flow will be in place from
the Texas/Louisiana border into eastern Kansas and western
Missouri, with potential for south-north axes of training.
Portions of these areas have been wetter than average over the
past 1-2 weeks which has lowered flash flood guidance values to 2
inches or less in 3 hours. The best timing for flash flooding will
be with the diurnal cycle and increase of available instability,
from the early afternoon to early overnight hours. 2-4 inches will
be possible for locations within the Slight and Marginal Risk
contours.


...Eastern North Dakota into northwestern Minnesota...
A quasi-stationary front is expected to remain situated across
eastern North Dakota into northern Minnesota through Thursday
night. While higher moisture will reside off toward the east into
eastern Minnesota and Wisconsin, Precipitable water anomalies near
the Red River are forecast to be near +2, or near 1.25 inches.
Thunderstorms are expected to redevelop with afternoon heating
between 18-00Z near and just north of the stationary front.
Beyond 00Z, 850 mb flow is forecast by the 00Z model consensus to
strengthen to near 20 kt, which is near or in excess of the
deeper-layer mean wind, supportive of training. Given locally
heavy rain across eastern North Dakota into northwestern Minnesota
from late Wednesday, a Marginal Risk remains to cover the
localized potential for flash flooding.

Roth/Otto


Day 2
Valid 12Z Fri May 21 2021 - 12Z Sat May 22 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EXTREME SOUTHEASTERN TEXAS AND SOUTHWESTERN LOUISIANA...

...21Z Excessive Rainfall Discussion...

Few changes needed to the on-going ERD. Did expand the Slight
Risk area a bit across parts of Texas and Louisiana to account for
some of the east/west possibilities still suggested by the
guidance...combined with the antecedent conditions. Overall,
though, the changes did not reflect a significant shift in the
forecast reasoning.

Bann

...08Z Excessive Rainfall Discussion...

With the upper-level high continuing to build over the Northeast
and Appalachian Mountains, the plume of moisture over the Gulf
Coast will drift to the west during this period. As such, the axis
of convective activity will also move westward to an area that has
reduced upper-level support. However, showers may still move
toward the Texas and Louisiana coast and advance inland over an
area that has been water logged for the better part of this week.
Potential remains for moderate to heavy rain during this period,
especially with precipitable water values holding steady near the
2 inch mark. With the

Maintained the inherited small Slight Risk area straddling the
Texas and Louisiana border with minor eastward expansion further
into Louisiana. A similar adjustment was also made to the eastern
bound of the Marginal Risk area to reflect the latest WPC QPF
trend.

Campbell

Day 3
Valid 12Z Sat May 22 2021 - 12Z Sun May 23 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEASTERN TEXAS...

...21Z Excessive Rainfall Discussion...
Spread remains in the model QPF across parts of the Texas coast.
Expanded the Marginal Risk area a bit based on coherent plume of
anomalously high precipitable water values being drawn inland
shown by the ensembles might imply better coverage than shown by
the deterministic models or that there could be higher rainfall
amounts brought farther inland. Given the lack of support from
the operational runs, it was decided to maintain the Marginal Risk
area with only minor adjustments to the boundary.

Bann

...08Z Excessive Rainfall Discussion...

The moisture plume and axis of precipitation will continue to
drift westward through the day 3 period, with showers and
thunderstorms moving inland from the coast of Texas. There is some
spread with where the maximums fall, with some of the guidance
keeping it just offshore and others bringing it in the vicinity of
the Houston metro. Although amounts are largely less than 1 inch,
it will fall over an area very sensitive to additional
accumulations. A Marginal Risk area was hoisted for parts of the
Southeast Coast and points west.

Campbell


$$
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FOUS30 KWBC 220835
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
435 AM EDT Sat May 22 2021

Day 1
Valid 12Z Sat May 22 2021 - 12Z Sun May 23 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEASTERN TEXAS, THE HIGH PLAINS, AS WELL AS THE SOUTHERN AND
CENTRAL ROCKIES...

...Southeastern Texas...
A surface low pressure system moving onshore early this morning
will slowly lift north and west between the weakening trough in
the West and ridging in the East. As a result, bands of heavy
rain circulating around the low will be ongoing to start the
forecast period. Anticipate convection to become more widespread
through the afternoon with diurnal heating with multiple rounds of
heavy rain potentially leading to localized flash flooding.

While the best moisture gradient is still directed into LA this
morning, as the aforementioned ridge in the East continues to
retrograde west, moisture transport will become better aligned
with the well-defined low pressure system. Precipitable water
values will increase to over 2 inches (aided by 20-30 knot
southeasterly low level flow) which is over 2.5 standard
deviations above the mean. Coincident with this transition will be
diurnal heating leading to better instability through the
afternoon. MUCAPE values will range between 500-1000 J/kg. Thus,
anticipate convection to blossom with rain rates exceeding 1.5
inches/hour. Areal average precipitation will range from 0.5-1.5+
inches with locally higher amounts anticipated.

With portions of southeastern TX saturated from the past weeks
precipitation activity, some locations may be more sensitive to
heavy rain and thus may flash flood. Therefore, the Marginal Risk
area was retained and refined based on the latest trends and 00Z
model data.

...Southern/Central Plains and Southern/Central Rockies...
The trough in the west will continue to weaken as it shifts north
and east through the forecast period. Mid-level shortwaves will
round the trough axis interacting with pooling moisture and
instability through the afternoon. In addition, mid-level impulses
will interact with existing surface boundaries helping to focus
convection, especially across western portions of the Southern and
Central Plains. This activity will likely continue through the
evening hours with the strengthening low level jet. Thus,
anticipate multiple rounds of heavy rain could lead to localized
flash flooding.

Precipitable water values will increase across the region to
around 0.75-1.25 inches aided by southern flow (both from the
Pacific and Gulf of Mexico). Values will be 2+ standard deviations
above the mean. With instability climbing to above 2000 J/kg
MUCAPE by the afternoon anticipate convection will develop,
especially across the higher terrain with low level upslope
enhancement. In addition, subtle mid-level impulses aloft will
interact with lingering surface boundaries which will likely act
as a focus for convection from the afternoon into the
evening/overnight hours. Rain rates may exceed 1 inch/hour with
multiple rounds of precipitation resulting in localized higher
hourly storm totals. Areal average precipitation will vary quite
a bit with some locations observing 3+ inches.

Given fairly wet antecedent conditions within the region,
localized flash flooding is possible. Therefore, the Marginal
Risk area was retained, but expanded north based on the latest 00Z
model guidance. Burn scars were also taken into consideration
when refining the risk area.

Pagano




Day 2
Valid 12Z Sun May 23 2021 - 12Z Mon May 24 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL/SOUTHEASTERN TEXAS AND THE UPPER MIDWEST...

...Central Texas to the Middle Texas Coast...
In the wake of a remnant low to mid-level low tracking northward
through northern Texas/Oklahoma early Sunday morning, an axis of
high precipitable water values (1.7 to 2.1 inches, or +2 to +3
standardized anomalies) will remain focused from the western Gulf
of Mexico into the middle Texas coast and portions of central
Texas. Aloft, a mid-level shear axis will remain oriented from
south to north across the region with embedded vorticity maxima.
850-300 mb mean flow is forecast to be on the weak side across
central/southern Texas, ranging between 5 and 15 kt. Meanwhile,
850 mb flow will remain roughly perpendicular to the coast and
vary between 10 and 25 kt, with the Gulf of Mexico acting as a
source region for moist and unstable air. Stronger 850 mb flow
allows for backward propagating Corfidi vectors, supportive of
backbuilding and training.

While there are differences with the degree of instability
forecast for the afternoon hours, the high moisture environment
should be able to support tall/skinny CAPE profiles as depicted by
the NAM. Forcing for ascent will be aided by smaller scale
vorticity maxima along the mid-level shear axis. The coarser
resolution models support a relative max in QPF across central
Texas to the coast with roughly 0.5 inches to 1.5 inches, but the
NAM_nest and FV3_LAM are perhaps too aggressive with smaller scale
maxima of 3-6 inches (NAM_nest locally higher). The potential for
2-4 inches on a localized basis atop wetter than average soils
supports a Marginal Risk.

...Upper Midwest...
A Marginal Risk was added to southern Minnesota, northeastern Iowa
and central/southern Wisconsin to account for the potential of
localized flash flooding. Anomalous precipitable water values will
travel northward through the Plains into the Upper Mississippi
Valley, on the west side of a ridge centered over the Tennessee
Valley. At the surface, a front will extend roughly west to east
across Minnesota into Wisconsin, with the nose of the moisture
axis intersecting the front near the Minnesota/Wisconsin border
(precipitable water values along the front of 1.5 and 1.8 inches).
Deeper-layer mean flow will be roughly parallel to the frontal
boundary which may support some repeating/localized training of
thunderstorms.

Differences in the latitude of the front remain, with the GFS and
FV3_LAM north of the remaining model consensus. 12 hour hi-res QPF
was availabe through 00Z/24, showing unanimous consensus for 2-4
inches locally across the Marginal Risk area. Additional heavy
rain will be possible beyond 00Z in the vicinity of the slow
moving frontal boundary. Despite the relative lack of rain over
the past two weeks, flash flood guidance values are only about 1.5
to 2.5 inches in 3 hours across a majority of the region, and
these values could be surpassed Sunday into Sunday night.

Otto

Day 3
Valid 12Z Mon May 24 2021 - 12Z Tue May 25 2021

...THE PROBABILITY OF RAINFALL EXCEEDING FLASH FLOOD GUIDANCE IS
LESS THAN 5 PERCENT...

Otto


$$
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FOUS30 KWBC 230848
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
447 AM EDT Sun May 23 2021

Day 1
Valid 12Z Sun May 23 2021 - 12Z Mon May 24 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL/SOUTHEASTERN TEXAS...

...Central Texas to Middle Texas Coast...
With the mid/upper level ridge anchored over the Southeast, a
decent fetch of deep layer moisture from the Gulf of Mexico will
funnel anomalously high precipitable water into the central TX
coast through the forecast period. Given this region has observed

600% of normal precipitation over the past week, soils will be

sensitive to additional heavy rain. Therefore, localized to
scattered flash flooding may occur.

An axis of high precipitable water values (ranging from 1.8 to 2+
inches aided by southeasterly low level flow) will focus from the
western Gulf of Mexico into central TX. Aloft, a mid-level shear
axis will remain oriented from south to north across the region
with embedded vorticity maxima. In addition, 850-300 mb mean flow
is forecast to be on the weak side across central/southern Texas,
ranging between 5 and 15 kt. Meanwhile, 850 mb flow will remain
roughly perpendicular to the coast and vary between 10 and 25 kt,
with the Gulf of Mexico acting as a source region for moist and
unstable air. The stronger 850 mb flow will allow for backward
propagating Corfidi vectors, supportive of backbuilding and
training. Given sufficient moisture and instability, this will be
enough to support rain rates in excess of 1.5 inches/hour. And
with the potential for training/backbuilding, hourly storm totals
may exceed 2 inches. It should also be noted that there will be
waves of activity through the forecast period based on the timing
of mid-level impulses into the region.

With saturated soils (FFG values as low as 1.5 inches/3 hours),
anticipate scattered flash flooding to occur closer to the central
TX coast and more scattered flash flooding possible farther inland
away from the more focused plume of precipitation.

...Upper Midwest...
A cold front dropping south through the Great Lakes region will
start to slow through the day. Meanwhile, a shortwave aloft will
interact with this front and the pooling moisture/instability to
the south. As a result, convection will focus along this frontal
zone with the potential for training/backbuilding storms that
could lead to localized flash flooding.

Precipitable water values along the front will range between
1.5-1.75+ inches aided by southwestern low level flow with
instability hovering around 500 J/kg MUCAPE. Given sufficient
moisture, instability and mid-level forcing for ascent, convection
should blossom this morning, centered over WI. Deep layer mean
flow will be roughly parallel to the frontal boundary resulting in
multiple rounds/training of storms. The aforementioned front will
also be slow to sink south and thus there could be a prolonged
period of convection within this region. Rain rates could exceed
1.5 inches/hour with hourly storm totals climbing above 2 inches.

Antecedent conditions vary quite a bit within this region with
near normal precipitation observed over the past week across
portions of central/eastern WI with wetter soil conditions toward
MN/WI border. With a bulk of the heavier activity expected across central/eastern WI, opted to retained the Marginal Risk area with
minor refinements made based on the latest model guidance and WPC
QPF.

...Western Dakotas...
A slow moving trough in the West will supply deep layer moisture
into the Norther Plains with mid-level shortwaves activity to
focus convection at the surface. The first shortwave, which is
exiting the region this morning, will have dropped 1-2 inches
across the region over the past 6-12 hours. Another stronger
shortwave will round the trough later this afternoon atop better
moisture and instability resulting in a more vigorous line of
convection and the potential for localized flash flooding.

Precipitable water values will try to exceed 0.75 inches aided by
southerly flow level flow. Meanwhile, assuming sufficient
destabilization, instability should climb to over 2000 J/kg
MUCAPE. As the mid-level impulses approaches it will interact
with the pooling of rich moisture and instability helping to
enhance/focus convection during the afternoon across portions of
the western High Plains. Shear will be ample to support more
organized convection with the line advancing quickly east. Thus,
hourly rainfall totals should not exceed 1.5-2 inches. So, while
lower FFG exists across the region due to the recent precipitation
activity, the progressive nature of this system should limit flash
flooding. Therefore, the Marginal Risk area was retained but
refined based on the latest thinking/00Z model guidance.

Pagano


Day 2
Valid 12Z Mon May 24 2021 - 12Z Tue May 25 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE ALLEGHENY PLATEAU/MOUNTAINS...CENTRAL/EASTERN TEXAS AND THE
SOUTHERN HIGH PLAINS INTO SOUTH-CENTRAL NEBRASKA...

...Portions of Central and Eastern Texas...
There is good large scale agreement in the 00Z guidance for a
mid-level vorticity max to track northward from the middle Texas
coast on Monday, after 12Z. Showers and thunderstorms are likely
to be ongoing near this feature at the start of the period, within
an axis of PWATs ranging between 1.8 and 2.1 inches. Low level
flow from the southeast will be similar to deeper-layer mean flow,
allowing for the potential of training thunderstorms. While
mesoscale boundaries, with placement undetermined at these longer
lead times, will likely play a role in the exact location of heavy
rainfall, the pattern broadly supports the potential from the
middle to upper Texas coast into north Texas, including the
Dallas-Fort Worth Metroplex. Portions of the outlook area have had
much above average rainfall over the past 1-2 weeks, which has
increased susceptibility to flash flooding. Localized potential
exists for 3-6 inches across the Coastal Plain, largely prior to
00Z. An upgrade to Slight Risk remains possible with future
forecast cycles if confidence increases on placement.

...Allegheny Plateau/Mountains...
Convection is expected to be ongoing near a front extending
southeastward across the upper Ohio Valley. Flow aloft will be
northwesterly and 850 mb flow is expected to be of a similar
direction and magnitude, ranging between 15-20 kt, roughly
perpendicular to the terrain. Increasing instability after sunrise
is expected to support an increase in convective coverage on the
heels of a departing, low amplitude, impulse aloft as seen in the
00Z NAM/GFS/ECMWF. Forecast PWATs of ~1.5 to 1.7 inches would
support standardized anomalies of +2 to +2.5 across the upper Ohio
Valley. The potential exists for very localized heavy rainfall in
the vicinity of the northern West Virginia/Ohio border into
southwestern Pennsylvania and the Maryland Panhandle, prior to the
loss of daytime heating/instability.

...Southern High Plains into western Kansas and south-central
Nebraska...
Afternoon heating ahead of a slow moving front/dryline in the
Plains will support convection in the evening from portions of
eastern New Mexico into the central Plains. Forecast PWAT values
of 1 to 1.5 inches are on the lower end of the standardized
anomaly scale (+1 to +2), but sufficient shear and instability
will exist for organized convection which often carries a low end
threat for flash flooding on a localized basis. However, an
increase in 850 mb flow into the 30-40 kt range just after 00Z
across western Kansas will potentially support expanding
convective coverage and a threat for training along the
southwestern portion of any convective clusters that may develop.
This potential combined with the mixture of lower flash flood
guidance values (1.5 to 2.5 inches in 3 hours) allows for a
somewhat broad Marginal Risk for the region.

Otto

Day 3
Valid 12Z Tue May 25 2021 - 12Z Wed May 26 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS INTO NORTHERN MISSOURI AND SOUTHERN IOWA...

...Central Plains/Midwest...
As a mid-level shortwave tracks eastward across the central
U.S./Canadian border, a cold front will push east across the upper
Mississippi Valley. However, a lack of height falls to the south
will support a quasi-stationary boundary across Kansas and
Nebraska. Similar to Monday, moisture within the warm sector of
the front will only have standardized anomalies of PWATs between
+1 and +2 (1.2 to 1.7 inches...higher across eastern Kansas into
Missouri).

00Z forecast soundings from the NAM and GFS show a capping
inversion during the day, breaking in the late afternoon with
daytime heating. Convective coverage is expected to rapidly
increase late in the day on Tuesday ahead of the front and near
any lingering outflow boundaries present from previous convection.
Lift should be augmented given the presence of the right-entrance
region of a 70-90 kt upper level jet located on the eastern side
of the shortwave trough axis to the north. Southwesterly 850 mb
flow of 25-35 kt should align with deeper-layer mean flow allowing
for periods of training from west to east.

The potential will exist for localized 2-4 inch rainfall totals
from central Kansas, east-northeastward across the Missouri River.
Outside of the bullish and possibly convective feedback-ish UKMET
(3-4 inches), model QPF forecasts for the 24 hour period are not
alarmingly high (1-2 inches), but the pattern is supportive for an
increased threat of flash flooding for the central Plains into
portions of the Midwest from late Tuesday afternoon into Tuesday
night.

Otto

$$
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FOUS30 KWBC 232037
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
436 PM EDT Sun May 23 2021

Day 1
Valid 1901Z Sun May 23 2021 - 12Z Mon May 24 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL/SOUTHEASTERN TEXAS...

...Central Texas to Middle Texas Coast...
With the mid/upper level ridge anchored over the Southeast, a
decent fetch of deep layer moisture from the Gulf of Mexico will
funnel anomalously high precipitable water into the central TX
coast through the forecast period. Given this region has observed

600% of normal precipitation over the past week, soils will be

sensitive to additional heavy rain. Therefore, localized to
scattered flash flooding may occur. Aloft, a mid-level shear axis
will remain oriented from south to north across the region with
embedded vorticity maxima. In addition, 850-300 mb mean flow is
forecast to be on the weak side across central/southern Texas,
ranging between 5 and 15 kt. Meanwhile, 850 mb flow will remain
roughly perpendicular to the coast and vary between 10 and 25 kt,
with the Gulf of Mexico acting as a source region for moist and
unstable air. The stronger 850 mb flow will allow for backward
propagating Corfidi vectors, supportive of backbuilding and
training. Given sufficient moisture and instability, this will be
enough to support rain rates in excess of 1.5 inches/hour. And
with the potential for training/backbuilding, hourly storm totals
may exceed 2 inches. It should also be noted that there will be
waves of activity through the forecast period based on the timing
of mid-level impulses into the region. With saturated soils (FFG
values as low as 1.5 inches/3 hours), anticipate scattered flash
flooding to occur closer to the central TX coast and more
scattered flash flooding possible farther inland away from the
more focused plume of precipitation.


...Upper Midwest/Lower Peninsula of Michigan...
A cold front dropping south through the Great Lakes region will
start to slow through the day. Meanwhile, a shortwave aloft will
interact with this front and the pooling moisture/instability to
the south. As a result, convection will focus along this frontal
zone with the potential for training/backbuilding storms that
could lead to localized flash flooding. Precipitable water values
along the front will range between 1.5-1.75+ inches aided by
southwest low level flow with instability of up to 1000 J/kg
ML/MUCAPE. Given sufficient moisture, instability and mid-level
forcing for ascent, convection is blossoming over portions of WI
and the Lower Peninsula of MI. Deep layer mean flow will be
roughly parallel to the frontal boundary resulting in multiple
rounds/training of storms. The aforementioned front will also be
slow to sink south and thus there could be a prolonged period of
convection within this region. Rain rates could exceed 1.5
inches/hour with hourly storm totals climbing above 2 inches.

Antecedent conditions vary quite a bit within this region with
near normal precipitation observed over the past week across
portions of central/eastern WI with wetter soil conditions toward
MN/WI border. This update extended the Marginal Risk area east
into the Lower Peninsula of Michigan per radar trends which show
extremely slow movement to activity in that area and 12z HREF
output which suggest eventual southeast movement through Detroit
and into portions of northwest OH as a cold pool sets up over the
next 2-3 hours and forced forward propagation.


...Western Dakotas...
A slow moving trough in the West will supply deep layer moisture
into the Norther Plains with mid-level shortwaves activity to
focus convection at the surface. The first shortwave, which is
exiting the region this morning, will have dropped 1-2 inches
across the region over the past 6-12 hours. Another stronger
shortwave will round the trough later this afternoon atop better
moisture and instability resulting in a more vigorous line of
convection and the potential for localized flash flooding.

Precipitable water values will try to exceed 0.75 inches aided by
southerly flow level flow. Meanwhile, assuming sufficient
destabilization, instability should climb to over 2000 J/kg
MUCAPE. As the mid-level impulses approaches it will interact
with the pooling of rich moisture and instability helping to
enhance/focus convection during the afternoon across portions of
the western High Plains. Shear will be ample to support more
organized convection with the line advancing quickly east. Thus,
hourly rainfall totals should not exceed 1.5-2 inches. So, while
lower FFG exists across the region due to the recent precipitation
activity, the progressive nature of this system should limit flash
flooding. Therefore, the Marginal Risk area was retained but
refined based on the latest thinking/00Z model guidance.

Roth/Pagano


Day 2
Valid 12Z Mon May 24 2021 - 12Z Tue May 25 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
EAST-CENTRAL TEXAS...

...Middle Texas Coast through East/North Texas into eastern
Oklahoma...
A mid-level impulse tracks northward from the middle Texas coast
around 12Z. This should focus further Monday thunderstorm activity
in a corridor from the northern middle/southern upper TX coast
north through the Dallas-Fort Worth Metroplex and into
east-central OK within an axis of 1.8 to 2 inch PWATs. Southerly
low level flow will be similar to deeper-layer mean flow, allowing
for the potential of training thunderstorms. With an expected
focus near the coast, upgraded to a Slight Risk south of Houston
while expanding the Marginal Risk up into east-central OK. Most of
these outlook areas have had well above average rainfall over the
past 1-2 weeks, which has increased susceptibility to flash
flooding. Localized potential exists for 3-6 inches across the
Coastal Plain, largely prior to 00Z with potential further
activity Monday night.

...Allegheny Plateau/Mountains...
Convection is expected to be ongoing near a front extending
southeastward across the upper Ohio Valley to the Allegheny
Plateau. Flow aloft will be northwesterly and 850 mb flow is
expected to be of a similar direction and magnitude, ranging
between 15-20 kt, roughly perpendicular to the terrain. Increasing
instability after sunrise is expected to support an increase in
convective coverage on the heels of a departing, low amplitude,
impulse aloft. Forecast PWATs of ~1.5 to 1.7 inches have
standardized anomalies of +2 to +2.5 across the upper Ohio Valley.
The potential exists for very localized heavy rainfall in the
vicinity of the north-central West Virginia/Ohio border into
southwestern Pennsylvania and the Maryland Panhandle, prior to the
loss of daytime heating/instability. Expanded the Marginal Risk a
bit south for lower FFG and more instability which may promote
southward shifting precipitation.

...Southern High Plains into western Kansas and south-central
Nebraska...
Afternoon heating ahead of a slow moving front/dryline in the
Plains will support convection in the evening from portions of
eastern New Mexico into the central Plains. Forecast PWAT values
of 1 to 1.5 inches are on the lower end of the standardized
anomaly scale (+1 to +2), but sufficient shear and instability
will exist for organized convection which often carries a low end
threat for flash flooding on a localized basis. However, an
increase in 850 mb flow into the 30-40 kt range just after 00Z
across western Kansas will potentially support expanding
convective coverage and a threat for training along the
southwestern portion of any convective clusters that may develop.
This potential combined with the mixture of lower flash flood
guidance values (1.5 to 2.5 inches in 3 hours) allows for a
somewhat broad Marginal Risk for the region. Expanded the southern
portion of the Marginal Risk a bit to account for heavy early
activity on the western side in eastern NM an for propagation on
the eastern side in the lower TX Panhandle.

Jackson/Otto

Day 3
Valid 12Z Tue May 25 2021 - 12Z Wed May 26 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
THE CENTRAL PLAINS...

...Central Kansas through Northwest Missouri...
A mid-level shortwave trough over the northern Plains will slowly
push a cold front from southeast Neb into northwest MO through the
forecast period. Similar to Monday, moisture within the warm
sector ahead of the front will have PWATs of 1.5 to 1.7 inches (2
standard deviations above normal) with southwesterly 850 mb flow
of 25-35 kt aligning with deeper-layer mean flow allowing for some
training activity.

The potential exists for localized 2-4 inch rainfall totals from
central Kansas, east-northeastward across the Missouri River
despite the 12Z consensus of an areal average of 1 to 2", so the
Marginal Risk was kept and expanded southeast to account for some
advancement from mesoscale processes.

Jackson/Otto

$$
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FOUS30 KWBC 241555
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1154 AM EDT Mon May 24 2021

Day 1
Valid 16Z Mon May 24 2021 - 12Z Tue May 25 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
EAST-CENTRAL TEXAS...

1600 UTC update

Only some minor changes made to the previous Excessive Rainfall
Outlook based on the 1200 UTC hi res run. The marginal risk was
extended eastward into far southwest Louisiana and the slight risk
was extended slightly farther eastward over southeast Texas.

Oravec

0900 UTC discussion

...Middle Texas Coast through East/North Texas into eastern
Oklahoma...
The moisture feed out of the Western Gulf of Mexico into the
Southern Plains continues through the forecast period as southerly
flow funnels between the trough to the West and ridging in the
East. Therefore, another couple of rounds of convection are
anticipated. Given the saturated soils, localized to scattered
flash flooding is possible from the Central TX coast across
portions of east TX and OK.

Another mid-level impulse is forecast to track northward this
morning focusing thunderstorm activity in a corridor from the
middle/upper TX coast toward the Dallas-Fort Worth Metroplex and
into east-central OK within an axis of 1.8 to 2 inch precipitable
water values. Anticipate convective coverage to increase through
the afternoon with better instability. Southerly low level flow
will become aligned with the deep-layer mean flow, allowing for
the potential of training thunderstorm activity. Hourly storm
total precipitation may exceed 2 inches/hour.

With an expected focus near the coast, retrained the Slight Risk
south of Houston while expanding the Marginal Risk across TX and
into OK. Most of these outlook areas have had well above average
rainfall over the past week or so, which has increased
susceptibility to flash flooding. Localized potential exists for
3-5+ inches across the Coastal Plain, largely prior to 00Z with
potential further activity Monday night as another impulse may
approach the region.

...Allegheny Plateau/Mountains...
A surface front dropping south through the Mid-Atlantic will be
the focus for convection through the afternoon as a mid-level
impulse rounds the ridge axis over the region. Anticipate showers
and storms to develop through the day, quickly diving south along
the western periphery of the ridge toward better
instability/moisture south of the boundary. Precipitable water
values will climb toward 1.75 inches aided by weak westerly flow
which is around 2-3 standard deviations above the mean.
Instability should increase through the afternoon to over 1000
J/kg MUCAPE. Thus, with sufficient pooling of instability and
moisture, the mid-level forcing for ascent should help trigger
convection that could drop 2 inches of rain in an hour. Some
enhancement may take place along the terrain as low level flow
becomes perpendicular to the slopes. Areal average precipitation
from southwestern PA to north-central NC will range between 1-2+
inches. Given lower FFG across the Central Appalachians/Allegheny
Plateau, opted to keep the Marginal Risk area confined to this
region with the better potential for localized flash flooding.

...Southern High Plains into western Kansas and south-central
Nebraska...
As the closed mid-level low over the Northern Rockies lifts north
and the ridging becomes settled over the Southeast, shortwaves
will ride atop the stationary boundary draped from the Southern
Plains into the Upper Midwest. Given the pooling moisture and
instability to the south of the boundary, these impulses will act
to promote brief training of convection that may lead to localized
flash flooding.

To start the forecast period, a decaying line of convection ahead
of a cold front will continue to track out of the Northern/Central
Plains. Debris clouds may initially inhibit diurnal heating, but
eventually clearing will allow instability to grow through the
afternoon. Thus, afternoon heating ahead of this slow moving
front/dryline in the Plains will support scattered convection.
However, the bulk of the activity will not develop nor become
better organized until the evening in response to the
strengthening low level jet. Forecast precipitable water values
of 1 to 1.5 inches are on the lower end of the standardized
anomaly scale (+1 to +2), but with sufficient instability
thunderstorms could produce over 1.5 inches/hour of rainfall. In
addition, southerly low level flow could become aligned with the
deep layer mean wind resulting in periods of training, especially
on the southwestern flank of any convective line. This potential
combined with lower flash flood guidance values (1.5 to 2.5 inches
in 3 hours) may lead to localized flash flooding and thus the
Marginal Risk was retained.

Pagano


Day 2
Valid 12Z Tue May 25 2021 - 12Z Wed May 26 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
THE CENTRAL PLAINS INTO SOUTHERN IOWA AND NORTHERN ILLINOIS...

...Central Kansas through Northwest Missouri...
An eastward tracking mid-level shortwave trough over the northern
Plains will slowly push a cold front southeastward through the
middle Missouri River Valley through Tuesday afternoon. PWATs of
roughly 1.4 to 1.7 inches (standardized anomalies of +2) should be
present within the warm sector over the central Plains and Midwest
with thunderstorms possibly ongoing early Tuesday morning, with a
second round of storms expected with afternoon
heating/destabilization. 850 mb and deep-layer mean flow will be
from the SW or WSW, parallel to the orientation of the frontal
boundary, which may support some repeating or training of
convection.

The potential exists for localized 2-4 inch rainfall totals from
central Kansas, east-northeastward across the Missouri River with
decent agreement from the available higher resolution guidance
(NAM_nest, FV3_LAM, regional_GEM). However, the coarser resolution
guidance, such as the NAM, GFS and ECMWF, show only 1-2 inches for
the 24 hour period ending 12Z Wednesday and are a little south of
the hi-res guidance mentioned above. The exception to the coarser
resolution models is the heavier UKMET which has been consistent
with 2-4 inches from eastern Kansas into western Missouri. A
Marginal Risk covers the range of solutions, much of which
overlaps with flash flood guidance values that are 1.5 to 2.5
inches in 3 hours.

Otto

Day 3
Valid 12Z Wed May 26 2021 - 12Z Thu May 27 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS INTO SOUTH DAKOTA AND IOWA...

...Central Plains into parts of Iowa and Missouri...
The models show good agreement regarding the idea of a potent
mid-level shortwave crossing the northern Rockies on Wednesday,
and ejecting eastward into the Dakotas and northern Nebraska late
Wednesday evening. There were some latitude differences to content
with the GFS/GEFS north of the remaining consensus. At the
surface, a well-defined front should extend ESE from a low near
the western South Dakota/Nebraska border into central Missouri for
00Z Thursday. PWATs are forecast to increase through the afternoon
within the warm sector, with values near 1.25 inches in the
pre-convective environment.

The eastward moving shortwave is expected to support an increase
in the low level jet to near 50 kt from the south just after 00Z
over Nebraska and Kansas. The models also support a strengthening
of an west-east upper-level jet streak across the upper
Mississippi Valley, placing a divergent and diffluent region aloft
across the central and northern Plains. A gradient in MUCAPE is
expected to lie across central Nebraska, parallel to the front,
with thunderstorms rapidly increasing in coverage Wednesday night.
Motion of any organized complex should follow the CAPE gradient
toward the ESE, in line with forecasts of Corfidi vectors, and
regeneration of convection to the west could support training and
repeating cell motions.

There are obvious uncertanties with existence of mesoscale
boundaries that may be in present prior to the ejection of the
shortwave, which may play a role in higher precipitation axes to
the south over northern Kansas and southern Nebraska. Given the
model QPF spread and uncertainty involved for this day-3 forecast,
a broad Marginal Risk was introduced across portions of the
central to northern Plains, with an upgrade to Slight possible as
confidence increases with furture forecast cycles.

Otto

$$
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FOUS30 KWBC 252013
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 PM EDT Tue May 25 2021

Day 1
Valid 16Z Tue May 25 2021 - 12Z Wed May 26 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
EAST TEXAS AS WELL AS THE LOWER RED RIVER VALLEY REGION...

16z Update: Main change with this update was to connect the Slight
risk areas over eastern TX. Given how saturated ground conditions
are, it does look like isolated to scattered flash flooding will
develop this afternoon over this region. As of 16z we have an
eastward progressing convective line over north central TX, and
beginning to see cells develop in the southerly flow off the Gulf
as well. Would expect these south to north moving cells to
continue to grow in intensity with diurnal heating...and will
likely see some build into the southern flank of the eastward
progressing squall line as well. Where these cell mergers occur a
more focused flash flood risk could evolve this afternoon.

There is also a growing high res model signal for convection
tonight along the Lower Red River Valley, likely on the tail of
the departing MCV where convergence ends up maximized overnight.
Will need to monitor this area closely, as setups such as this can
lead to impressive localized rainfall totals...and thus a focused
area of potentially significant flash flooding. Confidence on
these details remains low...but the trend in the 12z CAMs is
concerning, and certainly something to watch.

Otherwise expanded the marginal a bit westward in west TX to match
the expected axis of dry line convective initiation. Often
guidance under does QPF along the dry line, incorrectly moving
cells too quickly off to the east. So wanted to capture the
potential of a few slow moving/merging cells closer to the dry
line placement.

Chenard

...Previous Discussion...

...Texas into eastern Oklahoma/western Arkansas...
As a strong mid-level trough shifts east across the central
Canadian provinces and the ridge settles farther into the
Southeastern U.S. the once strong moisture feed into the Southern
Plains will begin to weaken through the forecast period. However,
there will still be periods of heavy precipitation that could
result in localized to scattered flash flooding in some locations.

Persistent tropical moisture along the southwestern extent of the aforementioned deep ridge will continue to feed anomalously high
precipitable water values (>1.75 inches) into the Central TX
coast. While the plume is starting to reduce in magnitude
relative to last week, there remains ample moisture and filtered
insolation to increase instability off the Gulf. Regardless, the
uncertainty within this region is related to the mesoscale forcing
for ascent. As weak shortwaves continue to move aloft they may
interact with existing instability/surface boundaries that would
trigger convective activity. Based on the latest observational
data and model trends, there is growing consensus that this could
occur just inland of the Central TX coast and also across portions
of the Lower Red River Valley region.

While most of all the global models suggest less focused
convection near the Central TX coast, the high resolution guidance
is starting to highlight the potential. With impulses aloft
interacting with the instability and sea breeze at the surface,
this may be enough to promote slow moving convection atop very
saturated soils from the last several days. Rain rates of over
1.5 inches/hour is possible this afternoon with training leading
to hourly totals of over 2 inches in some locations. Areal
average precipitation could range between 3-6 inches. As a result,
rapid runoff is anticipated with scattered flash flooding
possible. Therefore, a Slight Risk was introduced in this narrow
corridor across from Gonzales to northeast of Houston.

Farther north, shortwave activity (and resultant strengthening low
level jet) will interact with an outflow boundary from this
mornings MCS that tracked across northern TX. These two features
alone should promote strong forcing for ascent within a very moist
and unstable airmass. Convection has already started to develop
early this morning with more anticipated as the ingredients start
to converge. Expect training/backbuilding convection along the
surface boundary as the Corfidi vectors briefly align with the
mean wind. This could result in hourly rain totals exceeding 2
inches. HREF supports this with fairly decent probabilities of 2
inches/1 hour and 3 inches/6 hours. Another round of convection
may occur during the late afternoon/evening as a surface boundary
moves through the region. With these two rounds of heavy rain and
sufficient soil saturation, some locations may observe scattered
flash flooding. Therefore, a Slight Risk was introduced within
this region.

More scattered convection is expected this afternoon/evening
across central/southern TX focused on residual boundaries/dryline
within a fairly moist and unstable environment. Slow moving
convection could lead to localized flash flooding. Farther
norther into OK, confidence is a bit uncertain with respect to
another possible MCS overnight. Development is possible under
strengthening low level flow as depicted by some of the high
resolution guidance. It appears the location of this may be too
displaced from the best moisture/instability, but we will continue
to monitor this potential.

...Eastern Kansas to Southern Wisconsin...
As a cold front advances east across the Northern Plains,
mid-level impulses aloft will track north interacting with pockets
of relatively high moisture and instability closer to the surface
from the Central Plains into the Upper Midwest. In response,
periods of heavy rain are anticipated within this region through
the forecast period, some of which could lead to localized flash
flooding.

Precipitable water values will climb above 1.5 inches aided by
20-30 knot southerly low level flow. This is generally 2 standard
deviations above the mean. Instability ahead of the front will
surge to over 1000 J/kg MUCAPE with ample shear within the low/mid
levels helping to organize convection that does develop, most
notably ahead of the cold front. Scattered to more widespread
convection should develop through the afternoon/evening ahead of
the boundary. Therefore, multiple rounds of >1 inches/hour rain
rates could result in flash flooding with some wet antecedent
conditions in place. Therefore, retained the Marginal Risk areas
and refined the spatial extent based on the latest 00Z guidance
and HREF probabilities.

Pagano


Day 2
Valid 12Z Wed May 26 2021 - 12Z Thu May 27 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
CENTRAL PLAINS...

...2030Z Update...

The main change to the Day 2 ERO was to add a small Marginal Risk
area to portions of southeastern Texas. Ample instability plus
diurnal heating should lead to scattered thunderstorms across the
region during the afternoon/evening, which could contain locally
heavy rainfall. Given how wet the region has been during the last
couple of weeks, with generally 300-600% of normal rainfall
amounts, soils are saturated and streamflows remain high, so
localized totals over an inch or two could cause isolated flash
flooding issues. There is also some chance for potentially heavy
rainfall around eastern Mississippi/western Alabama around
Wednesday evening as shortwave energy moves across, but appears
below excessive rainfall thresholds at this time.

Elsewhere, the Slight and Marginal Risks were maintained for the
Central Plains and adjusted minorly based on the most recent QPF
issuance, as well as HREF probabilities for exceeding flash flood
guidance. See the previous discussion below for more details on
the meteorological setup there.

Tate

...Previous Discussion...

...Kansas, Nebraska into South Dakota and Iowa...
The 00Z guidance continues to show good agreement on the timing,
strength and position of a mid-level shortwave, forecast to track
into Nebraska and the Dakotas Wednesday evening. A well-defined
front will be in place across the Midwest, curving
west-northwestward across Nebraska into a surface low along the
South Dakota/Wyoming border at 00Z Thursday. High Plains
thunderstorms should develop between 18Z-00Z with sufficient speed
shear supporting organization. PWATs are forecast to be near 1.25
inches in the pre-convective environment over northern Kansas into
southwestern Nebraska. The guidance is unanimous in the
strengthening of the low level jet to near 50 kt between 00Z-06Z
Thursday from Kansas into Nebraska, which is expected to support
an expansion of convective strength and coverage across the
Kansas/Nebraska border as an MCS lively develops. Forecasts of
Corfidi vectors show movement toward the ESE, parallel to the
gradient in MUCAPE.

Current thinking is for renewed convective development along the
southwestern flank of the expected MCS, near the Kansas/Nebraska
border, with training and repeating of cells. Flow aloft is
expected to be divergent and diffluent, helping to support
vertical motion as heavy rain shifts east through 12Z Thursday.
Rainfall rates of 1-2 in/hr and 2-4 inches of event total rainfall
are expected near the central Kansas/Nebraska border.

...South Dakota/southwestern Minnesota...
While instability is expected to be weak with northward extent
into South Dakota, strong upper level divergence ahead of the
shortwave mentioned above will be present within the right
entrance region of a strengthening upper level jet streak along
the U.S./Canadian border between 00Z-12Z Thursday. Moisture values
will be marginally anomalous across South Dakota, but the
combination of weak instability and strong dynamic lift could be
enough to support rainfall in excess of area flash flood guidance.
2 inches of rain may fall on a localized basis over a 3 to 6 hour
window.

...Southern to Central High Plains...
Convection is expected ahead of a dryline which will extend
roughly north to south from a surface low along the
Wyoming/Nebraska border. The 00Z model consensus forecasts PWATs
of 0.7 to 1.2 inches would represent standardize anomalies of +1
to +2. While coverage of storms is a bit uncertain with better
height falls occurring to the north, overlap of any slow moving
storms with recent heavy rainfall across the region may result in
localized flash flooding.

Otto

Day 3
Valid 12Z Thu May 27 2021 - 12Z Fri May 28 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN KANSAS
AND OKLAHOMA INTO THE MIDDLE MISSISSIPPI VALLEY...

...2030Z Update...

No major changes were needed to the Day 3 ERO based on the recent
model guidance and QPF cycle. The Slight Risk was expanded
westward and northward slightly in the Middle Mississippi Valley
as convection moves out of the Central Plains late Wednesday night
(Day 2 period) and into the area on Thursday. See the previous
discussion below for details on the meteorological setup.

Tate

...Previous Discussion...

...Southern/Central Plains into portions of the Ohio Valley...
Thunderstorms are expected to be ongoing across parts of the
middle/lower Missouri River Valley at the start of the period,
ahead of a surface low which is expected to be in eastern Nebraska
12Z Thursday. Northward placement of instability will be limited
given the presence of a strong warm front which will be in place
across the Mid-Mississippi Valley into the Ohio Valley. Movement
of storms are expected to be roughly parallel to the frontal
boundary which will support repeating rounds of heavy rain from
Iowa and northern Missouri, eastward into Illinois and Indiana.

Farther south, convection is expected to become reinvigorated with
daytime heating, within the warm sector of an advancing frontal
cyclone. There are some questions with thunderstorm initiation
along any remnant outflow boundaries from previous convection, but
a cold front trailing from the advancing low to the north should
be slower to move across Oklahoma, southern Missouri and northern
Arkansas. The SW to NE orientation of the front will align with
storm movement, supporting the possibility of repeating and
training. An open Gulf of Mexico will allow for solid moisture
transport from the south and PWATs of 1.5 to 1.8 inches across the
Plains into the Mid-Mississippi Valley. Winds aloft are expected
to be diffluent between a southwesterly jet to the north and
northwesterly flow in the wake of a departing upper trough in the
Southeast. Pockets of 2-4 inches seem likely across portions of
the central to southern Plains, eastward into southern Missouri,
supporting a Slight Risk at this time.

Otto

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FOUS30 KWBC 261603
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1203 PM EDT Wed May 26 2021

Day 1
Valid 16Z Wed May 26 2021 - 12Z Thu May 27 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF
CENTRAL PLAINS...

16z Update: No major changes to the general thinking and ERO areas
for today. The Marginal risk was removed over the lower MS Valley
where morning convection has dissipated. Localized flash flooding
is possible over far southeast TX as cells develop in the
southerly flow off the Gulf. Any heavier rains here will be very
small scale in nature...so probably not much more than a highly
localized flood risk if any urban/low lying areas get underneath a
cell that drops a quick 1-2" of rain.

The Slight risk over the central Plains looks in good shape. A
decent amount of uncertainty remains with the overall convective
evolution here. Already have some elevated cell development in
northeast KS that is not being handled well by any model guidance.
This activity may very well try to persist and become increasingly
surfaced based with time as it tracks eastward. Nonetheless we
should still see additional development by later this afternoon
over the High Plans as the better forcing ejects eastward. Unknown
how this earlier convection will impact this later development,
but would still expect to see organized development into an
eventual squall line. The squall line will be
progressive...however cell mergers may lead to a flash flood risk,
especially on the southern flank. Also will have to watch for
upstream development behind the squall line as southerly moisture
transport into the region should persist. Overall pretty good
confidence in an isolated, to perhaps, scattered flash flood risk
over the Slight risk area, even though confidence on the exact
details/location is lower. Not seeing too much of a signal for
anything more numerous/significant in nature...so the Slight
should do for now, but will continue to monitor trends and update
as needed.

Chenard

...Previous Discussion...

...Kansas, Nebraska, Missouri into South Dakota and Iowa...
A mid-level low will track across the Northern Rockies today
emerging into the Northern High Plains by the evening. In
response, strong divergence aloft coupled with mid-level impulses
will promote strong forcing for ascent. As a result, a surface
low will develop this afternoon, eventually riding along a
residual surface boundary through the evening/overnight hours.
With ample moisture and instability, convection will blossom this
afternoon, eventually merging into a possible MCS. Anticipate
scattered flash flooding with this system and the potential more
widespread impacts.

Ahead of the aforementioned upper level trough and surface low,
precipitable water values will climb to over 1.5 inches aided by
30-40 knot low level southerly flow. This is 1.5 to 2 standard
deviations above the mean. With sufficient diurnal heating,
instability will also soar to over 3000 J/kg. Given ample large
scale lift and high bulk shear, scattered convection will quickly
grow with cell mergers likely. This combined with the
strengthening low level jet will likely lead to the development of
an MCS near the NE/KS border during the evening hours. This system
will track east along the instability boundary with the cold
pool/outflow boundary shifting the heaviest rain south and east
fairly quickly. As is typical with these MCSs, anticipate
training to occur along the southwestern flank of the outflow
boundary. Rain rates will likely exceed 1.5 inches/hour and with
cell mergers, training and/or multiple rounds of heavy rain,
hourly storm totals could easily eclipse 2 inches in an hour and
3+ inches within 3 hours in some locations. However, given the
current model spread, HREF probabilities are not as high as one
may expect. The ARWs keep a bulk of the heavier precipitation
activity well south of the model consensus across southern KS.
Based on the current observational trends, feel the frontal zone
will be farther north and thus opted to lean toward the 00Z HRRR,
NAMNest and FV3 with the heavy rain occurring across southern NE
and northern KS. It should be noted that the model spread did
negatively influence the confidence when considering a Moderate
Risk for this event.

While there are small pockets of lower FFG values based on recent
precipitation activity, generally speaking a bulk of the region is
observing near normal soil conditions. Therefore, antecedent
conditions was also not a factor to support an introduction of a
Moderate Risk nor was the areal average of 2-4 inches of rainfall
expected. However, if models come into better agreement and/or
QPF increases, there may need to be an upgrade at subsequent
updates.

Farther north of the MCS, likely into eastern SD the potential for
flash flooding diminishes. However, there was enough high
resolution support to suggest brief periods of training.
Therefore, the Marginal Risk was retained over this area, despite
the modest FFG values.

...Central TX Coast...
Another round of convection is possible during the afternoon aided
by sea breeze/outflow boundary convergence. Confidence on the
coverage of precipitation is fairly low with not much mid/upper
level support. However, if convection is able to develop, heavy
rain over the saturated soils could lead to isolated flash
flooding. With sufficient moisture (precipitable water values over
1.5 inches aided by 30 knot southeasterly flow off the Gulf) and
instability climbing to over 2000 J/kg MUCAPE, there could be
periods of slow moving thunderstorms. Rain rates could climb over
1 inch/hour and while this is not anomalous, over saturated soils
it could lead to localized flash flooding. Therefore, retained
the marginal Risk area which is draped just inland from the TX
coast.

...Northern TX Panhandle...
With ample afternoon heating, convection is expected ahead of a
dryline which will extend roughly north to south from a surface
low over eastern WY. The 00Z models forecast precipitable water
values surging to over 1.25 inches aided by 40-50+ knot low level
flow. This is 1-2 standard deviations above the mean. While
coverage of storms is a bit uncertain with better height falls
occurring to the north, overlap of any slow moving storms with
recent heavy rainfall across the region may result in localized
flash flooding. Therefore,

...Lower Red River Valley...
An MCV lifting north through AR has dropped a residual outflow
boundary draped across far northeast TX into southern AR. As the
low level jet increased overnight, it drew higher moisture and
instability north aiding in the development of convection along
the boundary. Therefore, ongoing convection will train across
portions of southern AR through early morning. Rain rates of over
2 inches have been observed with storm totals near 5 inches in
some locations. However, as the low level jet starts to diminish
through the morning and the outflow boundary advances anticipate
the convection to propagate southward fairly quickly. As is
typically the case, these types of systems can be slow to
devolve/diminish. Therefore, opted to introduce a Marginal Risk
area for any lingering heavy rain/training within this region over
the first couple of hours of the forecast period. Please see MPD
#240 for more information.

Pagano


Day 2
Valid 12Z Thu May 27 2021 - 12Z Fri May 28 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN KANSAS
AND OKLAHOMA INTO THE MIDDLE MISSISSIPPI VALLEY...

...Central/Southern Plains into the Midwest...
The 00Z models show good agreement on the track of a notable
mid-level shortwave tracking eastward through the central/northern
Plains into the Upper Midwest from Thursday into Friday. An MCS is
expected to be ongoing at the start of the period (12Z Thursday)
over IA/MO/KS, with training of heavy rain along its southwestern
flank from eastern KS into western MO. While some weakening of the
50+ kt low level jet over eastern KS is expected shortly after 12Z
Thursday, 850 mb flow of 30+ kt is forecast to persist through 00Z
from KS/OK into MO/IL, before restrengthening Thursday night. An
axis of PWATS ranging from 1.5 to 2.0 inches should be in place
out ahead of a cold front from OK into the Midwest (+2 to +3
standardized anomalies) beneath a diffluent flow regime aloft.

The MCS should weaken and/or move away from eastern KS/western MO
beyond 18Z Thursday, but additional thunderstorm development is
likely in the 21Z-00Z window later Thursday ahead of a surface low
in northern MO and the trailing cold front into northwestern OK.
Just beyond 00Z Friday, re-strengthening of the low level jet
should occur from eastern OK into MO/AR and southern IL. This will
be coupled with increasing ascent within the right entrance region
of a strengthening upper level jet max located on the southeastern
side of the advancing upper level shortwave. Low level flow will
be of a similar magnitude and direction to the 850-300 mb mean
flow, supportive of slow or backward propagating Corfidi vectors.
Given potential overlap of early Thursday morning rainfall with
Thursday evening rainfall, an upgrade to Moderate could occur if
confidence increase within the model guidance from portions of
eastern KS into MO. Where overlap of heavy rain from early
Thursday and rainfall later in the day occurs, rainfall totals of
3-6 inches will be possible. Farther south into northeastern OK,
convection will have the potential for flash flooding rains in the
00Z-06Z window before the front progresses south and east of the
region.

...Iowa into the Upper Mississippi Valley...
As a surface low tracks eastward across northern Missouri Thursday morning/afternoon, the northern end of a region of strong moisture
flux will overlap central and eastern IA. PWAT values will vary
from about 1.6 inches over southern IA to about 1 inch near MSP.
Given the favored low track of the 00Z GFS/ECMWF/UKMET,
instability should be limited with northward extent (north of the
warm front) which should tend to limit rainfall rates to 1 inch in
1-3 hours. However, there will be strong support aloft with
diffluent flow and lift within the entrance region of a departing
130 kt upper jet streak. Potential for 1-2 inches of rainfall from
central IA into southern Wisconsin exists, with the bulk likely
occurring within the 12Z-00Z time frame.

Otto

Day 3
Valid 12Z Fri May 28 2021 - 12Z Sat May 29 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE SOUTHERN
PLAINS INTO THE LOWER MISSISSIPPI VALLEY AS WELL AS THE
TENNESSEE/OHIO VALLEY INTO THE MID-ATLANTIC REGION...

...Southern Plains into Arkansas and Louisiana...
Thunderstorms are expected near a cold front forecast to drop
southward rather progressively for locations east of the
Mississippi River, but the front should track more slowly...even
remain stationary...for locations west of the Mississippi into the
Southern Plains. A consensus of 00Z models shows a weak mid-level
shortwave tracking southeastward from the CO/NM border through the
period. PWATs will remain high across the Southern Plains and
eastward along the front, with broad anomalies of +1 to +2.

While the focus for the heaviest rainfall will likely be mesoscale
in nature, forecast storm motions are expected to be less than 20
kt, slowest with southward extent, reaching less than 10 kt closer
to the Rio Grande. While thunderstorms may be ongoing at the start
of the period, coverage and intensity of storms should increase
notably during the afternoon/peak heating hours as the shortwave
impulse arrives. Forecasts from the 00Z GFS show the low level jet
intensifying overnight across parts of central TX which may help
to prolong rainfall and support organization and/or
intensification of any clusters that develop. Local rainfall
totals of 2-4 inches are expected through Saturday morning, and
any overlap with areas of recent heavy rainfall will increase the
potential for flash flooding.

...Tennessee/Ohio Valley into the Mid-Atlantic...
A strong shortwave is expected to track east into the Ohio Valley
on Friday/Friday night with an accompanying surface low to move
eastward toward the central Appalachians by Friday evening. A cold
front will trail southwest of the surface low and a
quasi-stationary front will be in place to the east, crossing the
east coast near the VA/NC border. Secondary surface low
development is expected by Friday evening over the Mid-Atlantic
region before the low tracks off into the western Atlantic for
Saturday morning.

Despite expected cloud cover, daytime heating should allow for an
increase in instability during the afternoon/early evening hours
ahead of the progressive cold front as it tracks eastward through
KY/TN into the Appalachians. PWAT anomalies of +1 to +2 (1.5 to
1.8 inches) will be present and forecast soundings from the NAM
and GFS support unidirectional flow from the WSW which could allow
for brief periods of training and rainfall rates of 1-2 in/hr.
Farther east, while model QPF guidance is not too aggressive, the
presence of a slow moving front and forecast CAPE values of
1000-2000 J/kg could support a few areas of flash flooding,
especially near the coast where 850 mb flow increases in the late
evening/early overnight to 25-35 kt, in excess of 850-300 mb mean
flow.

To the north, from much of PA into southern New England,
instability is expected to be limited, and this is depicted in
even the more aggressive 00Z NAM output. However, forcing for
ascent will be strong with a defined region of 850-700 mb
frontogenesis extending from west to east across the northern
Mid-Atlantic region, coupled with lift within the right-entrance
region of a 110 kt upper level jet max. While 24 hour rainfall
totals of 1-3 inches are expected within this region of strong
lift, limited instability and fairly dry antecedent soils should
prevent any organized areas of flash flooding given reduced
rainfall rates. Therefore, the Marginal Risk for Day 3 does not
overlap with some of the heaviest forecast rainfall for the Day 3
period from PA into southern New England.

Otto

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FOUS30 KWBC 272031
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Thu May 27 2021

Day 1
Valid 1832Z Thu May 27 2021 - 12Z Fri May 28 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM OKLAHOMA
INTO SOUTHWEST MISSOURI AND ADJACENT AREAS...

1830z Update: Based on latest observational trends we decided to
go ahead with a Moderate risk upgrade for portions of
central/eastern OK into southwest MO. Given the PWs in place, the
pool of upstream MLCAPE over 3000 J/kg, and persistent moisture transport...continued convective development will result in
multiple convective rounds with periodic training/backbuilding.
Thus would expect numerous flash flood issues to evolve with time
over the MDT risk area...with the expected flash flood coverage
just too much to justify keeping the risk at Slight. Still lower
confidence on whether the magnitude of impacts will quite get to
the level our MDT risks often get too...but certainly have the
potential for localized higher end impacts, especially if any
urban areas are involved.

Chenard

16z Update: Forecast still looks in pretty good shape. Isolated to
scattered flash flooding seems likely from OK into IL. There may
be a swath of more numerous flash flooding somewhere across
portions of central/eastern OK, western AR and far southwest MO.
Multiple rounds of convection are expected across this area, with
CAPE values increasing towards 4000 J/kg in spots. Cells will
generally be quick moving...but periodic cell mergers and the
multiple rounds may still end up producing a more focused corridor
of flash flooding. Even progressive cells will produce very high
instantaneous rainfall rates given the PWs/instability in place.
Contemplated a Moderate risk...but the general model consensus is
for swaths up to 3-5" of rain...which probably falls just below
the numbers needed for the more numerous and significant impacts a
Moderate risk would imply. And if higher totals in the 4-8"
amounts do fall, confidence on that preferred swath is just not
there at the moment...especially given the generally poor
initialization of the 12z HREF members and recent HRRR runs. Thus
for now opted to stick with the Slight risk and continue to
monitor...and keeping in mind that a Slight risk still implies the
expectation of scattered flash flooding, which could still be
locally significant in nature.

Chenard

...Previous Discussion...

Widespread heavy to locally excessive rainfall possible day 1
stretching from portions of the Southern Plains into the Lower
Missouri Valley and Mid Mississippi Valley in association with
height falls pushing eastward across the mid section of the
nation. These height falls will be accentuating large scale uvvs
in an axis of increasing PW values, 2 to 2.5+ standard deviations
above the mean, along and ahead of the associated frontal boundary
pressing southeastward across the risk areas day 1. There is the
potential for training of cells in a west to east direction as the
moist low level west southwest flow becomes parallel to this
boundary. The greatest threat for training appears to be early in
the forecast period across northern portions of Missouri and then
Thursday evening into Thursday night across northern to central
Oklahoma. Overall, not a lot of changes to the marginal and
slight risk areas from the previous outlooks for this period. The
slight risk was extended farther to the south into North Texas to
cover the latest hi res model qpf spread and the marginal risk was
extended north into southern Minnesota given model qpf and lower
ffg values across this area. The slight risk area encompasses the
regions of where the latest HREF neighborhood probabilities are
fairly high for 2"+ totals this period, generally in the 40-70%+
range. Probabilities for 3"+ totals fall to the 20-50%+ range and
to 10-30% for 5"+ amounts, but focus on the two above mentioned
region of greatest training potential. Inside the risk areas
drawn, there is a lot of spread with the qpf details, but not
unexpected given the widespread region of potentially heavy
totals. Given this spread, confidence is not high for the
depiction of any risk greater than slight, at the moment.

Oravec


Day 2
Valid 12Z Fri May 28 2021 - 12Z Sat May 29 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE SOUTHERN
PLAINS INTO THE LOWER MISSISSIPPI VALLEY AS WELL AS THE OH/TN
VALLEYS, CENTRAL AND SOUTHERN APPALACHIANS AND MID-ATLANTIC...

2030Z Update...
Overall, no significant changes were made to the previous outlook,
with Marginal Risk areas maintained for both regions.

One minor adjustment was to extend the Marginal Risk farther south
into the southern Appalachians. Latest model runs are presenting
a significant signal for locally heavy amounts within the region
-- with the 12Z HREF showing high neighborhood probabilities for
accumulations of 2-inches or more as far south as northern
Georgia. Still monitoring the potential need for an upgrade to a
Slight Risk across portions of the central Appalachians into the
Mid-Atlantic I-95 corridor, but given the remaining uncertainty as
described below, opted to maintain the Marginal Risk for now.

Also, remain confident that locally heavy amounts are likely to
occur within the Marginal Risk area over the southern Plains and
lower Mississippi Valley, and an upgrade at some point may be
needed. However, model spread remains significant, and therefore
confidence on the placement of heavy amounts is limited.

Pereira

Previous Discussion...
...Southern Plains into Lower Mississippi Valley...
A lot of uncertainty remains within this region through the
forecast period as an upper level trough moves through the eastern
half of the country with an associated cold front becoming draped
from NM/TX east into the Lower MS Valley region. With shortwave
activity expected to move atop this surface boundary and other
residual outflow boundaries, convection should develop during the
day and track to the south and east. Where this convection
develops and its overall propagation is hard to discern at this
update as models are struggling to determine the placement of the aforementioned shortwave (likely coming out of NM) and various
surface boundaries. The environment south of the front will be
moist and unstable with precipitable water values climbing to over
1.75 inches and MUCAPE surging to over 3000 J/kg. Therefore,
whatever convection does develop may produce over 1.5 inches/hour
rain rates. Training of thunderstorms is also possible which
could lead to areal average precipitation of 3-5+ inches in some
locations. With decent bulk shear, organized convection and
potential MCS could develop.

With the wet antecedent conditions in mind and the uncertainty as
described above, kept the Marginal Risk area broad. If models
depict a more robust area of organized convection, an upgraded
risk area may be needed. At this time localized flash flooding
seems to be a high likelihood within the Marginal Risk.

...OH/TN Valleys, Central Appalachians and Mid-Atlantic...
A mid-level trough moving from the Midwest toward the OH Valley
will sharpen through the forecast period. Meanwhile, the upper
level jet will begin to strengthen with strong divergence noted
within the right entrance region. This combined with mid-level
shortwaves will lead to strong large scale forcing for ascent just
south of the Great Lakes region. As a result, a surface low will
deepen as it moves from the OH Valley to just off the Mid-Atlantic
coast by Saturday morning. A trailing cold front will sweep
through the OH/TN Valleys and the Mid-Atlantic. Ahead of this
front, rich moisture and instability will promote convection ahead
and along the front.

There is quite a bit of uncertainty with respect to high far north
the warm front will lift, thus impacting how far north the
instability will surge. Regardless, precipitable water values
will climb to over 1.75 inches aided by 35+ knot low level
southwesterly flow. This is 1-2 standard deviations above the
mean. Meanwhile, destabilization is expected ahead of the front
to over 1000 J/kg in many locations. One limiting factor for
heavy rain leading to flash flooding will be the alignment of
higher precipitable water values with the instability. Based on
the current model guidance, rain rates of 1.25 inches/hour is
expected with some of these storms. However, with training and
multiple rounds of heavy rain, hourly storm totals may be higher.
Also, despite locations north of DC/BWI being more stable, with
multiple rounds of heavy rain, the I-95 corridor could see 2-4+
inches of rain through the forecast period.

While much of the east has been dry over the past couple of weeks,
which is clearly noted by the exceptionally dry soil conditions as
seen via NASA SPoRT, there could still be locations that observe
localized flash flooding. Portions of western PA and much of WV
have lower FFG and could be more vulnerable to heavy rain. Also,
the urban (I-95) corridor could also be sensitive to multiple
rounds of heavy rain. Therefore, the Marginal Risk was expanded.
If models come into better agreement, lift the front farther north
and increase QPF, there may be a need to upgrade a portion of the
area to a Slight Risk. However, uncertainty and dry conditions
have negatively influenced the confidence that more widespread
issues would arise.

Pagano

Day 3
Valid 12Z Sat May 29 2021 - 12Z Sun May 30 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL PLAINS AS WELL AS THE SOUTHERN PLAINS...

2030Z Update...
Overall, no significant changes were made to the previous outlook.

Pereira

Previous Discussion...
...Central Plains...
A trough axis moving through the Northern Rockies/Northern High
Plains will usher in rich moisture northward. Anticipate
destabilization through the afternoon with upslope flow toward the
higher terrain. As a result, convection will develop over the
terrain and move out into the Plains where better moisture and
instability will reside helping to strengthen the storms.
Sufficient shear will also be available to allow convection to
become better organized. With precipitable water values of over
1.5 inches and instability above 1000 J/kg (MUCAPE), rain rates of
over 1 inches/hour will be likely. In addition, training and slow
moving convection could exacerbate hourly storm totals. Given
this region has observed above normal precipitation over the past
few weeks (some locations over 400% of normal), heavy rain may
lead to localized flash flooding. Therefore, a Marginal Risk was
introduced at this update.

...Southern Plains...
Ongoing convection from a possible MCS may lead to localized flash
flooding across portions of eastern TX and LA. Typically this
time of year is challenging when trying to forecast surface
features, such as residual outflow boundaries. With this event,
we are assuming that convection will initiate across portions of western/central TX Friday night/early Saturday and translate east
into eastern TX. So, it is expected that the placement of this
risk area will be adjusted as models and trends are better
understood. Regardless, anticipate rich Gulf moisture and
lingering instability to aid in continued convection atop
saturated soils. Therefore, localized flash flooding is possible.

Pagano

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FOUS30 KWBC 012004
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 PM EDT Tue Jun 01 2021

Day 1
Valid 16Z Tue Jun 01 2021 - 12Z Wed Jun 02 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL TO
NORTHERN LOUISIANA, ACROSS ARKANSAS INTO SOUTHERN MISSOURI...

...Southern Missouri into Arkansas, northern to Central
Louisiana...
1600 UTC Update -- Minor adjustments were made to the Slight Risk
area, mainly to include a bit more real estate across eastern TX
and the Upper TX coast, based on the latest observational
(convective) trends and 12Z high-res CAMs, including the HREF
exceedance probabilities.

0800 UTC Discussion...

A MCV will continue to press eastward out of Oklahoma and across
northern Arkansas/southern Missouri Tuesday and toward the Lower
Ohio Valley by early Wednesday. Model consensus is for an axis of
heavy rains and isolated excessive rainfall amounts in the axis of
above average PW values stretching from southern Missouri, through
much of Arkansas into northern to central Louisiana to the east
and southeast of this MCV. The best instability will be to the
south of the MCV, across much of Arkansas into Louisiana. The
slight risk was extended farther to the south from the previous
issuance into northern and central Louisiana given this. This is
supported by the latest HREF neighborhood probabilities that are
high (50-90%+) for 2"+ amounts into northern to central Louisiana
and 30-60%+ for 3"+ amounts this period. On the hourly time
scale, the greater probabilities for 1 and 2"/hr amounts are from
central AR into northern to central Louisiana given the location
of the better instability. We were tempted to trim the northern
portion of the slight risk area, but given the model consensus for
heavy amounts farther to the north and current watches, we did
not. Models do have a heavy bias closer to the MCV track, with
the heavier totals often farther south into the better
instability. The marginal risk was also expanded farther eastward
into the Middle Tennessee Valley and central Gulf coast to cover
model spread with heavy rains across these areas.

...South Texas, northwestward into West Texas and eastern New
Mexico...
The previous marginal risk area over portions of central Texas was
expanded into West Texas and eastern New Mexico and pushed
southward into South Texas. The extension of the marginal risk
into South Texas was to cover current convection pressing
southeastward through the Lower Rio Grande Valley. This activity
may affect South Texas for the first few hours of the day 1 period
after 1200 UTC Tuesday. Across West Texas into eastern New
Mexico, additional scattered convection likely to fire this
afternoon from southeast Colorado into eastern New Mexico and West
Texas. With recent rains across these areas, FFG values have
lowered. While confidence is low with respect to details, there
is a model signal for locally heavy rains. HREF neighborhood
probabilities are greatest across West Texas, with 40-60%
probabilities of 1"+ totals this period, with lower probabilities
westward into eastern New Mexico.

Hurley/Oravec



Day 2
Valid 12Z Wed Jun 02 2021 - 12Z Thu Jun 03 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL STRETCHING FROM
THE OHIO VALLEY TO THE LOWER MISSISSIPPI VALLEY AS WELL AS WESTERN
EDWARDS PLATEAU/RIO GRANDE VALLEY IN TEXAS...

21Z Update...Model trends continue to hit southern TX with heavier
amounts of precipitation during this time period. With soils
saturated at 300 to 600% above normal, it won't take much to see
any kind of flooding to occur, especially given the proceeding
days of precipitation leading up to this event. Left the Marginal
Risk area for this region as was issued during the midnight shift.
For areas of the Ohio River Valley down through lower Mississippi
River Valley, tightened in the Marginal Risk region to account for
model guidance, but did not see any major changes from what QPF
showed during the overnight hours. Solid moisture flux convergence
with favorable Corfidi propagation vectors could cause expansion
of cells upstream.

Chiari

Previous Discussion...

...Ohio River Valley through lower Mississippi River Valley...
At the start of the forecast period, 02.12z Wednesday, a
sharpening mid-level trof across the mid-Mississippi Valley with
an embedded shortwave bottoming out across KS/OK before lifting
into the central Great Lakes by the end of the period. Deep
nearly unidirectional south-southwesterly flow will exist from
return moisture stream off the central Gulf of Mexico pooled along
and ahead of slow eastward pressing frontal zone. A weak surface
wave along/ahead of the shortwave will lift out of the
Mid-Mississippi River Valley and track toward the central Great
Lakes, supporting warm-air advection and mild moisture flux across
the Ohio Valley. Though overall moisture will increase to 1.5"
and instability will build across central KY into S Ohio with
1000-1250 j/kg available. Limiting factor to convective coverage
will be the limited convergence given the deep parallel flow to
the boundary, but there will be spots for development across the
Ohio Valley with Hi-Res CAMs trending toward greater convergence
across N MS by midday. Any cells that do develop will have the
potential to expand upstream with favorable Corfidi propagation
vectors and solid moisture flux convergence. The best potential
for excessive rainfall, will be short-duration training given the
deep steering flow and slow eastward march of the frontal zone.
Additionally, cells from overnight on Wednesday into Thursday may
have tracked well in advance of the frontal forcing (pre-frontal
convective line) to pre-soak grounds for the new round in late
evening to potentially cross. Given this, little change was made
to the Marginal Risk of Excessive Rainfall area from IND/OH back
to the Lower MS Valley; however, did expand to the LA coast where
soil conditions remain saturated from prior 2 weeks per AHPS, as
well as an increasing signal for scattered slow moving cells in
the Hi-res CAMs.


...Western Edwards Plateau into Rio Grande Valley...
It is probable that a convective complex and associated boundaries
will be placed across southwest Texas from the Day 1 period, well
in advance of the stalling/tail-end of the frontal zone as it sags
southward across the Permian Basin/Edward Plateau. While the
initial complex is likely to overturn some of the more
unstable/deeper available moisture in the Rio Grande Valley, there
is solid model agreement in return response through the
morning/early afternoon to support 1.6-1.75" of total PWats and up
to 2000 j/kg CAPEs along/south of the frontal zone across the
lower Pecos Valley and Edwards Plateau. Solid moisture
convergence from 15-20kts of flow back up the valley should be
sufficient to break out a new round of scattered convection. Weak
steering flow, should allow for southeast propagation across areas
that have received well above normal rainfall over the last few
days, with more to come in the short-term forecast. Recent
guidance trends continues to slide further south, but with two
distinct solutions; one centered along the higher terrain of the
Sierra Madre Oriental (ECMWF) while GFS/NAM are more along the
Edwards Plateau into the South Texas Plains. Both suggest slow
motions to allow for isolated pockets of 3-5" totals (or greater)
suggesting a Slight Risk will potentially be needed, but with
contingency on placement of Day 1 activity/boundaries, will
continue to maintain a Marginal Risk of Excessive Rainfall at this
time.

Gallina

Day 3
Valid 12Z Thu Jun 03 2021 - 12Z Fri Jun 04 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN
PENNSYLVANIA & NEW JERSEY AS WELL AS SOUTHERN VIRGINIA SOUTHWARD
INTO NORTHERN SOUTH CAROLINA AND PORTIONS OF SOUTHERN TEXAS...

21Z Update...

Eastern Pennsylvania and New Jersey...Models continue to show the
warm moist flow from off the water coupled with the shortwave
trough pushing in from the west bringing ample amounts of
precipitation to an area with very low FFG values. Models pushing
1 to 3 inches of additional rainfall during this time period with
AHPS of 200 to 400% of normal along the coastal waters of NJ and
150 to 200% of normal over the past week as you head into PA.
Because of this, the Marginal Risk area introduced on the midnight
shift has remained in place.

Southern Virginia southward through central North Carolina and
into portions of northern South Carolina...Similar to PA and NJ,
moist air pushing along the coast gets enhanced as the trough axis
makes its way into the region bringing long periods of
precipitation. PWAT values sit near 1.25 to 1.5 inches placing
them around 1 to 1.5 std deviations above climo. FFG sits near 2
to 4 inches for this region with the average among models sitting
near 1 to 3 inches with each of these models showing pockets of 2
to 4 inches somewhere within this region. Exact locations of where
this heavier precipitation will fall is still a bit uncertain, but
because of the current forecast amounts have introduced a Marginal
Risk area to this region.

Southern Texas...At this point, this region feels like a broken
record with AHPS showing 300 to 600% of normal soil saturation
over the past two weeks with FFG sitting near 2 to 3 inches and
latest model guidance placing pockets of 1 to 3 inches during this
period alone. Take into account the precipitation currently
occurring for this region and additional amounts for Day 1 and Day
2, a Marginal Risk area seems the most reasonable at this time.

Chiari

Previous Discussion...

...Mid-Atlantic...
Strong return flow (~30kts at 850mb) out of the Western Atlantic
by the start of the forecast period 03.12z, Thursday, bringing
deep sub-tropical moisture back across the Mid-Atlantic with Total
PWats surging to near 1.75". Being sub-tropical in nature, the
low level profiles will be very warm supporting narrow skinny
unstable environment with deep warm-cloud for efficient rainfall
production. While sheared mid-level shortwave progresses through
the Great Lakes it will also direct deeper moisture stream from
the Ohio/Mississippi Valleys across the Central Appalachians by
mid-afternoon maintaining deeper moisture profiles throughout the
day. The question will be forcing necessary to break out
convective cells, there may be enough moisture convergence in the
morning for some activity, perhaps enhanced by frictional
convergence at the northern edge of the Chesapeake Bay. However,
by mid-morning, guidance starts to suggest filtered insolation and
increase in instability, up to 750-1000 j/kg which is solid for
stronger convection given moist adiabatic lapse rates.

For cells that do develop, slant-wise ascent into enhancing right
entrance to 250mb polar jet should provide a solid outflow
environment to enhance UVVs across the area. Solid 15-20kts of
slowly veering low level inflow will keep potential for
high-efficiency cells (up to 2"/hr), though updrafts may be narrow
to reduce overall coverage and potential for training/repeat
convection. Still, the quick burst through areas of very low FFG
values, suggest widely scattered flash flooding conditions are
possible, particularly in urban corridors with greater run-off.
Additionally, global guidance suite remains uncertain to placement
and highest rainfall totals across the area of concern, being
contingent on Day 1 & 2 evolution upstream and timing of
additional mid-level moisture and better synoptic forcing from the
Ohio Valley. As such, in coordination with local forecast
offices, have placed a small Marginal Risk in the best
intersection of forcing/moisture flux and low FFG values from a
variety of guidance solutions which is currently centered over
eastern Pennsylvania, New Jersey. Slower timing from the Atlantic
moisture return or faster forcing from the west, could increase
threat for DC to Baltimore into Central PA; opposite evoltuions
could place the Catskills and south-central NY into play as well.

Gallina

$$
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FOUS30 KWBC 021622
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1222 PM EDT Wed Jun 02 2021

Day 1
Valid 16Z Wed Jun 02 2021 - 12Z Thu Jun 03 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE LOWER
MISSISSIPPI VALLEY THROUGH THE LOWER TENNESSEE VALLEY, THE LOWER
OHIO VALLEY, ACROSS SOUTH TO SOUTH CENTRAL TEXAS, AND FROM
NORTHEAST SOUTH CAROLINA INTO EASTERN NORTH CAROLINA...

...Lower Mississippi Valley, Lower Tennessee Valley, Lower Ohio
Valley...
A broad marginal risk area was maintained for the upcoming day 1
time period from the Lower Mississippi Valley, northeastward into
the Lower Ohio Valley along and ahead of the slow moving north to
south oriented frontal boundary across these areas. Shortwave
energy moving northeastward on the eastern side of the mid to
upper level trof approaching the Mississippi Valley day 1 will
accentuate lift in an axis of slightly above average PW values
expected across these areas. Model consensus is for an axis of
moderate to heavy totals across this region with HREF
neighborhood probabilities high for 1" and 2"+ amounts this
period, 60-90% and 40-80% respectively. With observed rainfall
totals below average over the past two weeks across nearly all of
this region, save for portions of southern Louisiana, stream flows
as per the National Water Model begin the period below average.
Given this, have opted to keep the risk level at marginal.

...South to South Central Texas...
Additional shortwave energy expected to round the base of the mid
to upper level trof across the Southern Plains, accentuating lift
in the vicinity of the stationary front and associated axis of
instability (mu-cape values 1500 j/kg+) stretching west to east
across south central Texas. This will likely support additional
convection pressing southeastward along and to the south of this
front from late afternoon Wednesday into Wednesday night/early
Thursday over portions of south central to south Texas. There is
still a sizable amount of spread in the latest guidance to pin
point where the heaviest totals may fall. Still, HREF neighborhood probabilities are fairly high for 2"+ totals across the marginal
risk area, 40-80%. Much of this region has seen heavy rains over
the past few days, raising stream flow and lowering ffg values.
The previous marginal risk area was expanded approximately 150
miles to the northwest and 50-100 miles to the east toward the
central to upper Texas coast to cover the spread of heavy precip
output from the latest guidance.

...Coastal Plain from northeast South Carolina to eastern North
Carolina...
Hoisted a Marginal Risk area with the 1600 UTC Day 1 ERO update,
based on the current mesoanalysis and trends with the 12Z high-res
CAM guidance. Convective initiation along a slow-moving warm
frontal boundary (moving n-nw) will continue to blossom along this
boundary, with the moisture flux convergence aided by the sea
breeze component. While this area in general hasn't seen much rain
over the past couple of weeks (rainfall 50% or less percent of
normal per AHPS over much of this region), favorable
thermodynamics with the potential for training convection along a
slow moving boundary will foster the potential for isolated or
localized flash flooding, especially over more urban-sub-urban
locations. This as surface-based CAPEs between 1000-2000 j/kg
along with the positive low-level theta-e advection (PWs
increasing to 1.75+ inches) promotes hourly rainfall rates of
1.5-2.0 inches. A few of the 12Z CAMs show spotty 3-5+ inch
totals, including the NAM CONUS-Nest and the FV3.

Hurley/Oravec


Day 2
Valid 12Z Thu Jun 03 2021 - 12Z Fri Jun 04 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
MID-ATLANTIC INTO FAR NORTHERN CAROLINAS WELL AS PORTIONS OF
SOUTHERN TEXAS...

...Mid-Atlantic...
At the start of the forecast period, 03.12z Thursday, return
subtropical moisture along the western periphery of the
sub-tropical ridge has surged northward across the coastal
Mid-Atlantic with solid WAA regime. In the wake, a 1.75" Total
PWat axis extends from the Eastern Carolinas toward Long Island
and Southeast New England, yet much of the eastern Mid-Atlantic
states lays within the anomalous moisture gradient nearing 95th
percentile and 1.7-2 StdDev. Upstream, a weakening long-wave
trough is crossing the Great Lakes, with an embedded shortwave in
the Ohio Valley and favorable placement of the entrance region
across the region with better synoptic forcing/ascent expected
further north into NY/PA. Still, early morning clearing should
allow for solid insolation to enhance the narrow skinny CAPE
profiles with 1000 J/kg expected. As such, highly efficient
rainfall producing showers and thunderstorms are expected across
the central Mid-Atlantic advancing eastward by late evening,
entering deeper moisture as well as slight increase in low level
confluence to perk up rain rates, resulting in sub-hourly totals
of 1-1.5" and potential for flash flooding. There is some
uncertainty on the timing of the shortwave/frontal zone crossing
the Mid-Atlantic; some slower solutions suggest enough timing for
an additional round of showers/thunderstorms after dark,
potentially supporting repeats/cross tracks. So if the first cell
doesn't result in flash flooding, a second cell has increased
potential (generally typical of Northeast flash flooding days).

Still, generally the hydrologic situation across S NY/E PA toward
the Chesapeake Bay and the eastern necks of VA toward urban areas
of SE VA have seen recent above average precipitation and wetter
than normal ground conditions. The northern Mid-Atlantic and S NY
have the lowest FFG supported by highest 0-40cm Soil Saturation
values from NASA and the National Water Model (generally over
80%), however, there are more scattered pockets across MD/E VA
than further north, but remain at risk for isolated flash flooding
with these quick burst heavy warm-cloud downpours.


...Eastern North Carolina...
Synoptically, a similar situation is unfolding across S VA into
the Carolinas as described above, with one significant difference.
North Carolina has very dry ground conditions with 0-40cm soil
saturation below 10%. However, there is a solid signal for 1-3"
scattered totals across this region through the day 1 period
(ending 03.12z), potentially resulting in worsening ground
conditions for additional rainfall on Day 2. Additionally,
strong sea-breeze convection should slowly march west-northwest by
evening on Thursday. At the same time, stronger mid-level forcing
and southwesterly flow aloft is directing the western moisture
plume across the region, as the frontal zone/upslope convection
advances into the Piedmont at or just after 00z. As such, a
collision of thunderstorms and flow regimes is likely in induced a
very short but quite intense intersection/ascent with merging
thunderstorms. As a result, extreme but highly isolated rates
with potential over 3"/hr may manifest in the vicinity of the
major metro centers of central North Carolina. With this
combination of factors from Day 1 and mergers, WPC has shaped the
Marginal Risk area to best account for this scenario with the
Marginal Risk across the Mid-Atlantic.


...South Texas...
Tail end of the mid to upper level trough and stalled surface
boundary will continue to meander around West to Southern Texas.
Return moisture off the western Gulf, day time heating and
intersection with old MCS boundaries is likely to result in
scattered clusters of thunderstorms that may congeal and enhance
with rain rates up to and exceeding 2"/hr. There is generally low
confidence in precise locations given contingency on day 1
evolution, however, compromised solid conditions with above
average 2-week anomalies seen across much of the region support
maintaining a Marginal Risk of Excessive Rainfall across much of
South Texas.

Gallina

Day 3
Valid 12Z Fri Jun 04 2021 - 12Z Sat Jun 05 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN
TEXAS...

...South Texas...
By the start of the Day 3 period, 04.12z, the mid to upper level
trof has severed from the northern stream to a weak closed low
over the Big Bend of Texas. So once again, remnant convective
outflow boundaries will be intersecting with return moisture flow
off the western Gulf. Moisture will have also begun to
increase/pool from surges out of the Caribbean with 2" total PWats
in proximity to far south Texas over the Western Gulf; which is
about 2.5 StdDev values even for this time of year. Solar
insolation should be sufficient again for solid instability to
build for new development. Given the weakening of the upper level
wind flow, cell motions will become even slower increasing
duration over any given location. Even though weaker, upper level
flow should be supportive of broad scale ascent to support some
increase in scale of individual cells to clusters and with solid
low level moisture flux from the southeast, propagation is likely
to be generally eastward into the deeper moisture increasing
rainfall efficiency perhaps with some isallobaric enhanced
moisture flux as well. Given the manner of cell development,
there will remain widely scattered flash flooding concerns across
South Texas...much like prior days over above normal deep soil
saturation...so another Marginal Risk will be placed across the
region for day 3.

Gallina

$$
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FOUS30 KWBC 031917
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Thu Jun 03 2021

Day 1
Valid 1716Z Thu Jun 03 2021 - 12Z Fri Jun 04 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHERN TEXAS AS WELL AS THE CAROLINAS AND MID-ATLANTIC...

...17Z UPDATE...
Based on the latest high resolution guidance (which inevitably
lower WPC QPF), drier antecedent conditions, and through
coordination with WFO ALY, dropped the Slight Risk area for
portions of central NY. Locally heavy rain resulting in 1-2+
inches with isolated flash flooding is possible. Therefore, the
Marginal category should be more representative of the risk
potential through the overnight.

...16Z UPDATE...
Made adjustments to the risk areas based on the latest
observations and 12Z high resolution model trends.

Expanded the Slight Risk across much of the Carolinas and into
portions of the Mid-Atlantic region as new guidance indicates a
better signal for heavy rain with HREF probabilities increasing
for the 1 and 2+ inch per hour rain rates rates. While much of
Eastern NC has observed heavy rain from yesterdays thunderstorm
activity, there are pockets of lower FFG across NC/VA and into the
Mid-Atlantic that would suggest the potential for scattered flash
flooding. If this were to occur, it would likely be over urban
corridors.

Expanded the Marginal Risk back across portions of the OH/TN
Valleys as well as the Appalachians with models suggesting
multiple rounds of moderate to heavy rain. The MCV moving out of
southern IL and tracking north and east will help to focus
convection and allow for a better potential for training. HREF
probabilities have increased for 2+ inches per hour rain rates
which may occur over saturated soils. Therefore, isolated flash
flooding could occur within this region.

Slightly modified the Marginal and Slight Risk areas across
Southeast TX. Ongoing convection continues to sink south toward
the stationary surface boundary with destabilization anticipated
through the afternoon, especially south of the boundary. The
merging of this activity through the afternoon/evening could
result in training of heavy rain and scattered flash flooding.
MPD #269 covers this region through the next couple of hours.
Additional focused convection along the stationary front will
result in slow moving activity closer to the Gulf Coast of TX into
southwest LA. Therefore, the Marginal Risk was expanded into this
region.

Pagano

...New York...
As the warm front drapes across the region, the warm, moist air
brings areas of heavy precipitation to the region. Models over the

past day have continuously placed rainfall signatures of 2 to 4
inches for an area that has very low FFG with PWAT values of 1.75
inches. PWAT anomalies sit around 1.5 to 2.0 sigmas above early
June climatology. Concerns remain sufficient for scattered
pockets of flash flooding and with Hi-Res models placing heavy
rainfall signatures of 2 to 4 inches for these areas, have opted
to maintain the Slight Risk area for this region but it was
reshaped a little per the new guidance output and coordination
with BTV/Burlington VT, ALY/Albany NY, and BGM/Binghampton NY
forecast offices.


...Virginia down through the Carolinas...
Same story as above with very warm, moist air creating bands of
heavy prolonged precipitation signals over the region. Models are
in great agreement for the eastern portions of NC and VA with
strong rainfall signals of 2-4" and a modest signal for 5" in an
area that received heavy rainfall during the past 24 hours. This
led to an upgrade to a Slight Risk for portions of the eastern
Carolinas and southeast Virginia.


...Southern Texas...
Continued precipitation in an area already seeing 400 to 600% of
normal soil saturation means it will not take much to create
flooding. Model guidance keeps strong rainfall signals of 1 to 3
inches over these areas. Upgraded part of the area to a Slight
Risk as the strongest signal for 5" of rain after 12z was in this
area, covering the morning and perhaps early afternoon hours.

Roth/Chiari


Day 2
Valid 12Z Fri Jun 04 2021 - 12Z Sat Jun 05 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN
TEXAS...

21Z Update...Models are still indicating the upper low to linger
over the region bringing ample amounts of moist gulf air into
southern TX. With grounds already sitting at 400 to 600% of normal
and new rainfall totals of 1 to 3 inches with pockets of 3 to 5
inches showing up on some of the Hi-Res solutions coupled with
continued precipitation expected over the days leading up to this
period, have opted to include a Slight Risk area for portions of
southern TX. The Marginal Risk area was extended a bit to
encompass the latest model guidance with variance on placement.

Chiari

Previous Discussion...

...South Texas...
By the start of the Day 2 period, 04.12z, the mid to upper level
trof has completely severed from the northern stream into a weak
closed low over the Big Bend of Texas. So once again, remnant
convective outflow boundaries will be intersecting with return
moisture flow off the western Gulf. Given the weaker upper
features, low level wind response will be weak and support highly
localized convective development at the peak of instability which
is likely to be highly variable across the region. As such,
convection will not be as organized as prior days; however, deep
warm cloud processes will allow for higher rates for widely
scattered significant rainfall totals across areas that remain
well above normal for precip per 1 to 2 week AHPS anomalies and
deeper soil saturation with ratios over 80%. Weak steering flow,
propagation along outflow boundaries with mergers/collisions are
likely to be the norm, with potential highly localized flash
flooding conditions to be closer than normal proximity or areal
coverage across South Texas at or above 5% probability or above
Marginal Risk category. As such, little change from the Day 3
outlook area with this issuance though with slight eastward
expansion toward the Houston Metro to account for the GEM Regional
and higher HREF probabilities than yesterday.

Gallina

Day 3
Valid 12Z Sat Jun 05 2021 - 12Z Sun Jun 06 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTH
EASTERN TEXAS INTO PORTIONS OF SOUTHWEST LOUISIANA...

21Z Update...The closed low starts to slowly migrate eastward
through this period with ample moisture being pumped in from the
gulf. Models are in better alignment as to placement of the
heavier signals with the coastal areas of south-eastern TX and
southern LA seeing the heaviest amounts and extending offshore.
Consensus is 1 to 3 inches with a handful of models showing 3 to 5
inches right along the coast. FFG for these areas is on the lower
end except for portions of LA, but with the expected rainfall
leading up to this period, expect FFG to be even lower and grounds
to be even more saturated. AHPS does show 150 to 200% along the LA
coast with areas in TX much higher around 200 to 400% of normal
soil saturation. With this being said, have opted to place a
Slight Risk area where the heavier signals are along the coast of
TX and southern LA. The Marginal Risk area that was in place from
the previous issuance was tweaked slightly to account for model
differences.

Chiari

Previous Discussion...

...South Texas to Southwest Louisiana...
By the start of the day 3 period, 05.12z Saturday, the overall
pattern across Texas into the Western Gulf improves for the
potential for organized convection relative to prior day. Some
upscale enhancement to the upper-low from the prior day from
convection supported a slight expansion and slight eastward drift
across the Edwards Plateau. Low level flow response supports a
broadening of due southerly flow with slight confluence along the
southeast periphery of the vertically stacked mid-level
circulation across the Central Texas coast. Overall moisture will
increase toward or slightly above 2" total PWat over SE
Texas...stronger solutions such as the ECMWF even suggest values
nearing 2.25" by mid-morning. Strong low level frictional
convergence is likely to spark convection in this vicinity, though
there remains broad west to east spread. Given the persistence
throughout the day, frictional convergence may result in
back-building or near stationary redevelopment given the low level
flow regime. However, given the weakness of the mid to upper
level flow, current guidance suggests that southward propagation
into the Gulf may be a more likely scenario.

Additionally, instability downstream over SE Texas (county or two
off the Gulf Coast) is highly questionable given likely
convective debris blocking insolation during peak heating.
However, along the northern and western periphery of the cloud
cover, perhaps directly below the 7-85H low may result in very
slow moving thunderstorms capable of inducing flash flooding given
the deepening moisture profiles and deep warm cloud processes for
rates over 2"/hr. Additionally, slow cell motions may result in
highly localized but greater than 5" totals...which has been a
common evolution in a bulk of the larger scale global guidance
(and individual ensemble runs).

At this time, there is large model spread/variance contingent on evolution/placement of the upper-low and where low level moisture
flux convergence will result in convective activity (on coast or
just off shore). Still, with ample moisture, deep moist profiles,
slow cell motions and large area of soil saturation ratios over
80% per NASA SPoRT 0-40cm LIS product, a broad Marginal Risk is
placed over these hydrologically compromised ground conditions
along the generally agreed upon eastern hemisphere of the
upper-low in the ECMWF/GFS/UKMET/CMC and NAM guidance/ensemble
suite. An upgrade to a Slight may be on the table with
subsequent updates, particularly as placement of the upper-low and
moisture axis become better defined.

Gallina

$$
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FOUS30 KWBC 040820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Fri Jun 04 2021

Day 1
Valid 12Z Fri Jun 04 2021 - 12Z Sat Jun 05 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
NORTHWEST GULF COAST...

TX/LA...
Models indicate that the upper low will linger over the region,
interacting with ample amounts of moist gulf air, with daytime
heating adding to the instability in the region. With soils
remaining nearly saturated and new rainfall totals of 1 to 3
inches with pockets of 3 to 5 inches showing up on some of the
Hi-Res solutions, have opted to continue the Slight Risk area for
portions of TX. However, the guidance has shifted northeast with
the possible threat area through Houston, Beaumont/Port Arthur,
into the Lake Charles LA area. The Marginal Risk area was
extended a bit further to encompass the latest model guidance with
variance on placement.


Carolinas/Virginia coastal plain...
Heavy rainfall should be ongoing at the beginning of the period,
which may take until local noon to fade. Once it does, daytime
heating will interact with the moisture in the place to set the
stage for more heavy rainfall from coastal SC into NC, with some
uncertainty concerning how much heavy rainfall is expected closer
to Norfolk VA. A Marginal Risk area was introduced for this area,
as one week rainfall anomalies for about half the region is
200-400% of average. Other areas included were due to possible
urban issues with heavy rainfall despite lesser saturation as of
late (like Charleston SC). Probabilities of another 3"+ locally
were high in this area per the 00z HREF.

Roth/Chiari


Day 2
Valid 12Z Sat Jun 05 2021 - 12Z Sun Jun 06 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTH
EASTERN TEXAS INTO PORTIONS OF SOUTHERN LOUISIANA...

The 04/00Z numerical guidance continued the idea of nudging a
closed upper low and associated rainfall eastward across the far
northern Gulf of Mexico and adjacent portions of the Gulf Coast.
As a result, the best signal in the models for heaviest QPF had
shifted into Louisiana with lesser amounts still lingering in near
the Upper Texas Coast/Southeast Texas. Consensus remains for
roughly 1 to 3 inches of rainfall in the area along the
coast...with several operational models and ensemble members
generating isolated values between 3 and 5 inches. Given the low
FFG in place, with the potential for the guidance values to lower
even more by the time the Day 2 period begins, opted to expand the
eastern boundary of the previously-issued Slight Risk area deeper
into Louisiana while maintaining the Upper Texas Coast given
uncertainty. As previously mentioned, AHPS showed 150 to 200% of
normal soil saturation along the LA coast with areas in TX much
higher around 200 to 400% of normal. Given the fairly weak low
level flow, some of the cells capable of producing the heaviest
rainfall rates (2+ inches per hour) may well sag south towards the
better instability.

Bann

Day 3
Valid 12Z Sun Jun 06 2021 - 12Z Mon Jun 07 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL GRADUALLY
EXTENDING FROM LOUISIANA INTO MISSISSIPPI FROM SUNDAY INTO EARLY
MONDAY...

Locally heavy to excessive rainfall will be possible again on
Sunday as a closed low wobbles over Texas or Louisiana. The
heaviest model rainfall continues to be placed in the region of
best upper level difluence/divergence east of the upper
low...generally at or above 400 mb. At lower levels, precipitable
water values approaching 2 inches will continue to be drawn inland
by southerly low level winds of 20 to 35 kts...resulting in
isolated instances of 2 inch per hour rates and isolated rainfall
totals of 3 to 5 inches with some potential for overlap with areas
receiving heavy rain from Saturday into early Sunday. Tended to
follow the model consensus for rain to be spreading northward and
eastward across Mississippi. The potential for the upper low to
wobble keeps the threat of locally heavy rainfall going in Texas
despite the better moisture transport/moisture flux convergence
farther east. Even the GEFS and SREF each have a couple of
members showing 2+ inch amounts closer to the upper
low...presumably aided by the instability of a cold core system.
But the spread involved in placement of the low limits the
confidence...so a broader Marginal Risk was focused here.

Bann



Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt__;!!DZ3fjg!qw12kFDPYgML6w5vYhR27E3Dj0fq0jTQPzzsUKkK5ZdkZHCq1k3It68YSYApfQMElHLp1sP0$

Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt__;!!DZ3fjg!qw12kFDPYgML6w5vYhR27E3Dj0fq0jTQPzzsUKkK5ZdkZHCq1k3It68YSYApfQMElEosL59j$

Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt__;!!DZ3fjg!qw12kFDPYgML6w5vYhR27E3Dj0fq0jTQPzzsUKkK5ZdkZHCq1k3It68YSYApfQMElEA4dTix$



$$
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FOUS30 KWBC 071917
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Mon Jun 07 2021

Day 1
Valid 16Z Mon Jun 07 2021 - 12Z Tue Jun 08 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN NORTHEAST
TEXAS AND SOUTHERN/CENTRAL ARKANSAS...

1600Z Update...
A small Marginal Risk area was added for portions of the central
Appalachians. With ample moisture in place, recent hi-res
guidance members show the potential for south to north moving
storms across the region, producing locally heavy accumulations
across the region this evening and overnight. Latest HREF shows
high probabilities for localized accumulations of 2-inches or more
from portions of eastern West Virginia northward into southwestern Pennsylvania.

Elsewhere, no significant changes were made to the Moderate Risk
area centered from northeastern Texas into southern Arkansas. Did
extend the surrounding Slight Risk area further to the west across north-central Texas, where several hi-res guidance members show a
good signal for redeveloping convection and the potential for
additional heavy rain overnight.

Pereira

Previous Discussion...
Northeast TX/AR/western TN...
The main focus for excessive rainfall on Monday is expected be to
from northeast TX across southern and central AR towards western
TN. An upper low/trough slowly weakens while drifting eastward.
Inflow at 850 hPa is persistent across this region with some
uptick with time, which weakens forward propagation vectors and
increases moisture transport/IVT with time. Effective bulk shear
is sufficient to organize activity, which should lead to the
formation of convective organization and could lead to the
formation of mesocyclones. Activity should take advantage of an
instability pool which is already increasing early this morning,
with ML CAPE already 1000-3500 J/kg across LA and southern AR.
Ongoing convection Monday morning over parts of North Texas should
shift east to east-southeast before weakening. Thunderstorms
shift back closer to the upper low due to daytime
heating/increasing instability under the cold pool aloft. The
degree of moisture and instability combined with organized
convective clusters should be able to yield hourly rain totals to
3", which would be problematic regardless of soil conditions and
topography. There is a bit of dispersion in the 00z mesoscale
guidance, yet still there is a 30-40% chance of 5"+ indicated in
the 00z HREF. Many pieces of mesoscale guidance display 7"+
maxima which cannot be ruled out based on the above factors.
Areas of saturated soils are mottled across the region, making the
potential for excessive rainfall more difficult than usual. Some
areas in TX/OK/AR have witnessed rains over the past week
exceeding 200% of average. Flash flooding instances are expected
to be scattered within the Moderate Risk area. Coordination on
the Moderate Risk area was made with the LZK/Little Rock AR,
SHV/Shreveport LA, and MEG/Memphis TN offices.


Southeast...
The Marginal Risk of excessive rainfall continues throughout the
Southeast to western South Carolina. Precipitable water values in
excess of 2" given moist Gulf inflow in a secondary moisture plume
within the upper low's warm conveyor belt circulation could lead
to heavy rain in thunderstorms that form, and high res guidance
along with available ingredients indicate hourly rainfall totals
of 2"+ for Alabama, Georgia, North FL, and parts of South Carolina
are possible Monday evening into Monday night, with local amounts
in the 4" range expected. Greater threats for flash flooding will
exist over any areas that see multiple storms. The best signal
for 5"+ in 24 hours is near the AL/GA/FL border junction --
40-50%. The main limiting factor for flash flooding will be the
recent well below average rainfall/dry soils, so kept the threat
level Marginal per coordination with the FFC/Peachtree City GA
forecast office.


Dakotas/Minnesota...
There is a non-zero risk of heavy rainfall causing isolated
flooding issues in a stretch from the Dakotas to northern
Minnesota on the northern side of a significant instability pool
as convection ramps up there Monday night. At the moment, this
seems to be below threshold for a Marginal Risk given its
quick-hitting nature as 850 hPa noticeably veer with time and
recent dryness in that region.

Roth


Day 2
Valid 12Z Tue Jun 08 2021 - 12Z Wed Jun 09 2021

...A SLIGHT RISK OF EXCESSIVE RAINFALL CONTINUES OVER THE SOUTHERN
PLAINS TO LOWER MISSISSIPPI VALLEY ON TUESDAY AND TUESDAY NIGHT...

21Z Update...

...Southern Plains/Lower Mississippi Valley...A very slow moving
area of low pressure will impact this region during this period
bringing an additional 2 to 3 inches of precipitation to an area
already expected to see heavy rainfall in the day 1 period. PW
values near 2.25 inches which is a 2-2.5 std deviation anomaly
over an area seeing pockets of 150 to 200% of normal precipitation
over the past two weeks has led to the Slight Risk area being
issued from the previous shift. No changes were made to this
region at this time with newest guidance aligning well with the
overall footprint for highest rainfall amounts and greatest
threats to flash flooding.

...Southeast U.S...Synoptic pattern is similar to what was
mentioned above, with a swatch of QPF around 1 to 3 inches showing
on most models. There is some slight variance among the models as
how this precipitation will progress from the day 1 to day 2
period, but footprint suggests a more southwest to northeast
orientation. Did pull the Marginal Risk area in a bit from the
previous issuance to where the consensus among the models stands
and lower FFG remains.

...North Dakota/Montana Border Area...As the upper level low for
the Northwest CONUS progresses eastward, heavier amounts of
precipitation signals are showing amongst the models. Consensus
showing 2 to 3 inches of additional rainfall in areas with PWAT
anomalies of 2-2.5 std deviations. PWAT values sit near 1 inch
with AHPS showing pockets of 150 to 300% or normal precipitation.
The Marginal Risk area was left in place because of this.

...Northeast US...Latest model guidance still holds onto 1 to 3
inches of additional precipitation for these areas during this
time with lower FFG and rainfall amounts from previous days in
play, have opted to keep the mention of Marginal Risk in place for
now. The additional rainfall amounts will likely lower FFG even
more and with PWAT anomalies of 2-2.5 std deviation, do not think
flash flooding is out of the question.


Chiari

Previous Discussion...

...Southern Plains/Lower Mississippi Valley...
There is a Slight Risk of excessive rainfall from Tuesday through
Tuesday night/early Wednesday morning over roughly the same area
of the Southern Plains and Lower Mississippi Valley where moderate
to heavy rainfall is expected in the Day 1 period. The 07/00Z
suite of numerical guidance necessitated another subtle eastward
shift to the Slight and Marginal Risk areas.

The overall synoptic pattern, though, will be changing little
during the period. A mid- and upper-level low will remain caught
between ridging along the Southeast and along the West Coast
although the low should be weakening and opening up during the
latter part of the period. With deep moisture still in place
(precipitable water values of 1.75 inches to 2 inches/1.5-2.0
standard anomalies greater than climatology), heavy downpours will
remain possible...with ensemble members showing increasing
coverage of 1 inch rainfall rates in Arkansas during the
afternoon. While the HRRR was the most aggressive with the idea,
the HRW-ARW also agreeing on the development of 1+ inch rainfall
rates in Arkansas by late afternoon. Global guidance continues to
move this convection eastward Tennessee on by Tuesday night.

...Southeast U.S...
Precipitable Water values around 2 inches will remain in
place...allowing any convection that forms to generate some
intense downpours. Given weak flow aloft in proximity to an upper
level ridge axis, thinking is that some isolated flash flooding
will be possible. Flash flood guidance is high, so the
expectation is that any flooding problems will be fairly localized
and tied closely to the most intense rainfall rates.

...North Dakota/Montana Border Area...
There has been a signal for some locally heavy rainfall from the
operational models as well as ensemble members near the
MT/ND/International border area for several runs. Placement of the
heaviest rainfall was still bouncing around a bit (even north of
the border) but the potential remains for some 3+ inch rainfall
amounts somewhere close to the MT/ND/International Border.

...Northeast U.S...
Ingredients will be in place to support some locally heavy
rainfall rates over parts of the Northeast U.S. on Tuesday.
Precipitable water values of 1.75 inches or so are in the 95th
percentile for this time of year. In addition, the depth of warm
cloud processes should be able to produce some locally intense
rainfall rates over areas where Flash Flood Guidance was as low as
1.5 inches per 3 hours. Few operational models are producing much
widespread heavy rainfall amounts, but the ARW, HRRR-ext and
NAM-NEST do show signatures of slow moving heavy rainfall
producing sells. The predictability of such cells is low this far
out, so a Marginal Risk focused in regions of lower Flash Flood
Guidance and in areas of terrain.

Bann

Day 3
Valid 12Z Wed Jun 09 2021 - 12Z Thu Jun 10 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TENNESSEE VALLEY...

21Z Update...Much more variance among the models on how to
transition from day 2 convection with the upper low to day 3.
Heaviest signals are all over the place among global models so
finding a good place for the Slight Risk and Marginal Risk areas
for this period were a bit difficult. Left the Slight Risk area in
place over where the heaviest PWAT anomalies are showing. With the
slow motion of this upper level feature, do expect a larger
footprint to be in play with the Marginal Risk area, but given the
variance among the models, did not make adjustments at this time.
With so much depending on previous days convection, will rely on
later model guidance to make adjustments and opt to leave things
in place that were issued during the midnight shift.

Chiari


Previous Discussion...

...Tennessee Valley/Southern Mississippi Valley Northeastward to
the Ohio Valley...
The mid- and upper level low which has been persistent over the
Southern Mississippi Valley will be weakening and lifting
northeastward during the period. It will still have abundant
moisture to work with early in the period...and a Slight Risk area
was maintained over parts of the Tennessee Valley where convection
may be on-going on Wednesday morning. It were mainly ARW members
which indicated the risk of 2 to 3 inch rainfall in the Day 3
period, although the GEFS had a few members showing 2 inch amounts
as well after the period begins at 12Z on Wednesday. The general
model consensus is for 1 to 2 inch amounts along the path of the
upper system with some isolated maximum amounts of 3+ inches
extending into the Ohio Valley. Given dry antecedent conditions
with corresponding high flash flood guidance suggested a Marginal
was sufficient for areas outside of the Tennessee Valley.

...Mid-Atlantic...
Precipitable water values approaching 2 inches will be in place,
resulting in enough instability to support briefly intense
rainfall rates mainly on Wednesday and Wednesday evening.
Steering flow will be pretty weak so some decent rainfall amounts
are possible, but the lack of any good focusing mechanism leaves
predictability of which places have a greater potential for seeing
any heavy rainfall at this point. Tended to use the contour of
precipitable water vales being 2.5 standard deviations above
climatology as a first guess for placing the Marginal Risk.
Additional adjustments are expected.

Bann

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FOUS30 KWBC 081606
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1205 PM EDT Tue Jun 08 2021

Day 1
Valid 1605Z Tue Jun 08 2021 - 12Z Wed Jun 09 2021

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
ARKANSAS AND MISSISSIPPI...

...WIDESPREAD FLASH FLOODING...LOCALLY SIGNIFICANT AND LIFE THREATENING...EXPECTED OVER PORTIONS OF SOUTHERN AND CENTRAL
ARKANSAS INTO NORTHERN MISSISSIPPI...

...Southern Plains/Lower Mississippi Valley...
1600 UTC Update...Upgraded a small portion of the Moderate Risk
area to a High Risk, while expanded the Moderate Risk farther east
into northern MS. The High Risk takes into account the rain that
has already fallen (much lower FFGs and higher 0-10cm soil
moisture percentiles per the latest NASA SPoRT imagery), along
with the convective trends and latest (12Z) high-res CAMS
(including HREF exceedance probabilities).

Previous Discussion...
The flow around the southern side of a very slow moving and
elongated mid- to upper-level low/trough will lead to
unidirectional flow slightly off the surface, which will interact
with precipitable water values of 2" and ML CAPE of 1000-3000 J/kg
and plentiful effective bulk shear to produce excessive rainfall
this period. Soil conditions are getting increasingly saturated
in and around southern AR, where the most persistent 850 hPa
inflow/convergence is expected Tuesday and Tuesday night.
Activity should be focused in a couple rounds, but the most
significant round of heavy rainfall is expected to start early
this morning and persist through the morning hours. The 00z
Canadian, 00z NAM CONEST, and 00z HRRR produced prolific rainfall
after 12z -- 10-15". While those pieces of guidance can be high
biased, the synoptic pattern could support hefty totals. There is
some concern, like what happened on Monday -- that the heavy
rainfall could end up farther to the south as there is no capping
inversion to hem in the activity, which will need to be watched. Conservatively increased the threat level to Moderate for areas in
and around southern AR.


...Southeast U.S...
The guidance is showing locally heavy amounts within the
southwest-northeast warm conveyor belt circulation around the
upper low to the west-northwest. Precipitable water values around
2" and the steering flow is weak. Along with daytime heating
generating instability, locally heavy downpours should occur again today/Tuesday. Each day, heavy rainfall has been saturating soils
from northeast to southwest through the Carolinas into Georgia and
the FL Panhandle/Big Bend, so each day the threat of excessive
rainfall due to the disorganized convective activity slowly
broadens. To exemplify its spottiness, per CAE/the Columbia SC
forecast office, Augusta/Bush Field GA received 4.89" on Monday
which is their seventh highest daily rainfall total on record
(since 1871) but nearby Augusta/Daniel Field GA only received
0.50" -- heavy rainfall should again be mainly pulse with heavy
rainfall spotty.


...North Dakota/Montana Border Area...
1600 UTC Update...Upgraded much of the outlook area to a Slight
Risk, based on the latest observational/mesoanalysis trends, 12Z
CAMs (including HREF exceedance probabilities. A good Maddox
"Frontal" heavy rainfall signal remains in place within the right
entrance region of the upper jet streak, on the periphery of the
upper ridge axis. Some of the 12Z CAMs show isolated additional
totals of 3-7+ inches through 12Z Wednesday.

Previous Discussion...
As the upper level low for the Northwest CONUS progresses
eastward, heavier amounts of precipitation are being advertised by
the guidance as a capping inversion with 700 hPa at or above 12C
allows significant instability (3000+ J/kg) to well up underneath
across the Dakotas and southeast MT. Precipitable water values
are not shabby for the High Plains, reaching values of 1.25"+.
Hourly rain totals to 2" are possible where cells merge/train and
the guidance shows local amounts of 3-6" being possible in this
area. AHPS shows small pockets of 150 to 300% or normal
precipitation over the past week or two. The Marginal Risk area
from continuity remains in place.


...Northeast US...
1800 UTC Update...Also upgraded a portion of the Marginal Risk to
Slight over this region, especially in areas with lower FFGs. 12Z
HREF 40km neighborhood exceedance probabilities (QPF> 1 and 3 hr
FFGs) are 30-50%+ over portions of the region, as are the probs of
QPF exceeding the 5 year ARI.

Previous Discussion...
Ingredients will be in place to support some locally heavy
rainfall rates over parts of the Northeast U.S. on Tuesday.
Precipitable water values of 1.75 inches or so are in the 95th
percentile for this time of year and there is deep, unidirectional
westerly flow within this moist regime. The depth of warm cloud
processes should be able to produce some locally intense rainfall
rates over areas where Flash Flood Guidance was as low as 1.5
inches per 3 hours. Activity is expected to be on the move, so
believe the Marginal Risk level remains reasonable.

Hurley/Roth


Day 2
Valid 12Z Wed Jun 09 2021 - 12Z Thu Jun 10 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TENNESSEE VALLEY EXTENDING INTO THE SOUTHERN PORTION OF THE
OHIO VALLEY...

...Tennessee Valley/Southern Mississippi Valley Northeastward to
the Ohio Valley...
The mid- and upper level low which has been persistent over the
Southern Mississippi Valley during the past few days will begin to northeastward as it starts to weaken/fill. Abundant moisture will
be in place throughout the region with precipitable water values
ranging from 1.8 to 2.2 inches (values of 2 inches is roughly 2.5
standardized anomalies greater than climatology for early June).
At the surface, there is only a weak reflection of the system
aloft and no real boundary to provide a focus with which
convection will interact. As a result, am expecting some locally
heavy rainfall near the southwest flank of the upper system due to
confluent flow which is where a Slight Risk of excessive rainfall
remains in place. The convergence will become less of a factor as
the system weakens. However, ingredients are in place for some
downpours to cause problems.

...Mid-Atlantic...
Precipitable water values approaching 2 inches will be in place,
resulting in enough instability to support briefly intense
rainfall rates mainly on Wednesday and Wednesday evening.
Steering flow will be pretty weak so some decent rainfall amounts
are possible, but the lack of any good focusing mechanism leaves
predictability low regarding placement of the heaviest rainfall.
Tended to use the contour of precipitable water values being 2.5
standard deviations above climatology as a first guess for placing
the Marginal Risk. As mentioned previously, much of the convection
in this period will depend on convection in the Day 1 period...so
additional refinements are expected.

...North Dakota...
Another round of showers and thunderstorms should develop on
Wednesday as surface low pressure forms to the lee of the Northern
Rockies and moves eastward. The models tend to agree that the best
chance for any heavy rainfall will be located to the east of the
surface low in a region of upper level difluence. Given rainfall
from Monday night into early Tuesday morning, some issues with
runoff could occur in areas of repeat convection.

Bann



Day 3
Valid 12Z Thu Jun 10 2021 - 12Z Fri Jun 11 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TENNESSEE VALLEY...

With the synoptic scale system continuing to move slowly north and
east during the period, the models still depict some enhanced
rainfall amounts across parts of the Tennessee Valley. While
rainfall amounts do not look to be blockbuster values, there could
be some hydrologic sensitivity given recent days of rainfall. The
SREF and GEFS both showed probabilities of 2+ inch amounts, though
a consensus value from the global models was little more than an
inch.

The Slight Risk was embedded within a broad Marginal Risk area
extending eastward to the Mid-Atlantic coast where deep moisture
is in place, steering flow is weak and where there was generally a
lack of any real forcing mechanism (the exception being along the
Mid-Atlantic coast where a cold front will be dropping southward).
As a result, the overall low QPF values in the global guidance
does not preclude some locally intense downpours with isolated
places receiving a couple inches of rain. Predictability of where
heavy rainfall occurs is quite low even though ingredients to
support heavy rainfall remain in place and some shortwave energy
to work with in the Midwest. This will also likely mean
additional adjustments in future updates.

Bann

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FOUS30 KWBC 091606
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1205 PM EDT Wed Jun 09 2021

Day 1
Valid 16Z Wed Jun 09 2021 - 12Z Thu Jun 10 2021

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
ARKANSAS AND MISSISSIPPI...

...ONGOING HEAVY RAIN OVER EXTREMELY SATURATED SOILS FROM
TUESDAY'S PROLIFIC RAINFALL CONTINUES FLASH FLOOD EVENT...

Northern Mississippi Delta...
A stalled mid/upper-level low will continue to focus heavy amounts
of precipitation for the Mid-South/ArkLaMiss. Under the base of
this system, the southwesterly flow will remain fairly
unidirectional through the low levels with max magnitude of 25kt
around 850mb with lighter flow above and below (which aids
redevelopment). Across northern LA, southern AR, and and northern
MS...PWs of 2" to 2.25" will continue to interact with MUCAPE of
1500-3500 J/kg which will continue to produce hourly rain totals
of 1.5 to 2". Unlike Tuesday, activity looks to be starting to
progress southeast a bit. Due to ongoing activity and threat for
overnight activity over similar areas, kept the High Risk as is,
shifted the Moderate Risk east a bit from AR/into northern AL.
Further updates are likely once the progression of the activity is
known and there is a better handle on location and magnitude of
the overnight activity.


Rest the the Eastern Third of the Lower 48...
Near and ahead of the weakening upper level trough, abundant
moisture will be in place throughout the region with precipitable
water values near or above 2" (2 standard deviations above
normal). Low-level inflow and the mean 850-400 hPa wind are weak,
which could lead to locally heavy downpours though convection
should struggle to organize. Instability should allow for
convection to mainly concentrate in the afternoon and evening
hours. The Slight Risk covers both the heavy rain potential and
portions of TN/KY/WV that either have had 200-400% of average
rainfall the past week and/or have lower flash flood guidance
values. May need to raise a Slight Risk for a portion of the
Mid-Atlantic coast where bay breeze interaction should help focus
activity.


...Northernmost MN...
Ongoing activity over northern ND will shift east with a surface
low along the central US/Canadian border. 12Z guidance continues
to agree that the best chance for any heavy rainfall will be
located to the east of the surface low in a region of upper level
difluence. The eastward shift in the Marginal Risk area reflects
similar shifts in the guidance that raise 700 hPa temperatures
above 12C across the Dakotas, which should act as a fairly
effective cap for convection -- MN is not as capped and the
surface low should be in the vicinity. Given rainfall from Monday
night into early Tuesday morning across the Northland/Arrowhead of
MN, some issues with runoff could occur in areas of repeat
convection. Expanded the Marginal Risk a bit south over northern
MN to include Duluth proper per the 12Z HRRR/NAMNest.

Roth/Jackson


Day 2
Valid 12Z Thu Jun 10 2021 - 12Z Fri Jun 11 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE TENNESSEE
VALLEY INTO PARTS OF THE MID-ATLANTIC REGION...

...Ohio Valley into the Mid-Atlantic Region...
The slow-moving system will continue to tap a moisture-laden
atmosphere to produce additional heavy...and potentially
excessive...rainfall along its track. Additional amounts of 1 to
3 inches are expected for areas already seeing ongoing flooding
and well above normal soil saturation of 200%+ in the Tennessee.
Depending on how slowly the heavy rainfall moves out of the
area...there may be enough overlap with areas recently soaked to
warrant an upgrade to Moderate...but too much uncertainty at this
point as to where that overlap may or may not occur. In the large
scale, though, PWAT anomalies continue to sit near 2.0-2.5 std
deviations above normal over this region. Given the changes made
to the Slight Risk area and Marginal Risk areas on Tuesday...only
change needed at this point was to account for somewhat better
agreement in terms of the western extent of QPF and to the risk
area along the Mid-Atlantic region as a cold front slowly makes is
way southward.

...Upper Midwest and Adjacent Western High Plains...
An upper low pushes out over the Western High Plains to the lee of
the Northern Rockies...setting up an broadly difluent upper level
pattern aloft and strengthening low level surface pattern that
keeps a flow of low level moisture over the region. Additional
pockets of 1 to 3 inches are expected...which can worsen any
ongoing flooding and lead to flooding in additional areas through
early Friday morning.

Bann


Day 3
Valid 12Z Fri Jun 11 2021 - 12Z Sat Jun 12 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC ON FRIDAY...

...Appalachians eastward to the Eastern Seaboard in the
Mid-Atlantic Region...
The threat of heavy to potentially excessive rainfall will
continue on Friday as the mid- and upper-level system makes it's
way from the Ohio Valley eastward...and then southeastward towards
southeast Virginia by early Saturday morning. With deep moisture
already in place, any convection that develops has the potential
to produce excessive rainfall via intense downpours. Some 1 to 2
inch per hour rates are possible given precipitable water values
of 2 inches (about 2.5 standard deviations greater than
climatology). Given broad weakness aloft, slow cell motion and
the potential for repeat convection will locally enhance the risk
of excessive rainfall.

...Upper Midwest...
Placed a Marginal Risk area over parts of North Dakota and
Minnesota mainly for convection early in the period associated
with a closed low that heads north of the International Border.
Precipitable water values are not expected to be terribly
anomalous, with the 1.25 to 1.5 inch values being little more than
1.5 standard deviations greater than climatology. That...combined
with the expectation that cells will be progressive...should limit
that risk of flash flooding. However, there still looks to be
some potential for convection early in the period to occur over
areas soaked in the preceding couple of days.

Bann

$$
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FOUS30 KWBC 101625
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1225 PM EDT Thu Jun 10 2021

Day 1
Valid 16Z Thu Jun 10 2021 - 12Z Fri Jun 11 2021

...THERE ARE MODERATE RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MID-SOUTH AND MID-ATLANTIC...

...Lower MS Valley into the Southeast...
Backbuilding/training convection persists through midday over the
ArkLaMiss which is west of a stratiform shield associated with an
MCV that has drifted into north-central late this morning. Plenty
of upstream instability will help sustain convection as the focus
area continues to slowly shift south about 21Z. 16Z to 21Z an an
additional 2-4" of rain over the southwestern portion of the
current Moderate Risk which was essentially maintained from the
previous issuance. Numerous instances of flash flooding can be
expected into the afternoon hours...some of which may continue to
be significant in nature. Scattered activity overnight north of
the current area of precipitation, particularly from the 12Z 3km
NAM and recent HRRRs allows maintenance of the northern/eastern
part of the Moderate Risk as well as the Slight Risk.


...Ohio Valley through the Mid-Atlantic...
16Z Update...
Raised a targeted Moderate Risk over the immediate Washington DC
metro area, central to western MD into far southern PA for the
combination of ample heating raising instability, 1.8 to 2 inch
PWs which are 2 standard deviations above normal, and general
convergence from the slow moving cold front. Wind shear is the
lacking variable here as flow is so light through the column that
maintenance of heavy raining cells should be difficult. However,
the presence of bay breezes, the surface front, and terrain to the
west should allow for repeating cells to occur over the rather
urbanized area of Baltimore/Washington producing potentially
numerous instances of flash flooding.

A broad and slow moving mid/upper level trough and slow moving
back door cold front will result in scattered to widespread
convection this afternoon/evening from the Ohio Valley to the
central Mid-Atlantic coast. The environment will be conducive for
very heavy rainfall rates. PWs are above the 90th percentile, and
wet bulb zero heights around 13kft AGL are at the average maximum
for this time in June. Thus efficient warm rain processes will
likely dominate into tonight. Instability is likely a limiting
factor (where cloud cover has been thicker) for depth/intensity of cells...however the aforementioned moisture parameters would still
favor shallow low topped cells capable of very heavy rain rates.
The other limiting factor is longevity of cells at any one
location...as cells should generally pulse up and down rather
quickly. The weak instability potentially restricting the coverage
of heavier cells, and the short lived nature of most cells, may
prevent a more widespread flash flood risk today/tonight. However
slow cell motions and heavy short duration rates will likely still
be enough to result in scattered flash flooding through the Slight
Risk area.

There is certainly some potential for a more widespread and
significant event today over portions of WV and VA given the
efficient environment in place. A backdoor front dropping
southwestward will be a convective focus today, potentially
helping sustain a corridor of cell mergers...as it interacts with
terrain induced cells and slow moving cells downstream of the
front. The event today/tonight also has Moderate risk potential
over mountainous portions of WV/VA...but scattered flash flooding,
some locally significant, is the current expectation in this area.
We will continue to monitor trends this afternoon.

A bit better instability over the OH valley...but less of a focus
for cells. Thus cells should generally be smaller in scale and
thus any heavy rains will be quite localized. Expect isolated to
scattered flash flooding here this afternoon/evening...but on too
small of a scale to warrant anything more than a Slight Risk.


...Northern Plains...
Organized convection will push across the northern Plains this
evening into the overnight hours. Most indications are for a
progressive convective line generally near the slow moving frontal boundary...and thus would generally expect severe weather to be a
bigger risk than flash flooding. Cell merging at the onset of
initiation seems plausible over northeast MT and northwest ND and
12Z CAM guidance is strong enough to warrant an expansion of the
Slight Risk from central ND to the Canadian border. Portions of
north central SD and south central ND have however been very wet
of late...with a response noted in soil moisture and streamflow
anomalies. Thus if this area were to see 2"+ of additional rain
then some impactful flash flooding could occur, though 12Z
CAM/HREF guidance is less bullish for this area than overnight.
The 12Z HREF does highlight northern ND and eastern SD for FFG
exceedance and multi-year ARI potential. Therefore the Slight Risk
was expanded in ND where FFG is lower, but kept a Marginal in SD
where FFG is higher.


...Northern WI/MI...
Maintained a small Marginal risk across portions of northern
WI/MI. Convection this afternoon may result in locally heavy
rainfall across this region. Some of this area has seen locally
heavy rain of late, possibly making the region a bit more
susceptible...however long term conditions are still generally
dry. There may be some localized FFG exceedance. However, given
the overall antecedent conditions, and amounts likely just barely
exceeding FFG, impacts are likely to be minimal.

Chenard/Jackson


Day 2
Valid 12Z Fri Jun 11 2021 - 12Z Sat Jun 12 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC ON FRIDAY...

21Z Update...

...Appalachians eastward to the Eastern Seaboard in the
Mid-Atlantic Region...
The threat of heavy to potentially excessive rainfall lingers into
Friday as shortwave energy slowly makes its way eastward from the
Ohio Valley into the Mid-Atlantic region before being shunted
southeastward into southeast Virginia by Friday night/early
Saturday morning. The atmosphere will be plenty moist and
sufficiently unstable to support convection that produces some
intense downpours. Precipitable water values will be in excess of
1.75 inches along the track of the shortwave energy...with values
over 2 inches in place east of the Appalachians. Those
precipitable water values of or more inches are 2 to 2.5 standard
deviations above climatological average for mid-June.
Consequently, there is concern for flash flooding due to the
abundant moisture and slow cell motion. As mentioned previously,
eastern portion of VA and NC are among the most prone for flooding
considering that AHPS was showing soils at 300 to 600% of normal.

...Upper Midwest...
Maintained a low-confidence Marginal Risk area over parts of North
Dakota and Minnesota mainly for convection early in the period
associated with a closed low that heads north of the International
Border. Precipitable water values are not expected to be terribly
anomalous, with the 1.25 to 1.5 inch values being little more than
1.5 standard deviations greater than climatology. That...combined
with the expectation that cells will be progressive...should limit
that risk of flash flooding. However, there still looks to be
some potential for convection early in the period to occur over
areas soaked in the preceding couple of days.

Bann


Day 3
Valid 12Z Sat Jun 12 2021 - 12Z Sun Jun 13 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF
THE SOUTHEAST U.S. AND WEST TEXAS...

...Southeast U.S....
Maintained a Marginal Risk area over portions of the southeast
U.S. given some lingering rainfall before the slow-moving system
finally moves off-shore. The consensus of the global model
guidance is that any additional moderate to heavy rainfall in the
Day 3 period should be from central North Carolina or
southward...which would be south of the area that has had areas of
5 inches of rain in the previous week plus any additional rainfall
that falls on Day 2. Even so, the area of North Carolina into
South Carolina and Georgia have had some pockets of 2 to 5 inch
amounts, so any downpours could result in localized flooding
concerns. Given the poor track record of how the models have
handled the timing of this system over the past days, opted to
depict a broader Marginal Risk than might be expected for the
deterministic QPF simply based on uncertainty.

...Texas Panhandle and Adjacent Portions of New Mexico...
High pressure located over the central and southern Plains will
begin to weaken and shift eastward during the day, allowing low
level winds to become southeast across the Texas panhandle and
adjacent portions of New Mexico by late afternoon or early
evening. The increased moisture availability should allow for
enough destabilization to allow scattered showers and
thunderstorms capable of producing locally heavy rainfall to form.
The outlook area is farther north than some places in West Texas
that have received 1 to locally 2 inch rainfall amounts in the
past week. At the moment, the GEFS had more ensemble members
showing 1+ inch contour over the area than the SREF during the 24
hour period. However, given the arrival of increased moisture due
to the southeasterly flow in low levels, felt a Marginal was
warranted.

Bann

$$
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FOUS30 KWBC 191304
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
903 AM EDT Sat Jun 19 2021

Day 1
Valid 1301Z Sat Jun 19 2021 - 12Z Sun Jun 20 2021

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL OVER THE
CENTRAL GULF COAST AND A MARGINAL RISK FOR EASTERN PORTIONS OF THE
OHIO VALLEY...

1300Z Update...
A Marginal Risk area was added to the southern portion of Lower
Michigan. Latest radar imagery shows convection beginning to
train near a west-east oriented stationary boundary, with rainfall
rate estimates over 2+ in/hr within some of the heavier cells.
With guidance indicating conditions will remain favorable for
heavy rainfall rates to persist over the next few hours, and given
the relatively low FFG values across the region, a Marginal Risk
was added to the region. Refer to MPD #353 for further details
concerning the near-term potential for heavy rainfall and flash
flooding across the region.

A Slight Risk area was added to portions of the Ohio Valley. The
Slight Risk area was drawn to highlight areas where ongoing and
previous convection have lowered 3-hr FFG to an inch or less. In
addition to ongoing convection, there is some potential that
upstream convection and/or convection developing later today into
the overnight may track into this same region.

A Marginal Risk was added for portions of the central Plains.
Guidance shows the potential for heavy rainfall rates increasing
during the evening and overnight as convection developing upstream
moves into a more moist environment. Recent runs of the RAP show
PWs increasing to 1.5 to 1.75 inches along an axis of 30-40 kt
southerly inflow into the region. The 06Z HREF shows high
neighborhood probabilities for 2-inches or more within much of the
Marginal Risk area.

Pereira


0900Z Discussion...
Central Gulf Coast and Southeast...As the disturbance continues
its inland trek, heavy rainfall amounts of 4 to 6 inches with
locally heavier amounts will occur for these regions. A bit of
uncertainty remains on how this will track, but the overall
footprint did not change much from previously issued Moderate,
Slight, and Marginal Risk areas. Coastal areas have already seen
locally 8+ inches of rain.

Ohio Valley...Backbuilding convection remains the greatest threat
for these areas during the day 1 time period. Slow moving storms
dropping additional 1 to 3 inches of rains with Hi-Res models
showing stronger signals of 3+ inches are enough to create
flooding potential. The Marginal Risk area was adjusted to account
for latest QPF forecast.

Chiari

Previous Discussion...

2030Z Update...
For the central Gulf Coast and Southeast--reflecting a more
progressive track, shifted the previous risk areas a little
farther east, while extending the northern extent of the Marginal
and Slight Risk areas farther northeast into northern Georgia and
the western Carolinas.

Along the Ohio Valley--extended the Marginal Risk back to the west
across southern Illinois and Indiana. The 12Z hi-res guidance
showed a good signal for convection redeveloping during the
afternoon and continuing into the evening across the region. With
the ARW, ARW2, FV3LAM, and the 12Z HRRR all indicating some
potential for backbuilding -- the HREF is showing high
neighborhood probabilities for accumulations of 2-inches or more
across a large portion of southern Illinois into southern Indiana.


Pereira



Day 2
Valid 12Z Sun Jun 20 2021 - 12Z Mon Jun 21 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER THE
SOUTHEAST U.S....

Southeast U.S. and Southern Mississippi east into Louisiana...

The disturbance continues its progression across the southeast
CONUS bringing heavy precipitation to the area. The further in
time we go with this feature, the more model spread is observed,
but overall footprint for continued heavy precipitation exists for
portions of NC southward into central GA and AL. Additional QPF of
2 to 4 inches is expected through this region already saturated
from previous days making flooding a concern. Some of these areas,
especially closer to the coastal waters of the Carolinas, have
seen upwards of 400% of normal precipitation over the past two
weeks. Minor adjustments were made to the Slight Risk area for NC
down into SC and northern portions of GA. A Marginal Risk area
envelops this area as well encompassing further north into NC and
westward into portions of AL. Additional precipitation is expected
along the coastal waters of LA, MS, and AL with previous days QPF
expected to lower FFG significantly.

Upper Mississippi Valley into Upper Great Lakes...

Convection is expected to fire off for this region again Sunday
afternoon creating conditions favorable for heavy precipitation.
PWAT anomalies of 1.5-2 std deviations exist through an area
seeing an ongoing series of convection firing off through previous
evenings. With latest guidance showing signals of 2 to 4 inches
falling during this period the Marginal Risk area was left in
place for the region. A few models show signals of 3 to 5 inches
for the area.

Chiari


Day 3
Valid 12Z Mon Jun 21 2021 - 12Z Tue Jun 22 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL OVER THE LOWER
MISSISSIPPI VALLEY NORTHEASTWARD INTO THE OHIO VALLEY AS WELL AS
ALONG THE CENTRAL GULF COAST....

Lower Mississippi Valley into the Ohio Valley...Upper level trough
drapes across the central US during this period bringing areas of
heavier precipitation out ahead with marginal instability.
Guidance has 1 to 3 inches for portions of this region that have
seen quite a bit of precipitation over the past few weeks. PWATs
sit around 1.25-1.75. Portions of the Lower Mississippi Valley
have seen upwards of 400+% of normal over the past two weeks
meaning soils will not handle much rain to cause flooding
concerns. Have opted to place this region under a Marginal Risk
with the footprint encompassing the heaviest amounts of QPF among
the models.

Central Gulf Coast...Moist southerly winds will bring additional
instability out ahead of the upper level trough. Model guidance
has signals of 1 to 3 inches of additional rains on grounds that
are already experiencing heavy amounts during the day 1 and 2 time
periods due to Potential Tropical Cyclone THREE. Soils will be
primed and flooding potential will only increase. Because of the
precedent conditions, have opted to place this area under a
Marginal Risk area for this time period.

Chiari

$$
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FOUS30 KWBC 192049
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
449 PM EDT Sat Jun 19 2021

Day 1
Valid 2039Z Sat Jun 19 2021 - 12Z Sun Jun 20 2021

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FROM THE
FLORIDA PANHANDLE TO THE WESTERN CAROLINAS...

2030Z Special Update...
...Gulf Coast to eastern Tennessee and the western Carolinas
Made some small additional adjustments based on observations and
recent runs of the HRRR--mainly to trim away some more of the
western extent of the previous outlook areas where the heavy
rainfall threat has diminished.

...Ohio Valley...
Removed the small Slight Risk area over portions of southern
Indiana and Ohio. Flash flood guidance values remain quite low
due to earlier convection, however the siginal for any organized
heavy rains to return to the region through the overnight has
diminished.

Pereira

1600Z Update...
...Gulf Coast to eastern Tennessee and the western Carolinas...
Began trimming away the western extent of the previous outlook
areas where the heavy rainfall threat is now diminishing along the
Gulf Coast from southeastern Louisiana to Alabama. Farther to the north--reflecting trends in some of the hi-res guidance and the
NAM, adjusted the previous outlook areas a little farther north.
Did not shift the Moderate Risk area as far north as the NAM and
some other members would suggest, but did adjust it to encompass
much of where the HREF is indicating high neighborhood
probabilities for 3-inches or more. Will continue to monitor and
make adjustments as needed.

...Central Plains and Mid Mississippi Valley...
The Marginal Risk area was expanded from the mid Mississippi
Valley back into the central Plains and a Slight Risk was added to
portions of northern Missouri and southern Illinois. General
consensus from the 12Z hi-res guidance shows a good signal for
slow-moving, backbuilding convection to develop during the
afternoon and continue into the evening across the area. Guidance
shows a period of persistent southwesterly low level inflow
supporting deep moisture (PWs at or above 1.75 inches) and ample
instability along a slow-moving boundary. The 12Z HREF shows high
neighborhood probabilities (50 percent or greater) for
accumulations of 2-inches or more, along with 30 percent or
greater probabilities for amounts of 3-inches or more within the
Slight Risk Area.

...Ohio Valley to the Mid Atlantic...
Extended the Marginal Risk east of the Appalachians into the Mid
Atlantic, where there is some signal for convection producing
locally heavy amounts as it develops and moves across the region
later today. The signal for additional heavy amounts is limited,
but did maintain a small Slight Risk area across areas in southern
Indiana, southwestern Ohio, and northern Kentucky impacted by
heavy rains. While the immediate threat for additional heavy
rains has diminished, will continue to monitor the potential for
additional heavy rains later today and overnight. Will also
monitor the potential need for a Slight Risk area farther to the
east. There is some signal in the hi-res guidance for heavy
amounts developing across portions of eastern Ohio, western
Pennsylvania, and northern West Virginia -- which may warrant an
increase to a Slight Risk area for portions of the region.

Pereira

1300Z Update...
...Lower Michigan...
A Marginal Risk area was added to the southern portion of Lower
Michigan. Latest radar imagery shows convection beginning to
train near a west-east oriented stationary boundary, with rainfall
rate estimates over 2+ in/hr within some of the heavier cells.
With guidance indicating conditions will remain favorable for
heavy rainfall rates to persist over the next few hours, and given
the relatively low FFG values across the region, a Marginal Risk
was added to the region. Refer to MPD #353 for further details
concerning the near-term potential for heavy rainfall and flash
flooding across the region.

...Ohio Valley...
A Slight Risk area was added to portions of the Ohio Valley. The
Slight Risk area was drawn to highlight areas where ongoing and
previous convection have lowered 3-hr FFG to an inch or less. In
addition to ongoing convection, there is some potential that
upstream convection and/or convection developing later today into
the overnight may track into this same region.

...Central Plains...
A Marginal Risk was added for portions of the central Plains.
Guidance shows the potential for heavy rainfall rates increasing
during the evening and overnight as convection developing upstream
moves into a more moist environment. Recent runs of the RAP show
PWs increasing to 1.5 to 1.75 inches along an axis of 30-40 kt
southerly inflow into the region. The 06Z HREF shows high
neighborhood probabilities for 2-inches or more within much of the
Marginal Risk area.

Pereira


0900Z Discussion...
Central Gulf Coast and Southeast...As the disturbance continues
its inland trek, heavy rainfall amounts of 4 to 6 inches with
locally heavier amounts will occur for these regions. A bit of
uncertainty remains on how this will track, but the overall
footprint did not change much from previously issued Moderate,
Slight, and Marginal Risk areas. Coastal areas have already seen
locally 8+ inches of rain.

Ohio Valley...Backbuilding convection remains the greatest threat
for these areas during the day 1 time period. Slow moving storms
dropping additional 1 to 3 inches of rains with Hi-Res models
showing stronger signals of 3+ inches are enough to create
flooding potential. The Marginal Risk area was adjusted to account
for latest QPF forecast.

Chiari

Previous Discussion...

2030Z Update...
For the central Gulf Coast and Southeast--reflecting a more
progressive track, shifted the previous risk areas a little
farther east, while extending the northern extent of the Marginal
and Slight Risk areas farther northeast into northern Georgia and
the western Carolinas.

Along the Ohio Valley--extended the Marginal Risk back to the west
across southern Illinois and Indiana. The 12Z hi-res guidance
showed a good signal for convection redeveloping during the
afternoon and continuing into the evening across the region. With
the ARW, ARW2, FV3LAM, and the 12Z HRRR all indicating some
potential for backbuilding -- the HREF is showing high
neighborhood probabilities for accumulations of 2-inches or more
across a large portion of southern Illinois into southern Indiana.


Pereira



Day 2
Valid 12Z Sun Jun 20 2021 - 12Z Mon Jun 21 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF
GEORGIA AND THE CAROLINAS AND EASTERN IOWA TO SOUTHERN LOWER
MICHIGAN AND FAR NORTHWESTERN OHIO ...

2030Z Update...
...Mid Mississippi Valley to the Great Lakes...
Introduced a Slight Risk area extending from portions of eastern
Iowa to far southern Lower Michigan and the northwestern corner of
Ohio. The general consensus of the 12Z guidance shows convection
ongoing at the start of the period moving northeast from the mid
Missouri Valley to the Great Lakes. Models are in generally good
agreement indicating convection developing later in the day along
a trailing outflow and ahead of a cold front approaching from the
northwest. Models show this convection developing over the mid
Missouri and Mississippi valley and training to the northeast back
into the Great Lakes. Deepening moisture (PWs at or above 1.75
inches) along a 50+ kt southwesterly jet will help support heavy
rainfall rates. The 12Z HREF shows high probabilities (50 percent
or greater) for rainfall accumulations of 3-inches or more within
the Slight Risk area.

...Southeast...
Based on consensus of the 12Z guidance, trimmed back some of the
western extent of the previous Marginal and Slight Risk areas,
while extending the Slight Risk farther south along the South
Carolina coast into coastal Georgia. Models are offering a pretty
good signal that as Claudette moves through the Carolinas, an
inflow band setting up east of the center could produce heavy
amounts across the region.

Pereira

0900Z Discussion...
Southeast U.S. and Southern Mississippi east into Louisiana...

The disturbance continues its progression across the southeast
CONUS bringing heavy precipitation to the area. The further in
time we go with this feature, the more model spread is observed,
but overall footprint for continued heavy precipitation exists for
portions of NC southward into central GA and AL. Additional QPF of
2 to 4 inches is expected through this region already saturated
from previous days making flooding a concern. Some of these areas,
especially closer to the coastal waters of the Carolinas, have
seen upwards of 400% of normal precipitation over the past two
weeks. Minor adjustments were made to the Slight Risk area for NC
down into SC and northern portions of GA. A Marginal Risk area
envelops this area as well encompassing further north into NC and
westward into portions of AL. Additional precipitation is expected
along the coastal waters of LA, MS, and AL with previous days QPF
expected to lower FFG significantly.

Upper Mississippi Valley into Upper Great Lakes...

Convection is expected to fire off for this region again Sunday
afternoon creating conditions favorable for heavy precipitation.
PWAT anomalies of 1.5-2 std deviations exist through an area
seeing an ongoing series of convection firing off through previous
evenings. With latest guidance showing signals of 2 to 4 inches
falling during this period the Marginal Risk area was left in
place for the region. A few models show signals of 3 to 5 inches
for the area.

Chiari


Day 3
Valid 12Z Mon Jun 21 2021 - 12Z Tue Jun 22 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE GULF
COAST TO THE NORTHEAST....

2030 Update...
Models continue to present of expansive footprint for potentially
locally heavy amounts, supported by broad southerly inflow from
the northern Gulf into a strong cold front that will be dropping
southeast from the mid Mississippi Valley and southern Plains,
while sweeping east from the Great Lakes and Ohio Valley.
Anticipate a Slight Risk area(s) may be required in future
updates. Deep moisture pooling along the front, along with strong
forcing, will likely support periods of heavy rainfall, producing
broader areas of flash flooding concerns than a Marignal Risk
indicates. However, given the uncertainty as to where those heavy
amounts may occur, opted to withhold any upgrades for now.

Pereira

0900Z Discussion...
...Lower Mississippi Valley into the Ohio Valley...
Upper level trough drapes across the central US during this period
bringing areas of heavier precipitation out ahead with marginal
instability. Guidance has 1 to 3 inches for portions of this
region that have seen quite a bit of precipitation over the past
few weeks. PWATs sit around 1.25-1.75. Portions of the Lower
Mississippi Valley have seen upwards of 400+% of normal over the
past two weeks meaning soils will not handle much rain to cause
flooding concerns. Have opted to place this region under a
Marginal Risk with the footprint encompassing the heaviest amounts
of QPF among the models.

...Central Gulf Coast...
Moist southerly winds will bring additional instability out ahead
of the upper level trough. Model guidance has signals of 1 to 3
inches of additional rains on grounds that are already
experiencing heavy amounts during the day 1 and 2 time periods due
to Potential Tropical Cyclone THREE. Soils will be primed and
flooding potential will only increase. Because of the precedent
conditions, have opted to place this area under a Marginal Risk
area for this time period.

Chiari

$$
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FOUS30 KWBC 200746
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Sun Jun 20 2021

Day 1
Valid 12Z Sun Jun 20 2021 - 12Z Mon Jun 21 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE SOUTHEAST
U.S. AS WELL AS UPPER GREAT LAKES REGION...

...Southeast U.S....

With the remnants of Tropical Depression Claudette making it's way
northeast across the region, heavy precipitation can be expected.
Signals along the coast of NC and SC show upwards of 2 to 4 inches
of additional precipitation for these areas with PWAT values
sitting near 2.25. Precipitable water anomalies sit close to 2.0
std deviations above climo. The Slight Risk area was tightened
eastward a bit from the previous issuance.

...Upper Great Lakes...

Ongoing convection is expected to impact these areas as a system
makes its way in from the northwest. With deepening moisture and
instability in play, heavy bands of precipitation could produce
flooding potential. Latest guidance keeps 1 to 3 inches of
precipitation over an area with pockets of quite low FFG. PWAT
values sit near 1.75 inches. The Slight Risk area was adjusted
slightly to account for latest model guidance.

Chiari




Day 2
Valid 12Z Sun Jun 20 2021 - 12Z Mon Jun 21 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER THE LOWER
MISSISSIPPI VALLEY AND CENTRAL GULF STATES ...

...Lower Mississippi Valley into the Ohio Valley...
Upper level trough drapes across the central US during this period
bringing areas of heavier precipitation out ahead with marginal
instability. Guidance has 1 to 3 inches for portions of this
region that have seen quite a bit of precipitation over the past
few weeks. PWATs sit around 1.25-1.75. Portions of the Lower
Mississippi Valley have seen upwards of 400+% of normal over the
past two weeks meaning soils will not handle much rain to cause
flooding concerns. The Marginal Risk area was kept in play from
previous issuance and a new Slight Risk area was introduced with
the footprint encompassing the heaviest amounts of QPF among the
models over portions of TN southwest into central MS.

...Central Gulf Coast...
Moist southerly winds will bring additional instability out ahead
of the upper level trough. Model guidance has signals of 2 to 4
inches of additional rains on grounds that have already
experienced heavy amounts due to Tropical Depression Claudette.
Soils will be primed and flooding potential will only increase.
Because of the precedent conditions, have opted to place this area
under a Slight Risk area for this time period. Later consideration
will be given if areas along the coast of AL/MS/LA should be
upgraded to a Moderate Risk.

Chiari



Day 3
Valid 12Z Mon Jun 21 2021 - 12Z Tue Jun 22 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE
SOUTHEAST CONUS INTO THE GULF COAST....

As the front continues its progression across the CONUS, heavy
bands of precipitation set up. Guidance has 1 to 3 inches of
precipitation falling during this time period for areas that are
already expected to see heavy amounts from the day 1 and 2 time
period which will just lower FFG even more. PWATs sit near 2.25
inches for areas closer to the coast with anomalies of 1 to 2 std
deviations. With the majority of this region expected to see wet
conditions leading up to this period, have placed a Marginal Risk
area from VA southwestward through eastern portions of LA with the
westward border reaching eastern portions of the TN Valley.

Chiari

$$
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FOUS30 KWBC 202257
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
656 PM EDT Sun Jun 20 2021

Day 1
Valid 2246Z Sun Jun 20 2021 - 12Z Mon Jun 21 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST U.S. AS WELL AS PARTS OF THE MIDWEST AND UPPER GREAT
LAKES REGION...

...Southeast...
2245 UTC Update...Removed the small Slight Risk over eastern
portions of the Florida Panhandle and southern Georgia, given that
Claudette's circulation has departed to the northeast (across SC).
The Slight Risk over much of the Carolinas and portions of eastern
GA continues. Areas along Claudette's main spiral band, along with
periphery bands east-northeast, will continue to have the best
chance of receiving 3+ inches within 3 hours, based on the current
radar and mesoanalysis trends, along with recent CAM guidance.

...Texas and Louisiana coasts...
A Marginal Risk area extending from the Middle Texas coast to
southeastern Louisiana was added. Mid level energy drifting
northeast while interacting with deep moisture (PWs at or above
2-inches) is expected to support slow-moving storms with the
potential for heavy rainfall. Much of the guidance keeps most of
the heaviest rainfall offshore, however there is some signal that
some of these storms may impact coastal communities and areas
farther inland. Both the 12Z HRRR and ARW2 show very heavy
amounts developing along the Middle Texas coast overnight. Will
continue to monitor the need for a potential upgrade across the
region.

...Mid Mississippi Valley to the Great Lakes...
Extended the Slight Risk farther southeast back into northern
Missouri to encompass areas where heavy rains occurred yesterday.
Given the wet antecedent soil conditions, the additional
convection currently moving into the region, along with the
potential for more development tonight, are more likely to cause
short-term runoff concerns over this region.

Hurley/Pereira


Day 2
Valid 12Z Mon Jun 21 2021 - 12Z Tue Jun 22 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE LOWER
MISSISSIPPI TO THE OHIO VALLEYS AND CENTRAL GULF STATES...

...2030 Update...
Extended the Slight Risk area farther to the northeast from the
lower Mississippi and Tennessee valleys into the Ohio Valley.
Deepening moisture along the front, along with strengthening upper
level dynamics will help support the potential for heavy rainfall,
with training storms raising the threat for heavy accumulations
and localized flash flooding concerns. The 12Z HREF showed high
neighborhood probabilities for accumulations of 2-inches or more
within much of the extended Slight Risk area. This included some
of the area in Kentucky and southern Ohio recently impacted by
heavy rains.

Pereira


0900Z Discussion...
...Lower Mississippi Valley into the Ohio Valley...
Upper level trough drapes across the central US during this period
bringing areas of heavier precipitation out ahead with marginal
instability. Guidance has 1 to 3 inches for portions of this
region that have seen quite a bit of precipitation over the past
few weeks. PWATs sit around 1.25-1.75. Portions of the Lower
Mississippi Valley have seen upwards of 400+% of normal over the
past two weeks meaning soils will not handle much rain to cause
flooding concerns. The Marginal Risk area was kept in play from
previous issuance and a new Slight Risk area was introduced with
the footprint encompassing the heaviest amounts of QPF among the
models over portions of TN southwest into central MS.

...Central Gulf Coast...
Moist southerly winds will bring additional instability out ahead
of the upper level trough. Model guidance has signals of 2 to 4
inches of additional rains on grounds that have already
experienced heavy amounts due to Tropical Depression Claudette.
Soils will be primed and flooding potential will only increase.
Because of the precedent conditions, have opted to place this area
under a Slight Risk area for this time period. Later consideration
will be given if areas along the coast of AL/MS/LA should be
upgraded to a Moderate Risk.

Chiari



Day 3
Valid 12Z Tue Jun 22 2021 - 12Z Wed Jun 23 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST....

2030Z Update...
Given the ongoing and recent heavy rain impacts due to Claudette
and the potential for additional locally heavy amounts during the
Day 3 period as shown by some of the 12Z guidance, including the
ECMWF, UKMET and NAM, introduced a Slight Risk area covering
portions of Alabama and Georgia. Given the evolving antecedent
conditions due to Claudette and what is a fair amount of model
spread with respect to the QPF details, adjustments to the outlook
area(s) are likely forethcoming. In the interim, the initial
Slight Risk area was drawn based on where the guidance is
currently showing the better potential for heavy amounts and where
FFG values are lower due to the recent rains.

Pereira

0900Z Discussion...
As the front continues its progression across the CONUS, heavy
bands of precipitation set up. Guidance has 1 to 3 inches of
precipitation falling during this time period for areas that are
already expected to see heavy amounts from the day 1 and 2 time
period which will just lower FFG even more. PWATs sit near 2.25
inches for areas closer to the coast with anomalies of 1 to 2 std
deviations. With the majority of this region expected to see wet
conditions leading up to this period, have placed a Marginal Risk
area from VA southwestward through eastern portions of LA with the
westward border reaching eastern portions of the TN Valley.

Chiari

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FOUS30 KWBC 211903
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Mon Jun 21 2021

Day 1
Valid 16Z Mon Jun 21 2021 - 12Z Tue Jun 22 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE LOWER
MISSISSIPPI VALLEY TO THE OHIO VALLEY AND GULF COAST FROM THE
UPPER TEXAS COAST TO THE FLORIDA PANHANDLE...

...Lower MS Valley/TN and OH Valleys...
A trough moving across the Mid-West will amplify through the
period as it advances toward the east coast. In response, strong
southerly flow will usher in rich Gulf moisture into the region.
Precipitable water values will climb above 2 inches in some
locations (which is around 2 standard deviations above the mean)
aided by 20 to 30 knot southwesterly low level flow. Aloft, as
the trough sharpens, the right entrance region of the upper jet
will align over portions the TN/OH Valleys with mid-level
shortwaves moving atop the instability gradient/surface trough.
Therefore, plenty of synoptic scale ascent will be present to
produce scattered showers and thunderstorms. MUCAPE values will
surge to over 3000 J/kg ahead of the associated cold front. Given
the warm rain process, hourly rain rates could climb above 1.5
inches. And with multiple rounds of precipitation advancing from
southwest to northeast, areal average precipitation will range
between 1-3+ inches with locally higher amounts expected.

Fairly dry antecedent conditions exist across much of the Lower MS
Valley region should limit the overall flood threat. However,
heavier precipitation and training may occur within this region to
support isolated to scattered flash flooding. This is also evident
by higher and prolonged HREF probabilities of 1+/2+ inch hourly
rain rates. One adjustment in the morning update was to fill in
the gap between slight risk areas in east Texas and central LA,
where merging outflows could results in redevelopment of
showers/storms/locally heavy rain similar to surrounding areas
supported by the HREF blended mean QPF.
In FL, showers and storms in the panhandle have developed and are
moving east further than the prior outlook, so the slight risk was
expanded east in conjunction with both radar trends and higher QPF
in the HREF members/mean.

Across portions of the Deep South into the TN Valley and
Appalachians, FFG values are lower with higher soil saturation.
Therefore, it will take less rainfall to saturate the basins.

...Central Gulf Coast...
A weak surface trough positioned either along or just off the
coast which will act as the focus for convection through the
forecast period. As the upper level trough approaches from the
north/west, increased divergence aloft and mid-level shortwaves
will provide ample large scale lift. Precipitable water values
will climb above 2 inches aided by 20 to 30 knot low level
southerly flow off the Gulf. This combined with instability of
2000+ J/kg and very high freezing levels should support very
efficient rain rates of over 2 inches/hour. Areal average
precipitation will range between 1-3+ inches with locally heavier
amounts expected. Given portions of southeast LA and southern MS
observed well above average precipitation from Claudette, any
additional heavy rain could result in scattered flash flooding.

Pagano/Petersen






Day 2
Valid 12Z Tue Jun 22 2021 - 12Z Wed Jun 23 2021

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST....

An amplified trough and associated surface cold front will
continue to advance across the southeast U.S. through the forecast
period. Strong return flow will provide sufficient deep layer
moisture and usher in warm air advection and instability into the
region. Meanwhile, a shortwave moving atop the surface boundary
will also be a focus for convection across eastern VA and NC.

...Southeast to the Florida Panhandle...
Convection is expected across areas that have recently received
rains from Claudette. With rich deep layer moisture and
instability climbing above 2000 J/kg, anticipate training
convection over saturated soils could result in scattered flash
flooding. With the 12z NAM showing the moisture advection
resulting in precipitable water values increasing to 2-2.25 inches
ahead of the approaching cold front, a slight risk was extended
across central to eastern South Carolina and North Carolina.
Localized flash flooding is possible where cell mergers and brief
training result in locally heavy rain.

...Eastern VA...
Precipitable water values will surge close to 2 inches, aided by
25 to 35 knot southwesterly flow which is nearly 2 standard
deviations above the mean. This combined with instability above
1000 J/kg and mid-level shortwaves aloft should result in an area
of convection that moves through the region Tuesday/Tuesday
afternoon. While rain rates could exceed 1.5 inches/hour, the
progression of the activity may help to limit overall amounts.
However, some guidance is suggesting that the mean propagation
vectors will align with the approaching front allowing for
training to occur. With this in mind, areal average precipitation
should range between 1 to 3+ inches with locally higher amounts.
Given the very dry antecedent conditions, especially across
eastern VA, the potential for flash flooding may be limited.
However, if model guidance continues to highlight this region with
HREF probabilities increasing, an upgrade to a Slight Risk may be
needed.

Pagano/Petersen

Day 3
Valid 12Z Wed Jun 23 2021 - 12Z Thu Jun 24 2021



...Southeast/Florida panhandle...
A cold front will advance off the Carolina Coast Wednesday
morning. However, the front is expected to stall and linger
across southern SC into southern portions of GA/AL and into the
Florida Panhandle. As a result, convection will likely focus
within this region with activity training from west to east. With
precipitable water values above 2 inches and sufficient
instability, hourly rain rates may climb above 1.5 inches/hour.
Areal average precipitation is around 1-3+ inches.
Flash flood guidance is mostly in the 3-4 inch range in 3 hours,
so only isolated locations are expected to exceed flash flood
guidance. Consequently, the risk is only depicted as marginal.

Petersen/Pagano

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FOUS30 KWBC 272044
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
443 PM EDT Sun Jun 27 2021

Day 1
Valid 16Z Sun Jun 27 2021 - 12Z Mon Jun 28 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PERMIAN BASIN OF SOUTHEAST NEW MEXICO AND WEST TEXAS...

...Southern Rockies and Southern Plains into the Midwest, Northern
Ohio Valley, and Southern Lower Michigan...
16Z Update: An upgrade to a MDT risk has been added to the Permian
Basin after coordination with WFO MAF. Ongoing rainfall this
morning has accumulated to 2-4" along I-20, priming the soils for
heavy rainfall later in the D1 period. This rainfall is occurring
along and just north of a wavering front in response to impressive
low-level convergence and moist advection. Despite convective
overturning and a lack of morning instability, warm and moist
advection increasing from the Gulf of Mexico should provide a
resupply of renewed MLCape this evening. At the same time,
forecast soundings indicate a deepening of the warm cloud layer,
suggesting rainfall rates will become increasingly efficient as
warm-rain processes dominate, and HREF rainfall rate probabilities
spike to a high chance for 2-3"/hr. The 12Z high-res suite is well
clustered in a narrow corridor of 2-4" of additional rainfall,
with locally higher amounts possible, as mean cloud layer winds
fall below 10 kts and Corfidi vectors become increasingly
anti-parallel to the mean flow indicating backbuilding potential.
A lack of significant shear should preclude much storm
organization across this region, but slow moving pulse convection
with these intense rainfall rates will likely lead to at least
scattered incidents of flash flooding. This region has been
saturated by 14-day rainfall which is locally as much as 400% of
normal leading to NASA SPoRT 10cm soil moisture above the 98th
percentile and compromised FFG as low as 0.75"/3 hrs. This will
likely be exceeded in many locations, and after discussion with
WFO MAF, a MDT risk was raised for much of the MAF CWA.

Otherwise, high-res guidance continues to suggest a shortwave and
accompanying mid-level divergence lifting northeast along the
stalled front, and accompanied by intense upper diffluence within
the RRQ of an upper jet streak. This impulse lifting into
favorable thermodynamics will provide ascent for heavy rainfall,
and training of echoes through boundary-parallel mean winds and
Corfidi vectors indicates a good chance for flash flooding. While
there continues some longitudinal variation in the placement of
the heaviest rainfall, much of this region has severely reduced
FFG due to recent rainfall, so despite fast moving storms,
training of rainfall rates 1-2"/hr would likely lead to additional
areas of flash flooding.

Relevant Portions of Previous Discussion:
There will be little change in the upper level pattern during the
day 1 period. This as the mid-upper level trough remains draped
over the northern Plains SSW into the central High Plains and
southern Rockies. At the surface, the main SW-NE oriented front
will remain quasi-stationary, buckling a bit west and east at
times in response to the MCS activity, along with the diurnal
convection (ensuing outflow boundaries). Overall, a few changes
were made to yesterday's Day 2 ERO, including an expansion of the
Slight Risk a bit farther north into eastern NM based on the
latest guidance trends. Areas from NM into western TX are expected
to become underneath a mid level COL, with the main upper
trough/height falls north and ridge axes west and east. Within
this COL however the models continue to show a more focused area
of modest synoptic scale upper divergence and deep-layer ascent
downwind of the trough base.

Later today, the favorable synoptic pattern will once again favor
convective re-development with daytime heating along the front and
withing the mid-upper level COL region across the southern
Rockies. For most areas, PWAT and 850-700 mb moisture transport
are not overly anomalous per the GEFS/SREF, though the persistent,
modest low-level easterly flow will push 1.5+ PWATs close to the
TX-NM border if not into eastern NM, with 850 mb moisture flux
anomalies climbing to 3-4 standard deviations above normal for the
end of June across eastern NM. Instability meanwhile will continue
to remain plentiful ahead of the front -- at least 1000-2500 j/kg
within a corridor from southeast NM and north-central TX northeast
into southern-central MO-IL into central IN. Within the Slight
Risk area, the 12Z HREF indicates scattered probabilities of 30-40
percent for 24hr rainfall totals exceeding 5", with 12-hr
probabilities for 3" also high. This as the individual CAMs all
show pockets of 3 to 5+ inches of rain, especially along the
southern flank of the Slight Risk area (toward the greater
deep-layer instability) where 2+ inch/hr rainfall rates would be
most likely. Along the northern portions of the Slight Risk, while
rainfall rates/amounts may not be as robust, the wetter antecedent
soils (lower FFGs) will offset and result in a similar enhanced
(Slight) ERO risk.

...Southwest Louisiana/Upper Texas coast...
16Z Update: Analysis of 12Z high-res suite and current satellite
imagery prompted a subtle expansion eastward of the SLGT risk.
Otherwise, the previous forecast remains on track for an
increasing heavy rain threat this aftn through tonight as a TUTT
moves onshore Texas drawing increasing moisture and instability
westward, focused primarily north of this feature.

Previous Discussion:
A tropical upper tropospheric trough or TUTT on the southern
periphery of the upper ridge is expected to continue westward
toward the Mouth of the Rio Grande by Monday morning. Precipitable
water values of 2.25"+ along with instability wafting in from the
Gulf and enough low-level inflow/effective bulk shear (25-35 kts)
within a regime with 20-25 knots of 850-400 hPa mean wind should
lead to heavy rainfall. At this point, the best signal in the
guidance overlaps both the Day 1 and Day 2 ERO period, beginning
after 00-03Z Monday. We added a Slight Risk along the Upper TX
Coast into southwest LA in light of the consensus in the QPF
guidance (especially high-res CAMs), which show scattered totals
between 3-5+".

Weiss/Hurley/Roth


Day 2
Valid 12Z Mon Jun 28 2021 - 12Z Tue Jun 29 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHEAST NEW MEXICO AND WEST TEXAS...

...New Mexico and Texas...
A wavering front will persist across southern New Mexico and
northeast through Texas on Monday. This front will move little on
D2 as the parent mid-level longwave trough remains anchored SW to
NE from The Four Corners into Minnesota, blocked by expansive
Bermuda ridging to the east. Return flow around this ridge to the
east will transport tropical moisture from the Gulf of Mexico
northwestward across Texas through the Permian Basin and into New
Mexico. At the same time, modest mid-level impulses will
periodically lift northeast through the confluent mid-level flow,
which will work in tandem with the expanding tail of the upper jet
streak to provide deep layer ascent across the region.
Additionally, low-level SE flow will enhance ascent through
isentropic upglide atop the front and local orographic enhancement
into the terrain. This robust ascent will act upon favorable
thermodynamics for heavy rainfall, as PWs climb towards 1.5",
which is above the climatological 90th percentile and 2 standard
deviations above the mean, with MLCape forecast to rise above 1000
J/kg. Deepening warm clouds noted in forecast soundings in this
environment support increasingly efficient warm rain processes,
and the recent HREF rain-rate probabilities indicate a high
likelihood for 1-2"/hr rates Monday evening, with locally 3"/hr
possible.

While the rain rates themselves are concerning, the likelihood of
training of these rain rates has led to an upgrade to a MDT risk
for this region. Aligned mean winds with Corfidi vectors suggest
training from south to north, especially late aftn through the
evening, and the 12Z high-res suite is in pretty good agreement in
a swath of 2-4" of rainfall, with the HREF 12-hr probabilities
indicating a low-end risk for 5 inches or more, and the EAS
probabilities showing a 50% chance for 2 inches which has shown
some positive verification for flash flood instances. This
rainfall will occur atop soils that are relatively dry due to a
lack of recent rainfall outside of pockets, but heavy rainfall is
occurring (and more is expected) on D1, priming the soils beyond
what the current FFG would indicate. After coordination with
MAF/EPZ/ABQ, a targeted MDT risk was raised for the likelihood of
flash flooding, which locally could be significant.

Surrounding the MDT risk area, the SLGT risk was expanded to the
east into the TX Panhandle where additional slow moving storms are
expected Monday atop soils that are saturated from heavy rainfall
on D1. Additionally, the SLGT risk was expanding northward into
the Sangre De Cristos where upslope enhancement will likely create
more intense rainfall, some of which could occur across sensitive
burn scars.


...Central Plains through the Upper Midwest and Great Lakes...
A nearly stationary front will wobble only slight west or east on
D2 as it remains entrenched beneath a slow moving upper trough.
This trough is sandwiched between two impressive mid-level ridges
to the west and east, driving its nearly stationary movement.
Robust moisture advection on tropical return flow from the
Atlantic and Gulf of Mexico will stream into the Central Plains
and lift northeast, providing low-level ascent through convergence
along the front. PWs approaching 2" are progged to move into
OK/KS/MO/IA and IL during the evening, combining with MUCape of
2000 J/kg or more to provide extremely favorable thermodynamics
for heavy rainfall. Within this environment, ascent will occur
along the front through the aforementioned low-level convergence,
but enhanced by upper level diffluence within the RRQ of the tail
of an expanding jet streak, and through modest PVA as subtle
mid-level impulses lift northeastward within the confluent flow.
These together will create rounds of convection with heavy
rainfall, and mean winds aligned with propagation vectors and
parallel to the boundary suggest a high likelihood for training of
echoes.

The recent HREF for D2 suggests scattered probabilities for
1-2"/hr rain rates, with the 1"/hr probabilities as high as 50%.
Where these rates train to the NE, rainfall may exceed 3" in spots
as shown by the CONEST, HRRR, and HREF Blended Mean. This could
lead to scattered flash flooding as recent rainfall has been
significant. 7-day rainfall departures are as high as 600% of
normal from Missouri through Illinois, leading to NASA SPoRT 40cm
soil moisture that is above the 98th percentile. This suggests
that any additional heavy rain will quickly lead to runoff and may
produce flash flooding. There is the potential that a targeted MDT
risk may be needed if guidance can converge on a heaviest axis
falling atop the most saturated soils and lowest FFG. However, the
models have waved a bit back to the north this aftn, keeping
confidence too low for an upgrade at this time. Despite that, the
SLGT risk was expanded SW to better match the higher probabilities
and most favorable antecedent hydrologic conditions, and expanded
slightly longitudinally to account for the potential for the
high-res to be too far north as they tend to verify further south
into the better instability.


...Upper Coast of Texas into Southwest Louisiana...
A mid-level wave evident on satellite imagery moving westward
across the northern Gulf of Mexico is associated with a TUTT which
is progged to lift onshore the middle or lower Texas Coast early
Monday morning. As this feature moves westward, it will drag
exceedingly high PWs of 2-2.25" along with it, with low and
mid-level confluence driving these high PWs, along with increasing
instability, onshore the Louisiana and Upper Texas Coast. Within
this tropical airmass, ascent will begin to increase through an
upper divergence maxima intensifying atop the developing
convection, and through low-level convergence as the 850mb inflow
reach 20-30 kts, exceeding the mean cloud-layer winds of 15-20
kts. This setup suggests showers and thunderstorms with efficient
rain rates of 2"/hr or more will advect onshore and blossom in
coverage much of Monday and Monday evening, with backbuilding into
the better PW/instability offshore providing training as shown by
increasingly anti-parallel Corfidi vectors. There continues to be
some latitudinal spread amongst the models in the placement of the
heaviest rainfall, and have expanding the SLGT risk slightly
westward with this update. However, the highest probabilities for
intense rainfall continue to focus in the Houston to Beaumont
corridor where both HREF EAS and 40km neighborhood probabilities
maximize. Locally, more than 5" of rainfall is possible, and while
the coverage of flash flooding is not forecast to be great enough
to warrant an upgrade, should this heaviest rain occur atop an
urban area, flash flooding could become locally more significant.


...South Carolina, Georgia, Northern Florida...
Invest 96L is getting better organized east of South Carolina and
is progged to intensify as it moves westward, possibly making
landfall as a tropical system Monday evening near the GA/SC
border. The disturbance is small and is ingesting dry air on its
southern end, but a long fetch of tropical moisture being advected
westward along and north of the low center could produce heavy
rainfall Monday and Monday night. HREF probabilities for 3"/24hrs
are as high as 50% along the immediate coast of GA near Savannah,
with lower probabilities extending as far west as the Piedmont of
GA. Although the system is small and mostly weak, training of rain
rates of 1-2"/hr could produce isolated flash flooding, especially
in any coastal urban areas. The MRGL risk has been extended
westward a bit after coordination with FFC. A trimming of the
southern end of the MRGL risk may be needed with later updates,
but enough spread in the track and rainfall footprint precluded
that with this update.

Weiss

Day 3
Valid 12Z Tue Jun 29 2021 - 12Z Wed Jun 30 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST NEW MEXICO AND WEST TEXAS...

...Southeast portions of the Lower Colorado River Basin and
Southern Rockies into parts of the Southern-Central Plains and
Midwest...
20Z Update: Little change to the risk areas this aftn other than
to adjust for positions of heavier QPF axes based off the aftn
guidance. Modified the SLGT risk for SE NM slightly to account for
anticipated heavy rainfall D2 as the pattern continues to support
periods of excessive rain rates of 1-2"/hr over sensitive soils.
As the pattern evolves very little due to blocking of the ridge to
the east, it is possible further adjustments to this SLGT risk
area will be needed with later updates, and as the antecedent
rainfall actually occurs. Further to the north, expanded the MRGL
risk a bit northeast through lower Michigan where recent rainfall
has led to swollen rivers and saturated soils. Training of heavy
rainfall along the slow moving front could lead to isolated
instances of flash flooding anywhere along the front, but the
coverage and intensity does appear to wane D3 as the Bermuda ridge
to the east pushes westward cutting off some of the available
moisture.

Previous Discussion:
The outlook areas across these regions for the Day 3 ERO were not
much different from Day 2, owing to the fairly stagnant upper
level pattern. Toward the end of the period (12Z Wed), the main
(longwave) trough will have slowly lifted into the Upper Midwest
and Great Lakes region, with weaker flow and a more subtle,
compact vort energy within the mid-level COL region over the
central Great Basin and 4 corners region. The model signal for
heavy rainfall remains most pronounced once again across
eastern-southeastern NM into portions of western TX, where the
persistent easterly (upslope) low-level inflow maintains highly
anomalous moisture flux into this region (+3 to +4 standard
deviations). The consensus from the guidance is an additional 1-3+
inches of rainfall in a consolidated area within the Slight Risk
outlook.


...Georgia/South Carolina/Alabama...
No risk area was introduced with this update, but periods of heavy
rainfall are possible D3 as Invest 96L weakens and shifts westward
beneath the Bermuda ridge. GEFS and ECENS probabilities for 1" are
modest, but efficient rain rates within the tropical airmass could
produce locally torrential rainfall. 1-2" of rain is possible
across the Piedmont of GA and into Upstate South Carolina where
upslope enhancement could maximize rainfall.

Hurley/Weiss

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FOUS30 KWBC 281955
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Mon Jun 28 2021

Day 1
Valid 16Z Mon Jun 28 2021 - 12Z Tue Jun 29 2021

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHEAST NEW MEXICO AND FAR WEST TEXAS...

16Z Update...

Minor updates were made to the Marginal Risk threat area along the
South Carolina coast south into Georgia due to the latest updates
to the track of Tropical Depression Four. This area was expanded
further northward to account for this update. Convection along the
boundary through the central CONUS is ongoing and sits well within
the previously issued Moderate, Slight, and Marginal Risk threat
areas. Do not plan on making updates to these areas at this time.

Chiari

Previous Discussion...

...New Mexico and West Texas...
Frontal boundary along the Rio Grande will linger in the region
today beneath the SW-to-NE oriented longwave trough from the Four
Corners to Minnesota. Pattern remains largely stuck over the CONUS
between two strong positive anomalies over the Pac NW/southwestern
Canada and just east of the Mid-Atlantic, favoring a continuation
of below normal heights (500mb anomalies around -2 sigma over the
region). Around the surface high centered over the western
Atlantic, southeasterly flow off the Gulf will continue to supply
the region with increased moisture as PW values rise to 1.25-1.50
inches from west to east across southern NM (above the 90th
percentile). With long, skinny CAPE through a deeply saturated
atmosphere, warm rainfall processes will act to enhance rates,
which may exceed 1-2"/hr (40% prob >1"hr per 00Z HREF) with
locally 3"/hr possible per some CAM guidance. In addition,
training areas/cells may be possible in the deep southerly flow
between 300-700mb, with upslope enhancement an added factor
southeast of higher terrain. Though FFG was relatively high in the
Moderate Risk area due to recent dry conditions (2-3"/hr and
3-4"/6hr), rainfall ongoing and expanding through the morning will
lower those values. 00Z model guidance still showed spatial
differences of ~100 miles but were better aligned in total QPF
amounts of several inches (2-5" overall in max areas, with a
broader area of lighter amounts contingent on convective
initiation/path). Expanded the Slight Risk area westward along the
U.S./Mexico border per coordination with EPZ (where FFG was lower
as well). Burn scar areas in the vicinity will also be sensitive
to these heavy rainfall rates.

...Central Plains through the Upper Midwest and Great Lakes...
Continuation of the quasi-stationary surface front northeastward
toward the Great Lakes will remain a focus for showers and storms
again today; maintained and expanded the Slight Risk area from the
Midwest back into Oklahoma. Precipitable water values will
increase to around 2" from the upper TX coast into central/eastern
Oklahoma on surface southeasterly flow and deep southerly flow
from 850mb up to about 350mb. Right rear quadrant of a 70-80kt
upper jet will slowly lift northward through the day into the
overnight hours, providing larger-scale lift atop lower-level
convergence along the frontal boundary. Recent rainfall has been
heavy in some locations with 3-6" the last 36 hours between OKC
and TUL, resulting in low FFG values (1-1.5"/1hr and 2"/3hr).
Recent 7-day rainfall in excess of 400-600% of normal to the
northeast (through MO into IL) has some FFG values even lower.
Convection is forecast to expand in coverage later today as weak
mid-level vort maxes lift northward out of TX in an unstable air
mass (2000+ J/kg MUCAPE). Training of cells will be possible given
the southerly wind profile through much of the atmosphere. 00Z
guidance showed a general 1-4" of rainfall along the boundary
arced to the northeast from Oklahoma, where HREF neighborhood
probs of 1"/hr were 30-60%. Flash flooding will be possible
especially in some areas that see higher rainfall amounts/rates on
top of saturated soils.

...Upper Coast of Texas into Southwest Louisiana...
TUTT low will sink southwestward into Mexico but mid-level
vorticity will stream in off the Gulf into TX/LA today, providing
a focus for convection. High PW air (2-2.25") in a saturated
column coincident with ample instability will drive efficient warm
rain processes with rates 1-2"/hr. 00Z CAM guidance continued to
narrow in on the Houston to Lake Charles area but with simulated
cells off the Gulf to the west and east capable of heavy rainfall
through the day. WPC forecasts 1-3" QPF for day 1, locally higher
in areas that see training off the Gulf,

...Georgia and southern South Carolina...
Invest 96L will track inland later today into Georgia near the SC
border per the latest guidance. The disturbance is small and is
ingesting dry air on its southern end, but a long fetch of
tropical moisture being advected westward along and north of the
low center could produce heavy rainfall in a narrow axis. Guidance
continued to show rain rates of 1-2"/hr which could produce
isolated flash flooding, especially in any coastal urban areas.
Trimmed the southern portion of the Risk area out of FL to account
for a bit better agreement in the models.


...Eastern Maine...
Cold front will push through northern Maine atop the closed high
off the Mid-Atlantic with a weak area of low pressure moving
across the region. 00Z CAM guidance showed eastward-moving cells
this afternoon that could produce 1-2"/hr rainfall rates where
local FFG values were about the same. Isolated flash flooding may
occur with some heavier elements.


Fracasso



Day 2
Valid 12Z Tue Jun 29 2021 - 12Z Wed Jun 30 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST NEW MEXICO AND WEST TEXAS AND OVER THE CENTRAL PLAINS...

21Z Update...

Minor adjustments were made to account for latest model guidance,
but overall footprint of the Slight and Marginal Risk areas was
left in place. More consideration will be given to the track of
Tropical Storm Danny to see if any risk areas will be necessary
for the Southeast U.S.. As models stand right now, this does not
appear to be the case with QPF less than 1 inch for much of these
locations.

Chiari

Previous Discussion...

...Southeast portions of the Lower Colorado River Basin and
Southern Rockies into parts of the Southern-Central Plains and
Midwest...
Introduced a Slight Risk area over central OK into southeastern KS
and far western MO as the wavy frontal boundary remains in place.
Rather stagnant longwave pattern suggests ingredients for heavy
rain remain mostly in place from day 1, but with a bit of a tilt
aloft allowing some height rise into the area, nudging the
rainfall axis to the northwest. However, the region has seen quite
a bit of rainfall recently so FFG values remain lower than normal.
Convection is forecast to produce rainfall in excess of 1-2" with
rates 1"/hr on Tuesday but perhaps not as focused as Monday. Given
the recent rainfall, narrow Slight Risk area over the region with
the highest rainfall potential (HREF probs >1"/hr >50%) seemed
prudent. Expanded the Marginal Risk contour northward into WI/MI
as some rainfall could exceed relatively low FFG values around
1-1.5"/hr.

To the southwest, maintained the Slight Risk area for southeastern
New Mexico and far west Texas as the southern portion of the upper
trough (and weak upper low) meander over the AZ/NM border with
another over UT. Rainfall/convection on day 1 will likely modulate
the activity for day 2, but guidance remains focused on areas
along and east of the Sacramento Mountains in the Pecos River
Valley. PW values should remain near 1.25-1.50" with skinny CAPE
profiles but easterly flow below 600mb and SE to southerly flow
aloft as the pattern shifts just a bit. Still, moisture flux
anomalies remain above normal. Though lower FFG values suggest it
will be easier to reach these values, maintained a Slight Risk due
to uncertainty in the antecedent rainfall distribution. Larger
Marginal Risk area continues westward into eastern Arizona where
afternoon convection has a better chance of forming Tuesday than
Monday as the southern portion of the upper trough nudges westward
and PW values increase from the east.


Fracasso

Day 3
Valid 12Z Wed Jun 30 2021 - 12Z Thu Jul 01 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST AND FROM THE CENTRAL PLAINS THROUGH THE OHIO
VALLEY...

21Z Update...

Convection will continue along the frontal boundary as it slowly
makes its way across the CONUS. Minor adjustments were made to the
footprint of the Marginal Risk area with an additional 1 to 3
inches forecast to occur during this time.

Chiari

Previous Discussion...


...Southwest...

Precipitable water values will continue to slowly creep up over
Arizona as the upper trough changes shape and tips eastward over
the northern Tier and lifts northward over the Southwest.
Afternoon convection in the terrain could support locally heavy
rainfall >0.50"/hr along the Mogollon Rim into the White Mountains
that may move off into the lower deserts. Farther east and
northeast, lingering instability coupled with above normal PW
values >1" will sustain afternoon convection from CO into NM over
areas that will have seen several days of rain in some locations.
Marginal Risk area should cover the region for now with a
less-focused setup as the upper pattern finally changes.


...Central Plains to Ohio Valley...

Upstream upper pattern over western Canada will finally see some
movement Wednesday as anticyclonic wave-breaking southward out of
Canada into the Northern Plains/Upper Midwest helps push the jet
stream eastward to the Eastern Great Lakes. This will help move
the quasi-stationary frontal boundary draped over the Corn
Belt/western Great Lakes eastward as height falls move through
southern Quebec and northern NY/New England. In advance of the
front, moisture plume of 1.75-2.00" precipitable water will sink
southeastward into the Ohio Valley with sufficient instability for
at least isolated areas of heavy rainfall. Models indicate some
potential for 1-3" areas anywhere along and ahead of the front,
focused during daytime heating. However, areas along and north of
the Ohio River have generally seen little rainfall (<1") during
the last week which will limit saturation but could enhance
runoff. This suggests that a broad/extended Marginal Risk area is
sufficient at this lead time.

Fracasso

$$
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FOUS30 KWBC 302009
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Wed Jun 30 2021

Day 1
Valid 16Z Wed Jun 30 2021 - 12Z Thu Jul 01 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
KANSAS AND MISSOURI EASTWARD TO WESTERN PENNSYLVANIA...

16Z Update...

Slow moving boundary draped across the central CONUS will continue
to bring heavy rainfall to the region creating concerns for flash
flooding. Ongoing flooding products are in place through much of
eastern KS and MO. This is handled well with the Slight Risk and
Marginal Risk areas from the previously issued package. No updates
were made to the EROs at this time.

Chiari

Previous Discussion...

...Kansas/Missouri eastward to Western PA...
Maintained and expanded the previous day 2 Slight Risk area
eastward across the Ohio Valley for today. Upper high off the
Mid-Atlantic will weaken as troughing out of northern Quebec dives
toward the the Great Lakes. Dissipating surface front over the
central Plains will again act as a focus for showers and storms to
develop roughly along the 70F dew point line. Height falls to the
north will carry a new front to reinforce the gradient over the
Midwest during the afternoon as the right rear quadrant of the jet
dips southeastward across the eastern Lakes, promoting large-scale
lift through the Risk area. Precipitable water values are forecast
to remain well above normal: 1.75-2.00" which is about 2-2.5 sigma
above climo. Highest instability will lie from IL eastward,
coincident with the western end of the SPC day 1 Marginal
convective area, with CAPE > 2000 J/kg and a saturated column.
Westerly flow aloft and minimal movement of the surface boundary
suggests potential for training in an environment favorable for
warm rain processes and efficient precipitation production. 850mb
moisture flux, however, was modest at best and displaced well
east. FFG values vary from west to east as a result of recent
rainfall--FFG values about 1-1.5"/hr over KS/MO where it has been
quite a wet past seven days while parts of OH and western PA have
seen no rain the past week. 00Z HREF probs of >1"/hr exceed 60-70%
this afternoon over southern IL/IN with max 1hr rates 3-4"/hr in
some CAM guidance, suggesting flash flooding concerns. Larger
Marginal Risk area stretches eastward to New England where the
front will be more progressive and the column less saturated than
areas farther west.


...Southwest...
Weakening upper low over Utah will remain over the area today as
heights build into TX from the east. Precipitable water values
will remain elevated (anywhere from 1-2" from north to south which
is about +2 to +4 sigma over Nevada). Rainfall on Tuesday was
generally higher than most CAM guidance yesterday, which
overestimated the amount of dry air aloft. Some areas saw
0.50-1.50" rainfall over southern Nevada/Utah in slower-moving
cells. With not much change in the overall pattern, kept the
Marginal over higher terrain and adjacent areas across the Four
Corners region (noting a minimum in QPF over the Four Corners
itself), and including much of central/eastern Nevada. 00 CAM
guidance showed potential for 1-2"/hr rates again today over parts
of NV just north of the heaviest rainfall yesterday to the north
of Las Vegas. Additional rainfall over NM will add to the recent
days of modest amounts with lower FFG than normal.


Fracasso



Day 2
Valid 12Z Thu Jul 01 2021 - 12Z Fri Jul 02 2021

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
CENTRAL APPALACHIANS EASTWARD TO THE COAST AS WELL AS CENTRAL
OKLAHOMA WESTWARD INTO THE TEXAS PANHANDLE...

21Z Update...

As the boundary continues its progression across the CONUS, areas
of heavier precipitation are expected through the Southern Plains
eastward into the Mid Atlantic states. For portions of central OK
into the panhandle of TX, heavy signals of 1 to 3 inches of
additional QPF are forecasted to occur during this time period.
With soils already sitting at 300%+ over the past week and low
FFG, have hoisted a Slight Risk area. The Slight Risk area that
was previously in play for the TN Valley eastward into the
Mid-Atlantic states was modified slightly to account for latest
model output. No changes were made to the Marginal Risk areas with
footprints for these three areas aligning nicely with the model
trends.

Chiari

Previous Discussion...


...Central Appalachians to Atlantic Coast...
Troughing will dig southward-southeastward out of Ontario into the
Ohio Valley, allowing the cold front to move southward and
eastward as it gets reinforced from the northern height falls.
Right rear quadrant of the jet (~70-100kts) will dip into the
eastern Great Lakes early Thursday promoting larger-scale upper
divergence above the surface cold front. Axis of precipitable
water values 1.50-2.00" (about +2 sigma) will precede the front,
coupled with CAPE values 1000-2000 J/kg generally near and south
of 40N. 00Z guidance showed separate threat areas along/west of
the Appalachians and also along the I-95 corridor, but enough
uncertainty existed to combine the areas for simplicity. Highest
model QPF signal was over the central Appalachians via afternoon
convection in addition to overnight rainfall into Friday morning.
Models continue to show afternoon convection along the I-95
corridor from DC to NYC with potential for 1-3"/3h and >1"/hr
given the fairly saturated column and potential for some training.

Farther west, extended the Marginal Risk area westward to the
southern Plains and Colorado ahead of the southward-moving cold
front. Moisture will still be higher than normal with PW values
1.5-2" (+2 sigma) until the front moves through. Convection has
the possibility of higher rainfall rates over areas that have
received several inches of rain the past week, especially
Oklahoma.


...Southeastern Arizona...
Moisture may again increase from the south, due in part to the
remnants of Enrique, with precipitable water values rising to over
1.75" (+2 to +3 sigma) as heights slowly rise in the wake of the
weakening upper low/trough to the north. 00Z HREF guidance showed
40-50% probs of >1"/hr over portions of southern/southeastern AZ
Thursday afternoon and ~1000 J/kg CAPE along the Mexican border
with dew points in the low to mid 60s.


...Northern Continental Divide...
Upper low/trough will lift out of Utah early Thursday and drift
northward as it slowly weakens into western Wyoming. Precipitable
water values will rise a bit to near 1" which is about +2 to +3
sigma with CAPE in excess of 1000 J/kg. Guidance varied on how
well any convection can overcome drier air aloft, but should some
cells organize enough they could produce >0.5"/hr rates which
could lead to local flash flooding. 00Z FV3-LAM and NAM nest were
even showing 1"/hr rates which is in excess of FFG.


Fracasso

Day 3
Valid 12Z Fri Jul 02 2021 - 12Z Sat Jul 03 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER THE
SOUTHEAST AND THE SOUTHERN ROCKIES...

21Z Update...

The frontal boundary will continue its journey across the
southeast CONUS and Mid-Atlantic states bringing periods of heavy
rain. The overall footprint for the Marginal Risk areas is on
track with the latest guidance with only minor adjustments made.
Coastal areas of NC/SC still appear to show the strongest signals
for heavy rains and will be monitored in future updates to see if
a Slight Risk is warranted.

Chiari

Previous Discussion...

...Southeast...
Cold front will sink southward as the deep upper low (500mb
heights about 2.5 standard deviations below normal) swings through
the Mid-Atlantic. Though moisture levels will remain well above
normal south of the front, generally PW values around 2" (1-2
standard deviations above normal), the front will be progressive.
Highest ensemble signal for >1" rainfall Friday lies over eastern
North Carolina, but there remains spread in the guidance on how
the upper low and surface pattern evolve along/off the East Coast
which may shift that focus as currently seen. ECMWF ensemble mean
has trended southward until recently while the GEFS mean has been
less consistent.


...Southern Rockies...
Cold front from the north/northeast will likely stall east of the
Rockies and across the TX Panhandle as the upper high slips
westward to Arizona. With the tropical airmass (PW values still
1.25-1.50" or +2 to +3 sigma), heavy rainfall will still be a
possibility in the warm sector (roughly eastern AZ through NM into
W TX). Models varied spatially on QPF coverage and amounts, but
generally overlapped near eastern NM which has seen well above
normal rainfall amounts the past few days, lowering FFG values
that will likely take time to recover.

Fracasso

$$
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FOUS30 KWBC 161558
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1058 AM EST Sat Nov 16 2024

Day 1
Valid 16Z Sat Nov 16 2024 - 12Z Sun Nov 17 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE COASTAL
RANGES OF THE PACIFIC NORTHWEST...

Conventional GOES satellite imagery along with ALPW data shows an
offshore atmospheric river beginning to gradually impinge on the
Pacific Northwest. An offshore warm front will gradually begin to
overspread the region by this evening which will coincide with
strong warm air advection and moisture transport. The latest 12Z
HREF guidance suggests a 40 to 50+ kt low-level jet arriving with
an orientation gradually becoming more orthognal to the coastal
terrain of western WA including the Olympic Peninsula and extending
southward down into northwest OR. A cold front will then begin to
approach and gradually cross western WA by 12Z/Sunday. This
atmospheric river event will be progressive, but there will be a
corridor of IVT values locally exceeding 500 kg/m/s tonight across
these coastal ranges based off the 06Z GFS which seems reasonable
considering the elevated CIRA-LVT parameters offshore. Some
rainfall rates associated with the combination of strong warm air
advection and orographic forcing should reach into the 0.25" to
0.50"/hour range between 00Z and 12Z, and this will help facilitate
some rainfall totals of 2 to 3 inches by 12Z for the windward
slopes of the coastal ranges. Given the expected rates and totals
by early Sunday morning, and thus earlier arrival of this event, a
Marginal Risk of excessive rainfall has introduced for this period.

Orrison


Day 2
Valid 12Z Sun Nov 17 2024 - 12Z Mon Nov 18 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS...

A southern stream cutoff low is expected to eject into the Plains
and be accompanied with very impressive IVT, PWs and forcing as it
moves across Texas and Oklahoma by Sunday evening into the
overnight hours. This should be enough to overcome the limited
forecast instability and result in deep convection and heavy
rainfall rates across portions of the Southern Rockies and the
adjacent Southern/Central Plains. Guidance continues to suggest
2-4" of rain, but would expect isolated swaths as high as 4-6"
within this setup. The overall progressive nature of the system and
limited instability will cap the upper bound of rainfall
totals...but this system is dynamic enough with plentiful moisture
to suggest at least localized flash flooding is likely. A Marginal
Risk area remains in effect for central/eastern New Mexico,
northern Texas, most of Oklahoma and southern Kansas. The inherited
Slight Risk area saw a minor expansion to reflect the latest trends
and WPC QPF and spans from northern Texas/panhandle to western
Oklahoma.

...Coastal Oregon and northwest California...

A fairly progressive, weak/moderate atmospheric river will be
ongoing across the region. Total rain of 2-4" (through 12z Mon)
and hourly rainfall rates possibly approaching 0.5" within a narrow
southward shifting axis suggest that some minor flooding issues
could arise along coastal areas/mountains. A Marginal Risk area
remains in effect along the coastline.

Campbell


Day 3
Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024

...THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE GULF COAST AND THE PLAINS/UPPER MIDWEST...

...Central U.S. ...

The precipitation shield will shift eastward as the cold front
advances further into the Plains. Moderate to locally heavy
rainfall is expected from northern Texas to southern Minnesota with
areal average of 1 to 3 inches. The higher amounts will likely
concentrate over eastern Kansas, eastern Nebraska and western Iowa.
A Marginal Risk area was maintained with minor adjustments for this
issuance.

...Gulf Coast...

A plume of enhanced tropical moisture will likely advect northward
through the Gulf of Mexico well in advance of tropical cyclone
Sara; which will increase the potential for heavy rain and
localized flooding concerns along portions of the Gulf Coast. A
Marginal Risk area is in effect from southeast Texas to southern
Mississippi.

Campbell
$$
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FOUS30 KWBC 171556
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1056 AM EST Sun Nov 17 2024

Day 1
Valid 16Z Sun Nov 17 2024 - 12Z Mon Nov 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS...

1600Z Update...

The 12Z HREF guidance along with 06Z RRFS/REFS output continues to
support a locally heavy rainfall event across areas of the southern
Plains by this evening and extending into the overnight period. There
has been some shift farther west with some of the swaths of heavy
rainfall potential across west TX as some of the individual CAMs
are supporting the heavier rainfall/convection along the immediate
cold front and in close proximity to the deepening surface low
that will be evolving across the TX High Plains tonight. However, a complication to the rainfall forecast is the downstream evolution
of convection along the front ahead of the surface low involving
areas of northwest TX through southwest to central OK. This is
where strong moisture convergence and forcing may foster a threat
for some training areas of convection that become aligned in a
southwest to northeast fashion. Overall, still expecting some
swaths of 2 to 4 inch rainfall totals, with isolated potential for
heavier amounts and especially across northwest TX into southwest
OK where the better convective training threat will exist. Given
the latest CAM guidance, the Slight Risk area has been expanded
somewhat to account for the varying modes of convective evolution
that are expected this evening and overnight.

Elsewhere, no changes have been made to the previous D1 outlook
including the Marginal Risk area across western OR where there
continues to be some lingering, but modest atmospheric river
activity.

Orrison

Previous Discussion...

...Southern Plains...

A southern stream cutoff low is expected to eject into the Plains
today with very impressive IVT and accompanying anomalous PWs and
forcing as it moves across Texas and Oklahoma by this evening into
the overnight hours. This looks to be enough to overcome the
limited forecast instability and result in deep convection and
heavy rainfall rates across portions of the Southern Rockies and
the adjacent Southern/Central Plains. Guidance continues to suggest
2-4" of rain (per latest HREF and WPC QPF), but would expect
isolated swaths as high as 4-6" within this setup (indicated by
some of the heaviest CAMs). The overall progressive nature of the
system and limited instability will cap the upper bound of rainfall
totals, but this system is dynamic enough with plentiful moisture
to suggest at least localized flash flooding is likely. A Marginal
Risk area remains in effect for central/eastern New Mexico,
northern Texas, most of Oklahoma and southern Kansas. The inherited
Slight Risk area saw only minor adjustments to reflect the latest
trends and WPC QPF and spans from northern Texas/panhandle to
western Oklahoma.

...Coastal Oregon and northwest California...

A fairly progressive, weak/moderate atmospheric river will be
ongoing across the region. Total rain of 2-4" (through 12z Mon)
and hourly rainfall rates possibly approaching 0.5" within a narrow
southward shifting axis suggest that some minor flooding issues
could arise along coastal areas/mountains. A Marginal Risk area
remains in effect along the coastline.

Churchill/Campbell


Day 2
Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GULF COAST AND THE PLAINS/UPPER MIDWEST...

...Central U.S. ...

The precipitation shield will shift eastward as the cold front
advances further into the Plains. Moderate to locally heavy
rainfall is expected from northern Texas to southern Minnesota with
areal average of 1 to 3 inches. The higher amounts will likely
concentrate over eastern Kansas, eastern Nebraska and western Iowa.
A Marginal Risk area was maintained once again with only minor
adjustments for this issuance (not enough confidence for 2"+ totals
to introduce a Slight Risk).

...Gulf Coast...

A plume of enhanced tropical moisture will likely advect northward
through the Gulf of Mexico well in advance of tropical cyclone
Sara; which will increase the potential for heavy rain and
localized flooding concerns along portions of the Gulf Coast. A
Marginal Risk area is in effect from southeast Texas to southern
Mississippi (changed very little from inherited with 3" localized
totals possible).

Churchill/Campbell


Day 3
Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AND CENTRAL GULF COAST...

...Central Gulf Coast and Southeast...

By Tuesday significantly more tropical moisture will likely advect
northward through the Gulf of Mexico in association with the
remnants of Tropical Storm Sara; which will increase the potential
for heavy rain and localized flooding concerns along portions of
the Gulf Coast. An inherited Slight Risk area was maintained for
southern AL and surrounding portions of FL/GA/MS/LA, given the
potential for localized 3-5" totals (per ensemble guidance and WPC
QPF).

...Coastal Washington, Oregon, and northwest California...

Another weak/moderate atmospheric river is expected for region for
Day 3. Total rain of 2-4" (similar to Day 1) are expected with
hourly rainfall rates possibly approaching 0.5" once again suggest
that some minor flooding issues could arise along coastal
areas/mountains. An inherited Marginal Risk area remains in effect
along the coastline.

Churchill
$$
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FOUS30 KWBC 180756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Mon Nov 18 2024

Day 1
Valid 12Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST
LOUISIANA INTO FAR SOUTHWEST MISSISSIPPI...


An axis of much above average tropical PW values expected to be
drawn rapidly northeastward from the remnants of Sara, across the
central Gulf of Mexico and into the Central Gulf coast ahead of
the negatively tilted closed low ejecting northeastward from the
Southern Plains into the Upper Mississippi Valley day 1. These
anomalous tropical PW values, 3 to 4+ standard deviations above the
mean will enhance convection along and ahead of the cold front
pushing eastward across the Gulf coast, primarily during the second
half of day 1 from southeast Louisiana into southern Mississippi.
A slight risk area was maintained where the latest HREF
neighborhood probabilities are highest for 2 and 3"+ amounts. The
simulated hi res radars are in good agreement in showing potential
for a period of training of cells in this anomalous PW axis in the
0000-1200 UTC Tuesday time period across the slight risk area of
southeast LA into southern MS. In areas of training, hourly
rainfall amounts of 1-2"+ possible, resulting in flash flooding,
especially in the urbanized areas of New Orleans.

Across the Southern to Central Plains toward the Upper MS Valley...there
is fairly good agreement on an axis of heavy rain from northern
OK, across much of KS, eastern NE, northwest MO, southeast SD, much
of IA, southern MN into southwest WI in a region of strong
isentropic lift ahead of the deepening surface low moving quickly
northeastward across these areas. While widespread heavy amounts
are likely, hourly rates will are mostly depicted as .25-50"+ in
the latest hi res guidance. This should keep the threat of any
runoff issues at marginal. The previous marginal risk area was
trimmed on the eastern end, taking out much of IA, MO and eastern
OK. The current marginal risk coincides well with the axis of the
highest HREF 2"+ day 1 probabilities.

Oravec

Day 2
Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST
LOUISIANA, ACROSS FAR SOUTHERN MISSISSIPPI, FAR SOUTHERN ALABAMA
INTO THE WESTERN FLORIDA PANHANDLE...

The convection that is expected to enhance along the Central Gulf
coast late day 1, will continue into day 2 as the surface front
continues to push east along the Gulf Coast. The tropical PW
values entrained ahead of this front day 1, will continue to push
across central to eastern Gulf coast day 2. Training of cells in a
southwest to northeasterly direction likely to continue Tuesday
morning from southeast LA into far southern MS, far southern AL and
the western FL Panhandle. There is some timing differences as to
whether heavy rains will continue in the vicinity of New Orleans
early Tuesday, or have pressed just to the east. The New Orleans
metro area was kept in the slight risk given potential for an
overlap of day 1 and day 2 heavy rain areas. The latest HREF 12
hour neighborhood probabilities ending 00Z Wed show the highest
probabilities for 2 and 3"+ amounts remaining across far southeast
LA, far southern MS, far southern AL and the western FL Panhandle.
This was the region depicted in a slight risk for day 2, with the
previous risk area trimmed on the northern end across south central
AL after collaboration with WFO MOB. The slight risk area
corresponds with the hi res consensus for the greatest hourly
totals of 1-2"+ in regions of training.

...Coastal Washington, Oregon and far northwest California...

Models are in good agreement with the rapid cyclogenesis event
across the northeast Pacific off the Pacific Northwest coast. The
low level southerly flow along and ahead of the associated cold
front will become very anomalous Tuesday night into early
Wednesday, with 850-700 mb moisture flux anomalies of 3-4 standard
deviations above the mean impacting the Pac NW/far northwest CA
coast. A well defined area of frontal precip likely to push inland
across coastal Pac NW/far northwest CA after 0000 UTC. Hourly
rainfall rates in the .25-50"+ range possible, especially across
southwest Oregon into far northwest CA where the strong southerly
low level flow will have a better upslope component. For areas of
northwest California into far southwest Oregon, this will be the
beginning of a multi day Atmospheric River event. No changes made
to the marginal risk area from the previous issuance.

Oravec


Day 3
Valid 12Z Wed Nov 20 2024 - 12Z Thu Nov 21 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS FAR
Southwest OREGON INTO NORTHWEST CALIFORNIA...

The multi day atmospheric river event the began late day 2 for far
southwest Oregon into northwest California will continue day 3. The
rapidly deepening day 2 low over the northeast Pacific will anchor
a nearly stationary upper vortex off the Pac NW coast day 3. Strong
persistent deep layered west southwest flow on the south side of
this vortex will impact northwest California into far southwest
Oregon day 3. with IVT values of 500-800 km/m-1 s-2. and 850-700 mb
moisture flux anomalies of 2-4 standard deviations above the mean.
There is fairly good model agreement with heavy precip totals of
3-5" primarily over northwest California, with isolated totals of
6-8" possible. No significant changes made to the previous slight risk area.

Oravec

$$
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FOUS30 KWBC 190828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Tue Nov 19 2024

Day 1
Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL GULF COAST AND FOR PORTIONS OF NORTHWEST CALIFORNIA AND
SOUTHWEST OREGON...

...Central Gulf Coast...

Latest satellite imagery is showing enhancing convection to the
south of southeast LA associated with the anomalous tropical
moisture surge ahead of the cold front moving east across the
Central Gulf Coast. Trends in the latest hi res guidance suggest
this offshore enhancing convection will become the heaviest
frontal/pre-frontal rains as organized convection currently along
the front farther to the north begin to weaken after 1200 UTC. Hi
res guidance is in fairly good agreement that the heaviest rains
with the offshore enhancing convection will remain just to the
south of far southern MS, far southern AL and the western FL
Panhandle, but still affecting far southeast LA. HREF neighborhood probabilities show that the axis of the highest probabilities for 2
and 3"+ amounts will remain offshore with the northern edge of
these higher probabilities affecting areas from far southeast LA
and along the immediate far southern MS, far southern AL and
western FL Panhandle. Changes to the previous outlook were to trim
the northern extent of the slight risk area by approximately 20 nm
and to trim the western extent of the slight risk by
approximately 40nm to account for the hi res trends. At the
moment, the consensus on timing of convection supports removing the
New Orleans metro area from the slight risk, with the heaviest
rains likely to the east and southeast prior to 1200 UTC Tuesday.

...Coastal Washington, Oregon and far northwest California...

Latest models continue to be in good agreement on the rapid
cyclogenesis event across the northeast Pacific off the Pacific
Northwest coast, signaling the beginning of a multi day Atmospheric
River event for northwest CA into far southwest OR. Anomalous low
level southerly flow expected along and ahead of the associated
cold front Tuesday night into early Wednesday, with 850-700 mb
moisture flux anomalies of 3-4 standard deviations above the mean
impacting the coastal Pac NW, south into far northwest CA coast. A
well defined area of frontal precip likely to push inland across
coastal Pac NW/far northwest CA after 0000 UTC Wednesday. The
strong southerly low level flow will be mostly parallel to the
terrain along the Pac NW coast, limiting hourly rainfall rates to
.10-.25"+. Greater hourly rates likely across far southwest Oregon
into coastal NW CA where a more significant upslope component to
the low level flow is likely. Hi res guidance is fairly similar in
showing hourly rates of .25-.50"+ in the 0600 to 1200 UTC Wednesday
period across far southwest OR into far NW CA. HREF .50"/hr
probabilities subsequently are high across this area, with 1"/hr
probabilities much lower and primarily offshore. No changes made to
the previous slight risk area, which corresponds well to the above
mentioned axis of high .50"+/hr HREF neighborhood probabilities.

Oravec


Day 2
Valid 12Z Wed Nov 20 2024 - 12Z Thu Nov 21 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA...

The multi day atmospheric river event the began late day 1 for far
southwest Oregon into northwest California will continue day 2,
primarily across northwest California. The rapidly deepening low
over the northeast Pacific day 1 will anchor a nearly stationary
upper vortex off the Pac NW coast day 2. Strong persistent deep
layered west southwest flow on the south side of this vortex will
impact northwest California day 2, with IVT values of 500-800
km/m-1 s-2. and 850-700 mb moisture flux anomalies of 2-4+
standard deviations above the mean. There is fairly good model
agreement with heavy precip totals of 3-5" primarily over northwest
California, with isolated totals of 6-8" possible. The previous
moderate risk area was adjusted slightly southward by approximately
10-40 nm to account for model max axis consensus. Hourly rainfall
amounts of .25-.50"+ likely through the day 2 time period across
northwest CA with increasing flooding risks with time as soils
become saturated and streams and river levels rise. The HREF 1 hour probabilities for .50"+ totals for the first half of day 2 when
the data is available, remain high across northwest CA.

Oravec

Day 3
Valid 12Z Thu Nov 21 2024 - 12Z Fri Nov 221 2024

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...

The deep northeast Pacific Vortex remains nearly stationary day 3
off the Pacific Northwest coast, supporting the continuation of the
multi day Atmospheric River event for northwest CA into far
southwest OR. Anomalous deep layered southwesterly flow will
continue on the south side of this vortex. 850 to 700 mb moisture
flux anomalies will continue to be 2 to 4+ standard deviations
above the mean with IVT values also maintained in the 500 to 800 km/m-1
s-2 range across northwest CA into far southwest OR. There is a
likelihood of very heavy rains again falling day 3 over similar
regions from the day 2 period. Hourly rates of .25-.50"+ likely to
continue at times supporting additional 24 hour rainfall amounts
of 3 to 6 inches and isolated additional 24 hour totals of 6-8"
across northwest CA into southwest OR. Storm total amounts of 12 to
16+ inches possible across portions of northwest California. Given
this day 2-3 heavy rain overlap potential, a small high risk area
was introduced over northwest CA, across the west central portions
of the EKA's CWA. Flood risks will continue to increase day 3 along
with rock and land slides as soils become saturated and streams
and rivers continue to rise and overflow. There will be a
northward shift back into far southwest Oregon day 3 for the heavy
rain potential as the upper flow back and heights rise. The rising
heights will also keep much of the day 3 precip as rain, increasing
overall runoff.


Oravec
$$
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FOUS30 KWBC 200809
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
309 AM EST Wed Nov 20 2024

Day 1
Valid 12Z Wed Nov 20 2024 - 12Z Thu Nov 21 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA...

The latest model guidance remains consistent with the overall
evolution of the strong multi day atmospheric river event into
northwest California. The low that deepened rapidly over the
northeast Pacific Tuesday, will continue to anchor deep layered
west southwest flow on the south side of this system, impacting
northwest California day 1 with IVT values of 500-800 km/m-1 s-2.
and 850-700 mb moisture flux anomalies of 2-4+ standard deviations
above the mean. There is fairly good model agreement with heavy
precip totals of 3-5" primarily over northwest California and the
northern Sierra, with isolated totals of 6-8" possible. HREF
neighborhood probabilities are very high for 5"+ amounts across
northern California, with most areas showing 80-90%+, while 8"+
probabilities are 40-80% along the northwest CA coast and 40-60%+
over the northern Sierra. Hourly rainfall amounts of .25-.50"+
likely through the day 1 time period across northwest CA with
increasing flooding risks with time as soils become saturated and
streams and river levels rise. The HREF 1 hour probabilities for
.50"+ totals remain high in the max inflow band across northwest
CA for all of day 1. This band does continue to sink south for the
first few hours of day 1, but then become nearly stationary for the
remainder of the day 1 time period. No significant change to the
previous outlook. The slight risk was extended approximately 75
miles farther south in the lee of the northern Sierra where 2-4"
rainfall totals are possible.

Oravec


Day 2
Valid 12Z Thu Nov 21 2024 - 12Z Fri Nov 22 2024

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...

The strong Atmospheric river event into northwest California will
continue into day 2 as deep layered southwesterly flow continues
to the south of the northeast Pacific upper vortex. The anomalous
deep layered southwesterly flow will continue to support 850 to 700
mb moisture flux anomalies of 2 to 4+ standard deviations above
the mean and IVT values in the 500 to 800 km/m-1 s-2 range across
northwest CA. During the second half of day 2, a strong vort
rotating around the base of the northeast Pacific vortex will
support another rapidly deepening low moving northeast
approximately 200 miles off the Pacific Northwest coast. While this
second rapid cyclogenesis event will not be as strong as the
Tuesday event, it will help to begin to push the best onshore flow
axis back to the north during the second half of day 2, returning
heavy precip into southwest Oregon. There is a likelihood of very
heavy rains again falling day 2 over similar regions from the day 1
period. Hourly rates of .25-.50"+ likely to continue at times
supporting additional 24 hour rainfall amounts of 3 to 6 inches and
isolated additional 24 hour totals of 6-8" across northwest CA
into southwest OR. Storm total amounts of 12 to 16+ inches possible
across portions of northwest California. Given the 2 days of very
heavy totals in the same region, no significant changes were made
to the previous high risk area. Significant flood risks will
continue to increase day 2 along with rock and land slides as soils
become saturated and streams and rivers continue to rise and
overflow. Ahead of the next deepening low, rising heights over
northern California will keep much of the precipitation in the form
of rain. This will increase the runoff threats for areas in the
lee of the northern Sierra. Across this region, the risk area was
increased from slight to moderate from the previous issuance.


Day 3
Valid 12Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA...

The multi day atmospheric rive event will begin to weaken on day 3
as the onshore southwesterly flow weakens and the best onshore flow
axis shifts southward. The heaviest totals day 3 likely into the
upslope of the northern Sierra where high heights will again favor
a majority of the precip falling as rain. A moderate risk was added
for this region where additional totals day 3 of 2-4 inches
possible across areas that received 5-8+ inches days 1 and 2. Along
the northwest California coast, additional heavy preip totals
likely day 3, but amounts will be much less than the day 1 and day
2 amounts, generally 1 to 1.50"+, with localized 2"+ totals. A
slight risk was maintained here given the effects from the day 1
and 2 amounts, which will support additional runoff issues for
lesser than normal amounts to produce runoff concerns.

Oravec
$$
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FOUS30 KWBC 220747
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
247 AM EST Fri Nov 22 2024

Day 1
Valid 12Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHERN CALIFORNIA...

The long duration, multi-day atmospheric river event that has been
impacting northern California will finally begin to become more
progressive and start to wane in intensity as a cold front just
offshore of the West Coast advances inland and drives the axis of
deeper layer Pacific moisture transport south and eastward.

However, there will still be one more day of locally very heavy
rainfall totals which are expected to be excessive and capable of
driving locally significant flooding impacts. The 00Z HREF guidance
supports some additional 5 to 10 inch rainfall totals for the
24-hour period ending 12Z/Saturday, with the heaviest totals
expected to be over the windward slopes of the northern Sierra
Nevada, and perhaps locally a few of the coastal ranges just north
of the Bay Area where there has already been very heavy rainfall
totals over the last couple of days.

IVT magnitudes along and just south of the cold front will be rather
high at the start of the period this morning, with magnitudes over
coastal areas of northwest California upwards of 750+ kg/m/s, and
these values should decrease slowly and settle southward toward
00Z. However, as the front gets into the Bay Area and the Central
Valley by early this evening, there is some suggestion in the
guidance that the front will slow down again just a bit. Coinciding
with that will be arrival of a shortwave and associated upper-
level jet streak (130+ kts) rounding the base of the deep offshore
trough. This should foster a temporary resurgence/strengthening of
the IVT values in the 00Z to 06Z time frame across the Bay Area and
extending inland across portions of the Central Valley and
foothills of the Sierra Nevada. Thus expectations are that heavy
rains will overspread all of these areas along with some localized
increase in rainfall rates this evening. In general, the rainfall
rates will be capable of reaching 0.50" to 0.75"/hour across the
coastal ranges around the Bay Area and also into the upslope areas
of the northern Sierra Nevada. Given the additional rainfall
amounts, and considering the antecedent conditions at this point,
the Moderate Risk area is maintained across the northern Sierra
Nevada, with a separate Moderate Risk introduced just north of the
Bay Area involving Sonoma and Napa Counties. Other adjustments
include extending the Marginal and Slight Risk areas a bit farther
south to include the coastal ranges a bit south of the Bay Area and
also more areas of the Central Valley. Flooding impacts will again
be rather widespread and locally significant, with additional
concerns for debris flows, rock and landslide activity, and flash
flooding involving burn scar locations. In fact, the Park Fire burn
area involving parts of Tehama and Butte Counties will need to be
very closely monitored today for enhanced impacts given the amount
of rainfall that is forecast here.

Orrison


Day 2
Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SIERRA NEVADA...

At 12z Saturday the cold front and associated moisture plume will
be dropping south across the central Sierra Nevada. By this time
rainfall intensity will be decreasing, and the axis of better
moisture will be dropping south of the areas hardest hit over the
past few days. Thus the flood risk should be pretty low Saturday
morning along this southward dropping front.

Some weak post frontal instability is forecast which could allow for
locally heavy showers in the wake of the front over the western
slopes of the central Sierra Nevada. Not expecting anything too
intense or widespread, however there is at least a chance that a few
cells could drop a quick 0.5" or so of rain. This would be falling
over increasingly saturated ground, and so a localized flood risk is
possible. Thus we will maintain the small Marginal risk where a few
heavier post frontal showers are possible Saturday.

Chenard


Day 3
Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHWEST CALIFORNIA INTO FAR SOUTHWEST OREGON...

Locally heavy rain will be possible Sunday into Sunday night over
northwest CA into far southwest OR. A front slowing near the coast
will provide plentiful low level moisture convergence, and mid level
shortwave energy should provide some additional lift as well.
Moisture is a limiting factor for heavy rainfall with the better
moisture plume situated well south and not really getting drawn into
this system. Thus this is not expected to be a significant
atmospheric river, and normally would not expect a system of this
nature to pose a flood risk. However given the significant rainfall
over the past few days, this additional rainfall may be enough to
cause some localized additional flooding as the area will be more hydrologically sensitive than normal. Still some model differences
in the exact evolution of this system and thus rainfall amounts this
period are uncertain...but current indications suggest localized
amounts of 2-3" will be possible.

Chenard

$$
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FOUS30 KWBC 230748
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
248 AM EST Sat Nov 23 2024

Day 1
Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SIERRA NEVADA...

A cold front and the remnants of the long duration, multi-day
atmospheric river that has been impacting the West Coast will be
dropping south across the central Sierra Nevada early this morning.
As the deeper layer moisture flux parameters/IVT magnitudes
continue to weaken and thin out, the rainfall rates will be
continuing to drop off. However, there still may be a sufficient
level of upslope flow/orographic ascent in the foothills of the
central Sierra Nevada for a few more hours early this morning of
spotty 0.25" to 0.50"/hour rainfall rates. Some weak post-frontal
instability is also still forecast which could allow for some
scattered pockets of heavy shower activity going through midday to
impact some of the western slopes of the central Sierra Nevada.
This could result in some isolated quick- hitting 0.25" to 0.50"
rainfall amounts, but these convective cells are forecast to be
progressive. Overall, the additional storm total amounts based off
the 00Z HREF guidance may reach as high as 1 to 2 inches.

Given the very wet antecedent conditions, the additional rains may
foster some additional localized runoff/flooding concerns. Thus,
the Marginal Risk area over portions of the Sierra Nevada will be
maintained.

Orrison


Day 2
Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHWEST CALIFORNIA INTO FAR SOUTHWEST OREGON...

Locally heavy rain will be possible Sunday into Sunday night over
northwest CA into far southwest OR. A front slowing near the coast
and a weak low to mid level wave riding along it will support a
modest uptick in IVT and plentiful low level convergence. Moisture
is a limiting factor for heavy rainfall with the better moisture
plume situated well south and not really getting drawn into this
system. Thus this is not expected to be a significant atmospheric
river, and normally would not expect a system of this nature to
pose a flood risk. However given the significant rainfall over the
past few days, this additional rainfall may be enough to cause some
localized additional flooding as the area will be more
hydrologically sensitive than normal.

We are still seeing some spread in model QPF output over this
period. If some of the more aggressive solutions pan out then we
could see 1-3" of rainfall is spots with hourly totals peaking
around 0.3-0.4" in the typically more favored upslope areas.

Chenard


Day 3
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHERN CALIFORNIA INTO FAR SOUTHWEST OREGON...

Showery conditions will likely continue across portions of northern
CA and southwest OR Monday into Monday night as a deep layered low
and shortwave energy moves inland. Certainly not a classic setup
for flooding with limited moisture and dropping IVT
values...however this area remains more hydrologically sensitive
than normal following the significant atmospheric river event the
past few days. Additional rainfall over far northwest CA into
southwest OR is generally 1-3" in the guidance, with isolated
totals of this magnitude also possible over the western slopes of
the northern Sierra Nevada.

A more significant plume of moisture/IVT will actually move into
portions of southern/central CA...generally from portions of San
Luis Obispo and Monterey counties into the western slopes of the
southern Sierra Nevada. QPF has been trending up with this system,
although still seeing some model spread. The latest model consensus
suggests a possible 2-3" of rain along the coastal ranges and the
southern Sierra below snow levels. Not seeing any instability with
this system and large scale forcing is only modest, so hourly
rainfall rates are generally not expected to exceed 0.5". Given
this, and the fact that most of this area has not seen significant
rainfall recently, will continue with no areas in the excessive
rainfall outlook for now.

Chenard

$$
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FOUS30 KWBC 240816
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
316 AM EST Sun Nov 24 2024

Day 1
Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHERN CALIFORNIA INTO FAR SOUTHWEST OREGON...

Locally heavy rain will be possible today and especially tonight
over portions of northern CA into far southwest OR as an offshore
shortwave and attendant wave of low pressure arrives near the
coastal ranges. Generally the IVT magnitudes out ahead of this
surface wave will be modest with values increasing to as high as
250 to 400 kg/m/s. However, with a front nearby and favorable
upslope flow/orographic ascent involving the coastal ranges, there
will likely be some concentrations of heavier shower activity with
rainfall rates capable of occasionally reaching into the 0.25" to
0.50"/hour range. Some pockets of heavier rain may also make it
into the far northern Sacramento Valley and adjacent foothills of
the northern Sierra Nevada and Shasta/Siskiyou ranges. The 00Z HREF
guidance supports some spotty 1 to 3 inch rainfall amounts going
through early Monday morning, and thus this is not expected to be a particularly heavy event. However, given the significant rainfall
and lingering hydrological impacts from the most recent atmospheric
river, these additional rains may result in at least some
localized runoff problems and flooding.

Orrison


Day 2
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF CALIFORNIA AND SOUTHWEST OREGON...

Showery conditions will likely continue across portions of
northern CA and southwest OR Monday into Monday night as a deep
layered low and shortwave energy moves inland. Certainly not a
classic setup for flooding with limited moisture and dropping IVT
values... however this area remains more hydrologically sensitive
than normal following the significant atmospheric river event the
past few days. Additional rainfall over far northwest CA into
southwest OR is upwards of 1-3" in the guidance, with isolated
totals of this magnitude also possible over the western slopes of
the northern Sierra Nevada.

A more significant plume of moisture/IVT will move into portions of southern/central CA...generally from portions of San Luis Obispo and
Monterey counties into the western slopes of the southern/central
Sierra Nevada. IVT values are forecast to be around 500-600 kg/ms,
which at this latitude is starting to approach climatological max
values for late November. PWs are also forecast to increase to
around the 99th percentile for late November, and thus expecting
plenty of moisture and upslope flow to allow for decent rainfall
totals. Not expecting any instability, so rainfall intensity will be
solely driven by the aforementioned moisture transport and upslope enhancement...and these factors alone are unlikely to support
rainfall rates high enough for true flash flooding. However we will
have an extended period of time with elevated IVT values, and so a
prolonged steady rainfall could eventually result in some localized
minor flood concerns. Overall expecting 1-3" of rain over the
coastal ranges in San Luis Obispo and Monterey counties (with
localized totals towards 4" possible)...with totals ranging from
near 1" in the eastern valley locations to 2-4" over the western
Sierra below snow levels.

Chenard


Day 3
Valid 12Z Tue Nov 26 2024 - 12Z Wed Nov 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL AND SOUTHERN SIERRA NEVADA...

The atmospheric river event described in the day 2 discussion
continues into Tuesday over portions of the central and southern
Sierra Nevada. There is a bit more model spread by this time with
regards to the persistence of the IVT and strength of the mid/upper
forcing. A majority of the 00z deterministic models have trended a
bit weaker and faster with the shortwave energy pushing across CA
and into NV...and this trend would result in less additional QPF
over central CA on day 3. However the 00z GEFS and recent NBM
probabilities suggest there is still a chance for a slower/stronger
wave resulting in more robust rainfall amounts.

This will be the 2nd consecutive day of steady rainfall for these
areas, and so do expect some increased hydrologic sensitivity that
could result in at least some localized minor flooding issues. When
summing the two days of rainfall together, the latest WPC
deterministic QPF is calling for around 2" in Fresno to Visalia,
increasing to 3-6" in the foothills/western slopes of the southern
and central Sierra Nevada. Given the magnitude of IVT forecast even
even forecasting 1-2" spilling over to the east of the crest towards
Bishop and Independence. These forecast values average around a 2yr
ARI for 48 hr amounts, also supportive of a localized minor flood
threat. As mentioned above, there is both higher and lower end
potential with this forecast rainfall. The higher end scenarios
could even result in Slight risk level impacts, while the lower end
outcomes would be a minimal flood threat. So will need to continue
to monitor trends, and for now still think the Marginal risk is
appropriate.

Chenard

$$
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FOUS30 KWBC 250757
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Mon Nov 25 2024

Day 1
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN SIERRA NEVADA...

Moderate to locally heavy showers are expected to continue across
portions of northern CA and southwest OR today and tonight as a
deep layered low and associated shortwave energy moves inland. The
deep layer cyclonic flow coupled some steepening of the mid-level
lapse rates and orographic ascent/upslope flow over the coastal
ranges should favor some occasional 0.25" to 0.50"/hour rainfall
rates. Some of these heavier showers will also spread inland
across the northern Sierra Nevada. Overall, the additional rainfall
amounts here should be relatively modest, but portions of the
coastal ranges of northwest CA and far southwest OR may see some
1-3" rainfall totals going through early Tuesday morning. Given the wet/saturated soil conditions and high streamflows, these
additional rains may foster some renewed runoff problems and
localized flooding. A Marginal Risk will be maintained for these
areas.

Farther south into the upslope areas of the central and especially
southern Sierra Nevada, and also the coastal ranges of Monterey,
San Luis Obispo and Santa Barbara Counties, a moderately strong
atmospheric river event will be arriving today and continuing
through tonight and early Tuesday. This will be associated with the
arrival of a southern stream shortwave impulse which will be
embedded within the transport of deeper layer subtropical moisture
extending well offshore of southern CA and upstream over the
eastern Pacific. The GFS and ECMWF support increasing IVT
magnitudes especially by 00Z across the coastal ranges and into
higher terrain of the southern Sierra Nevada with magnitudes rising
to 500 to 700+ kg/m/s. The orientation of this will be generally
orthogonal to the terrain, and this should yield enhanced
orographic ascent that will be conducive for heavier rainfall
rates.

The 00Z HREF guidance favors some occasional 0.50"+/hour rainfall
rates with the higher probabilities of this focused over the
foothills of the Sierra Nevada involving Fresno and Tulare
Counties, and a small portion of northern Kern County. A consensus
of the 00Z HREF guidance supports 24-hour rainfall totals
(generally below 9000 feet/snow level) of 3 to 6 inches, with a
couple of models indicating some spotty 6 to 8+ inch amounts.
Meanwhile, over the coastal ranges from Monterey County down
through San Luis Obispo and Santa Barbara Counties, some 1 to 3+
inch rainfall totals are supported in the guidance. One key factor
in general that is supporting these higher rainfall totals, aside
from the upslope flow into the terrain, is the fact that much of
the moisture will be of subtropical origins, and this will yield PW
anomalies as high as 2 to 4 standard deviations above normal for
this time of the year. Given the forecast rainfall amounts and
associated rainfall rates, there may be some flooding concerns that materialize, and this will include at least a localized threat for
some burn scar flash flooding/debris flow impacts. As a result, a
Slight Risk for excessive rainfall has been introduced for portions
of the southern Sierra Nevada, with some localized expansion of
the prior Marginal Risk area to account for rainfall over the
coastal ranges and parts of the San Joaquin Valley.

Orrison


Day 2
Valid 12Z Tue Nov 26 2024 - 12Z Wed Nov 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL AND SOUTHERN SIERRA NEVADA...

Model differences remain in the 25/00Z guidance with respect to
the timing of a shortwave moving inland from the California/Oregon
border with the ECMWF being faster to bring the shortwave trough
across the Sierra Nevada range and into the Great Basin by
27/00Z...although most of the 00Z suite of guidance...including the GFS...trended trended faster and a bit weaker. With the exception
of the Canadian GEM and Regional...the 25/00Z GFS remained heaviest
across the central and southern Sierra Nevada for the 24 hour
period ending Wednesday morning. Still expect some increased
hydrologic sensitivity that could result in at least some localized
minor flooding issues given that this will be the second
consecutive day of steady rainfall for these areas. Even though the
non-25/00Z GFS/Canadian consensus is for 2 to 4 inches across the central/southern Sierra Nevada over the 24 hour period, the amounts
tend to be more in the 1 to 3 inch range below the expected snow
levels (approx 9000 ft per the WPC Winter Weather Desk forecaster)
which may mitigate some of the concern for flooding. Even so...the
concern remains that those values may be too low at least locally
considering the coarser resolution of the available guidance at
longer lead times so an upgrade may still be needed later. Rainfall
rates and areal coverage of rainfall should decrease late in the
period as the flow of moisture becomes disrupted and the better
dynamics pull away from the area.

Bann


Day 3
Valid 12Z Wed Nov 27 2024 - 12Z Thu Nov 28 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann
$$
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FOUS30 KWBC 020710
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
210 AM EST Mon Dec 2 2024

Day 1
Valid 12Z Mon Dec 02 2024 - 12Z Tue Dec 03 2024

Onshore flow across far south Texas will maintain a moist/unstable
airmass that could conditionally support heavy rainfall near the
Brownsville vicinity and adjacent coastal areas especially from the
afternoon onward. This risk is too isolated/conditional for
inclusion of a Marginal risk area at this time but will be
monitored through the day. The probability of rainfall exceeding
flash flood guidance is less than 5 percent.

Cook


Day 2
Valid 12Z Tue Dec 03 2024 - 12Z Wed Dec 04 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Wed Dec 04 2024 - 12Z Thu Dec 05 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

The Marginal Risk area for portions of eastern Texas and much of
western Louisiana that was inherited was removed with this update.
A fairly robust 30-40 kt LLJ will advect Gulf moisture into much of
the Mississippi Delta region and west into eastern Texas on
Wednesday. PWATs may exceed 1.75 inches, especially along the Gulf
coast. However, this is the only ingredient that is sufficient for
flooding rains. Instability, while non-zero...will be meager,
struggling in most areas to get to 500 J/kg of MUCAPE. This will be
the primary limiting factor against flooding, as a lack of
instability should keep most of the rain as stratiform. Secondly,
much of the eastern half of the U.S., including eastern Texas and
Louisiana have been very dry lately, so stream and creekbeds are
running low. The drawn out period of mostly light rain in this
area will therefore mostly be beneficial. While embedded convective
cells could briefly cause locally heavy rain, the dry conditions
and high FFGs should mitigate any flooding. Finally, at the tail
end of the event late Wednesday night into Thursday, cold frontal
passage will push any heavy rain south and out of where the
heaviest rains are expected over central Louisiana. This too should
diminish the flooding potential.

As always, increases in the guidance for rainfall amounts in the
next couple days or greater instability could require a reissuance
of the Marginal, but for now the rainfall in the guidance likely
falls in the sub-Marginal 0-5% chance category area-wide.

Wegman

$$
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FOUS30 KWBC 030729
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
229 AM EST Tue Dec 3 2024

Day 1
Valid 12Z Tue Dec 03 2024 - 12Z Wed Dec 04 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTH TEXAS...

A moist airmass (characterized by 1.4+ inch PW values) will migrate
slowly northwestward through the day along a surface trough located
near the south Texas coast. Convergence along that trough axis
will support areas of deep convection through the day, with
kinematic profiles supporting slow storm motions even as mid-level
flow weakens during the forecast period. Isolated spots of 2-4 inch
rainfall amounts appear possible per latest CAMS in the Marginal
Risk area. Isolated flash flood potential will exist in this
regime.

Cook


Day 2
Valid 12Z Wed Dec 04 2024 - 12Z Thu Dec 05 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Thu Dec 05 2024 - 12Z Fri Dec 06 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
$$
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FOUS30 KWBC 040721
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
221 AM EST Wed Dec 4 2024

Day 1
Valid 12Z Wed Dec 04 2024 - 12Z Thu Dec 05 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent. Strengthening warm/moist advection across east Texas
and Louisiana will result in widespread shower and thunderstorm
activity most of the day through early Thursday. Several areas of
2-4 inch rainfall totals are expected especially from southeast
Texas into central Louisiana. The rain will fall in areas that have
not experienced appreciable rainfall over at least the past week,
and models are inconsistent with depiction of any focused corridors
of heavier and/or training convection that could result in larger-
scale flash flood concerns. FFG thresholds are also relatively
high. <5% probabilities are maintained for this outlook with the
expectation that any flash flood concerns should be isolated at
best.

Cook


Day 2
Valid 12Z Thu Dec 05 2024 - 12Z Fri Dec 06 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Fri Dec 06 2024 - 12Z Sat Dec 07 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
$$
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FOUS30 KWBC 050738
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
238 AM EST Thu Dec 5 2024

Day 1
Valid 12Z Thu Dec 05 2024 - 12Z Fri Dec 06 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Otto


Day 2
Valid 12Z Fri Dec 06 2024 - 12Z Sat Dec 07 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Sat Dec 07 2024 - 12Z Sun Dec 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF
EASTERN TEXAS...

A low-level jet streaming Gulf moisture northward across eastern
Texas will interact with a strong shortwave trough which will move northeastward out of New Mexico and into the central Plains. PWATs
along the upper Texas coast are expected to increase to above 1.75
inches by Saturday night. While shower and light rain activity is
expected especially over western portions of the Marginal Risk area
during the day Saturday, the heaviest rain will be on Saturday
night due to the typical nocturnal strengthening of the LLJ.
Instability will be the primary limiting factor as MUCAPE values
struggle to even get above a couple hundred J/kg. Portions of the
Marginal Risk area will get some rain Friday and Friday night, such
that this much stronger second round may train over some areas
that due to previous rain may have more saturated soils. Isolated
flash flooding is possible, especially in urban and flood prone and
low lying areas.

Wegman

$$
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FOUS30 KWBC 061533
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1033 AM EST Fri Dec 6 2024

Day 1
Valid 12Z Fri Dec 06 2024 - 12Z Sat Dec 07 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...Far Deep South Texas...
The slow sagging frontal zone is nearly flat west to east across
the Northern Gulf of Mexico and a weak surface to boundary layer
cyclone has developed along the upstream edge near the southern tip
of Texas. Enhanced deep layer moisture resides along and north of
the boundary across this section of the front with total PWat
values ranging from 1.5-1.75". Warm sea surface and proximity to
the front allow for some weak instability in proximity to that low
with values of 750-1000 J/kg available along/offshore. So there
will remain some solid potential for thunderstorm activity across
far south Texas. However, the vast majority of guidance continues
to suggest best overall convergence and strongest updrafts will
remain offshore or at the near coast. It is possible some weakening
showers or rouge thunderstorm may make an appearance onshore in
proximity to the urban corridor of the lower Rio Grande Valley,
making the risk for flash flooding non-zero, but not rising to high
enough probability/coverage to reach 5% threshold for delineating
a Marginal Risk at this time.

Gallina


Day 2
Valid 12Z Sat Dec 07 2024 - 12Z Sun Dec 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN TEXAS AND WESTERN LOUISIANA...

A low-level jet streaming Gulf moisture northward across eastern
Texas will interact with a strong shortwave trough which will move northeastward out of New Mexico and into the central Plains. PWATs
along the Upper Texas coast are expected to increase to 1.5"+ and
instability should remain minimal (and mostly confined to the
coast). The heaviest rain is forecast to be on Saturday night due
to the typical nocturnal strengthening of the low-level jet. Soils
have become a bit more saturated in this region due to the recent
rainfall, and some rainfall is expected on Friday. Isolated flash
flooding is possible, especially in areas of more saturated soils,
urban areas, and flood prone/low lying areas.

Churchill/Roth


Day 3
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST...

Continued moisture flux transport via a moderately strong low-
level jet is expected to result in increasing coverage of flash
flooding on Sunday, as the bulk of the forcing and jet dynamics
via the aforementioned strong shortwave trough will not arrive
until midday (along with maximized instability). Localized training
is expected to result in 2-4" totals, suggesting a lower-end
Slight Risk based on the probability spectrum (which ranges from
15-40%). Given that most of the region is experiencing very dry
antecedent soil conditions (10th percentile or lower for much of
MS/AR/LA, per NASA SPoRT-LIS 0-100 cm soil moisture), any flooding
should be relegated to flood-prone areas (with the greatest risk
for any localized significant flash flooding over west-central LA,
where antecedent soil conditions and streamflows are locally higher).


Churchill
$$

d
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FOUS30 KWBC 070820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Sat Dec 7 2024

Day 1
Valid 12Z Sat Dec 07 2024 - 12Z Sun Dec 08 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 2
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

The Slight Risk has been maintained along and near the central
Gulf Coast for Sunday and Sunday night, confined to where more
appreciable deep-layer instability exists to support a higher
probability of more intense short-term rainfall rates (2"+/hr).
Still, this is considered a lower-end Slight Risk (based on the
wide 15-40% probability spectrum), suggesting widely scattered
instances of flash flooding at best (given expected localized
totals of 2-4" are near the corresponding 3-hr FFGs of 3-4").
Also, much of the Slight Risk area, particularly from Jasper TX
east-northeast to Alexandria LA, have received heavy rainfall over
recent days, and as a result the antecedent soil conditions and
streamflows are a bit higher compared to areas farther north (where
NASA SPoRT-LIS 0-100 cm soil moisture in the bottom 10th percentile
for much of MS/AR/LA). In addition to drier antecedent conditions
farther north and east across the Mid-South, very limited elevated
instability (MU CAPE ~200 J/kg at best) should prevent rainfall
rates from climbing too high (generally 1.5"/hr or less, driven by
impressive forcing via the shortwave and associated jet dynamics).
This suggests that a Marginal Risk should suffice with any flash
flooding being highly localized, despite areal average rainfall
amounts near 2" (expected to be spread out over 6+ hours).

Churchill/Hurley


Day 3
Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL GULF COAST...

The shortwave trough responsible for heavy rainfall on Sunday over
much of the Lower MS Valley looks to eject northeastward into the
central Appalachians and Mid-Atlantic on Monday, bringing largely
beneficial rainfall (with totals generally under 1") to much of the
Northeast. Farther south along the central Gulf Coast, there are
hints of another (weaker) shortwave trough within the southern
stream that may enhance heavy rainfall along a trailing cold front.
Given the slowing of the front, the opportunity for training areas
of heavy rainfall will be greater on Day 3, best depicted by the
00z ECMWF and ECENS (depicting localized banded totals of 2-5" and probabilities for 2" exceedance as high as 30%). This risk appears
greatest from southeast LA to southern AL and the tip of the FL
Panhandle, though the EC model system is the most extreme and
farthest east of all the models. Maintained the inherited Slight
Risk area with some adjustments, limiting the eastern extent a bit
given the lack of agreement from the other models. Uncertainty is
still relatively high, but subsequent outlooks should get a better
handle on things as these details begin to enter the CAM period.

Churchill
$$
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FOUS30 KWBC 080805
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
305 AM EST Sun Dec 8 2024

Day 1
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
LOWER MISSISSIPPI VALLEY...

The inherited Marginal Risk area was maintained for this update,
suggesting that any flash flooding should remain localized today,
as rainfall rates will generally remain capped below 1"/hr (with
lacking instability being the primary limiting factor). The
heaviest rains for the period are expected in the middle of the
Marginal Risk area over northern Mississippi, where an interaction
with a front in that area will lead to greater forcing than areas
farther south along the Gulf Coast. Where the front adds additional
forcing, localized convective bursts may be embedded within the
broader rainfall shield.

Hydrologically, most if not all of the soils in the are are at or
below average for moisture. Low/dry creek beds should easily
handle most of the light to briefly moderate rainfall rates. Any
isolated flooding will be in any urban areas, as well as other low-
lying or more flood-prone areas.

Churchill/Wegman


Day 2
Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN LOUISIANA AND COASTAL MISSISSIPPI...

The shortwave trough responsible for heavy rainfall on Sunday over
much of the Lower MS Valley looks to eject northeastward into the
central Appalachians and Mid-Atlantic on Monday, bringing largely
beneficial rainfall (with totals generally well under 1") to much
of the Northeast. Farther south along the central Gulf Coast, a
weaker shortwave within the southern stream may locally enhance
heavy rainfall along a trailing cold front. The inherited Slight
Risk in this area was maintained, mainly from the I-10 & I-12
corridors from Baton Rouge east to Biloxi, where the flooding
potential appears greatest in urban and more flood-prone areas
(given relatively dry antecedent conditions and streamflows normal
to below normal). There is still some question as to how far east
the heavy rainfall threat will extend (as the ECMWF and ECENS have
been persistent outliers suggesting an eastward extension into the
FL Panhandle and surrounding southeast AL and southwest GA), but
the signal for localized totals of 2"+ are maximized near the
Slight Risk (and confined to where instability and forcing are
greatest).

Churchill


Day 3
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST AND CENTRAL GULF COAST...

Large scale upper trough will be digging into the central US by
Tuesday, setting the stage for more organized heavy rainfall into
portions of the Southeast. Initially positively tilted, the trough
looks to quickly take on a more neutral tilt over the course of
the course of the day, as an associated 250 mb jet streak
strengthens from ~110 kts to ~170 kts over 24 hours (with our area
of interest for the Slight Risk located near the right-entrance
region). As with prior days, instability once again looks to be the
primary limiting factor, and this is particularly the case farther
north into the Carolinas. While the GFS/GEFS depict 2-4" localized
totals into northern GA and western SC, the ECMWF/ECENS largely
limit this threat (for 2" and 3" exceedance) a bit farther south
into central GA, southeast AL, and the FL Panhandle (where the
inherited Slight Risk was maintained and adjusted a bit based on
the new model data). Given relatively dry antecedent conditions and
associated 3-6 hr FFGs generally ranging from 3.0-4.0", the Slight
is on the lower-end of the 15-40% probability distribution,
suggesting widely scattered instances of flooding (at best).

Churchill

$$
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FOUS30 KWBC 090758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 AM EST Mon Dec 9 2024

Day 1
Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

Overall pattern evolution will continue to point to a benign
instability axis (MUCAPE ~500-1000 J/kg) being confined closer to
the Gulf with sufficient deep layer shear and moisture presence
(PWATs b/w +1 to +2 deviations) allowing for a small area of
convective development capable of locally enhanced rainfall rates
for flash flood potential. Given the overall antecedent dry
conditions prior, the prospects of flash flooding will likely be
relegated to more urban settings along the Central Gulf coast,
including places such as New Orleans, Biloxi, and Mobile. A few
other Parishes within Southeastern LA also contain areas that
experience a bit easier ability to flash flood given some of the
above urbanization factors, so decided to maintain the previous
SLGT risk inheritance with only some minor modifications made to
the overall risk area.

Latest 00z HREF neighborhood probabilities for >3" of rainfall are
within the low-end 20-25% range with highest prospects likely
between 1-2" when taking relevant CAMs into account. This is
similar to the previous forecast prob fields from the 12z HREF
iteration, and with coordination from the local WFOs along the Gulf
Coast earlier for the SLGT risk, there was no need to deviate from
what was proposed in the last update. Best chance will reside from
training echoes on persistent west-southwest flow during the
diurnal instability maximum, carrying into the evening before the
setup decays prior to the next period of relevant convection for
D2.

Kleebauer


Day 2
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN
ALABAMA TO UPSTATE SOUTH CAROLINA...

As the previous shortwave trough kicks to the northeast, a stronger
mid-level trough will dig quickly out of the Rockies into the
Southern Plains by Tuesday morning with sights downstream on areas
east of the Mississippi. Increasing meridional flow ahead of the
digging trough will aid in the advection of deep moisture poleward
with the initial surge propagating north out of the Gulf through
the Southeast by Tuesday afternoon, eventually spreading northward
into the Southern and Central Mid Atlantic the second half of the
period. The advection pattern will be sufficient in increasing
regional mid-level instability as profile moisture saturates deeper
into the profile across the areas above. PWAT anomalies on the
order of 2-3 deviations above normal will be common over parts of
GA spreading up through the neighboring Carolinas, eventually
surging into the Central Mid Atlantic and Northeastern U.S by the
end of the D2 cycle. This will prime several areas for heavy
rainfall prospects as the final ingredients come to together for a
locally enhanced precip pattern.

The trough to the west will begin tilting more neutral with a
strengthening upper jet positioning allowing for large scale ascent
to increase in earnest as we move into Tuesday afternoon and
beyond. The main trigger for a line of enhanced convection will
form at the hands of a strong surface cold front that will shove
eastward beneath an intensifying surface low that develops in
conjunction to the favorable upper level evolution. Current
deterministic sfc to 850mb wind field indicates a corridor of
backed flow across much of GA into the Carolinas during Tuesday
evening ahead of the cold front allowing for widespread rainfall to
develop downstream of the approaching front. As the cold front
swings eastward by late Tuesday night through early Wednesday, the
peak rainfall rates will be occurring across the Carolina Piedmont
down through Central GA, aligning with the cold front as it moves
through the region. The eventual heavy rain footprint will spread
northeastward into the Southern Mid Atlantic with locally heavy
rainfall impacting areas as far north as Virginia with some lighter
precip making headway into areas further north. Places across
Southwestern VA down to the I-40 corridor in NC will see anywhere
from 1-2" of rainfall with locally higher further south through the
zone. 2-3" with local totals to 4" will be plausible for the area
extending from Southeastern AL through Central GA into Upstate SC,
an area more prone to flash flood prospects thanks to the
complexity of the local terrain.

This signal has been prevalent over the past series of
deterministic model outputs and is now firmly within the ensemble
mean as bias corrected ENS now supports 2.5-3" through much of the
area outlined above in the 2-3" forecast. The saving grace in this
setup is the time of year allowing for lower surface based
instability parameters and very dry antecedent conditions leading
into the event. This allowed for a SLGT risk to be sufficient for
the setup, a carry over from the previous forecast. The MRGL risk
still extends into VA where the northern periphery of the heavy
rainfall is expected.

Kleebauer


Day 3
Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND...

As the longwave pattern evolves further, the neutral-tilted trough
from previous D2 time frame will enhance further with the mean
trough taking on a more negative tilt allowing for enhanced surface
low strengthening as the surface low progresses northward into New
England and eventually Quebec. PWAT anomalies on the order of 2-3
deviations above normal will quickly progress northward
encompassing much of New England by Wednesday afternoon, even
getting up towards +4 deviations as we move into Wednesday night
over Northern New England from NH through ME. This pattern is
textbook for a corridor of heavy rainfall to spread from the Mid
Atlantic through all of the Northeast with the primary areas of
interest location along and ahead of the cold front that will sweep
eastward during the forecast period. Portions of Western NY state
and Northern PA will eventually change to winter precipitation
allowing for a decay in flash flood concerns, however areas
downstream will be under a respectable deluge of rainfall prior to
the rain threat ceasing in wake of the front.

Totals of 1.5-2.5" of rain are forecast across much of Long Island
up through Southern New England, eventually moving up through ME
as we close out the forecast period. 1-2" is forecast for points
further west, but those areas also have the added snow melt factors
that will play into the flooding concerns as high dewpoints and
heavy rain will promote rapid snow melt for places in the interior
over VT/NH and the neighboring Berkshires in MA. The expectations
are for some creeks and smaller streams to become problematic in
the setup and could cause localized flood concerns during the peak
of the heavy rainfall. The rivers across New England can thankfully
take a decent surge of moisture after a very dry fall, so the
prospects of significant flooding are very low at this point.

The setup is still worthy of a large SLGT risk encompassing much
of New England with the best threats likely over urbanized areas in
LI and Southern New England and over those zones in the interior
where rapid snow melt has historical precedence for localized
flash flood concerns. A MRGL risk will encompass the SLGT risk
across Northeast with an extension down into Central and Eastern NC
as the early portion of the forecast period will still see
moderate to heavy rainfall Wednesday morning before pulling
northward.

Kleebauer
$$
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FOUS30 KWBC 100756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Tue Dec 10 2024

Day 1
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL ALABAMA
THROUGH PORTIONS OF THE WESTERN CAROLINAS...

The overall synoptic pattern remains on track to deliver a swath of
heavier rainfall to portions of the Southeast and Southern Mid
Atlantic during the period. A strong mid- level trough is currently
analyzed over the Four Corners region, digging quickly to the
southeast with sights on the Southern Plains by later this morning.
As the mean trough pivots across the Southern Plains near OK/TX,
increasing meridional flow ahead of the digging trough will aid in
the advection of deep moisture poleward with the initial surge
propagating north out of the Gulf through the Southeast by the
afternoon, eventually spreading northward into the Southern and
Central Mid Atlantic the second half of the period. The advection
pattern will be sufficient in increasing regional mid- level
instability as moisture saturates deeper into the profile across
the areas above. PWAT anomalies on the order of 2-3 deviations
above normal will be common over parts of GA spreading up through
the neighboring Carolinas, eventually surging into the Central Mid
Atlantic and Northeastern U.S by the end of the D1 cycle. This
will prime several areas for heavy rainfall prospects as the final
ingredients come to together for a locally enhanced precip pattern.

The trough to the west will begin tilting more neutral with a
strengthening upper jet positioning allowing for large scale ascent
to increase in earnest as we move into the afternoon hours and
beyond. The main trigger for a line of enhanced convection will
form at the hands of a strong surface cold front that will shove
eastward beneath an intensifying surface low that develops in
conjunction to the favorable upper level evolution. Current
deterministic sfc to 850mb wind field indicates a corridor of
backed flow across much of GA into the Carolinas during Tuesday
evening ahead of the cold front allowing for widespread rainfall to
develop downstream of the approaching front. As the cold front
swings eastward by late Tuesday night through early Wednesday, the
peak rainfall rates will be occurring across the Carolina Piedmont
down through central GA into portions of east-central AL, aligning
with the cold front as it moves through the region. The eventual
heavy rain footprint will spread northeastward into the Southern
Mid Atlantic with locally heavy rainfall impacting areas as far
north as southwestern Virginia with some lighter precip making
headway into areas further north.

Recent 00z HREF output remains steadfast with the prospects of a
widespread 1-3" of rainfall across a large chunk of the Southeast
into the Highlands of the Western Carolinas with the northern
extension of any maxima peeking into southwestern Virginia where
some of the terrain will make use of the southeasterly upslope
component to the wind field. Probability fields in both the
neighborhood and EAS realm are pretty solid when it comes to the
potential with the latter of which indicating a well-defined axis
of 30-50% for at least 2", a signal prevalent historically for
favored SLGT risks or higher for the current impact areas.
Neighborhood probabilities are of course higher in percentile given
how the prob field is calculated, however the most notable output
is the very high 60-80% coverage for >3" and low-end probs of
20-25% for up to 5" across the area southeast of Birmingham up
through the Atlanta metro. This was a small shift northward in
terms of the heaviest QPF axis from previous forecasts and align
well with the latest ECMWF and GFS ML outputs, a testament to
increasing agreement within the deterministic on where the heaviest
rainfall will occur. The saving grace from this setup being a more
prolific outcome is the lack of sufficient surface based
instability that tends to add a greater convective element to the
rainfall rates that shift the signal to a higher-end risk threat.
Thankfully, that is not the case here, so the threat does remain a
bit capped on the upper bound of potential.

The previous SLGT risk was adjusted a bit further northwest across
AL/GA to reflect the change within the axis of heaviest rainfall
putting the Atlanta metro squarely within the SLGT risk forecast.
The SLGT remains firmly in-place across Upstate SC with a general
northward extension into the Appalachian front of southwestern NC
as well as small eastward expansion into the Charlotte metro to
atone for the higher QPF being signaled as of the latest 00z
guidance (All in coordination with the local WFO in
Greenville/Spartanburg). The MRGL risk remains generally in a
similar place compared to previous forecast, but some minor
adjustments to the northwest across AL and northward a bit further
into western VA with the most notable inclusion of the Birmingham
metro where the urban flash flood threat will lie right on the edge
where guidance is depicting the heaviest rainfall.

Kleebauer


Day 2
Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND...

The previous forecast remains largely unchanged as the synoptic
regime anticipated for mid-week continues to trend favorable for
heavy rainfall across much of the Northeast and Mid Atlantic. The
neutral- tilted trough from previous D1 time frame will enhance
further with the mean trough taking on a more negative tilt
allowing for enhanced surface low strengthening as the surface low
progresses northward into New England and eventually Quebec. PWAT
anomalies on the order of 2-3 deviations above normal will quickly
progress northward encompassing much of New England by Wednesday
afternoon, even getting up towards +4 to +5 deviations as we move
into Wednesday night over Northern New England from NH through ME.
This pattern is textbook for a corridor of heavy rainfall to spread
from the Mid Atlantic through all of the Northeast with the
primary areas of interest located along and ahead of the cold front
that will sweep eastward during the forecast period. Portions of
Western NY state and Northern PA will eventually change to winter
precipitation allowing for a decay in flash flood concerns, however
areas downstream will be under a respectable deluge of rainfall
prior to the rain threat ceasing in wake of the front.

Totals of 2-3.5" of rain are forecast across much of Long Island
up through Southern New England, eventually moving up through ME as
we close out the forecast period. 1-2" with locally higher amounts
are forecast for points further west, but those areas also have
the added snow melt factors that will play into the flooding
concerns as high dewpoints and heavy rain will promote rapid snow
melt for places in the interior over VT/NH and the neighboring
Berkshires in MA. The expectations are for some creeks and smaller
streams to become problematic in the setup and could cause
localized flood concerns during the peak of the heavy rainfall. The
rivers across New England can thankfully take a decent surge of
moisture after a very dry fall, so the prospects of significant
flooding are lower than normal at this point.

The previously inherited forecast was largely unchanged as the
setup and subsequent forecast is still worthy of a large SLGT risk
encompassing much of New England with the best threats likely over
urbanized areas in LI and Southern New England and over those
zones in the interior where rapid snow melt has historical
precedence for localized flash flood concerns. A MRGL risk will
encompass the SLGT risk across Northeast with an extension down
into Central and Northeastern NC as the early portion of the
forecast period will still see moderate to heavy rainfall Wednesday
morning before pulling northward.

Kleebauer


Day 3
Valid 12Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer
$$
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FOUS30 KWBC 110803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Wed Dec 11 2024

Day 1
Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND...

Potent longwave trough is currently aligned over the Mississippi
Valley with a tilt closer to neutral as of the latest UA analysis.
A vigorous 160kt speed max at 250mb was analyzed over the Ohio
Valley on the leading side of the trough with sampled winds from
the 00z KILN sounding indicating those 160kt winds are verified.
This signals a very potent upper level evolution with an
anticipated holding of strength the next few hrs before a further
strengthening is anticipated later this morning and afternoon as
the trough axis begins tilting more negative allowing for a jet
buckling downstream as the speed max enters New England and
Quebec. At the surface, multiple waves of low pressure are centered
north to south from the eastern Great Lakes, down through the
Southern Appalachians as they lie within the elongated cold front
extending down from the northern low to the Central Gulf coast. The
forecast is for these waves of low pressure to propagate northward
during the next 3-6 hrs before the surface low over the lakes
becomes the dominant SLP feature, strengthening rapidly as the
trough buckles and the low pressure centered gets positioned firmly
within the right entrance region of the very powerful jet over
northeast North America.

The evolution above will lead to a large swath of rainfall to
encompass much of the area along and east of the Appalachian front
with a trailing axis of convection situated over the southern half
of the Carolinas down into FL as the precip aligns along and just
ahead of the surface cold front. Frontal progression will lead to
rainfall shifting eastward overtime, however the environment ahead
of the front will be significantly enhanced with deep layer
moisture advecting poleward well ahead of the mean trough. PWAT
anomalies are already over +2 deviations across a good portion of
the Northeast U.S down through the Southeast with anomalies
expected to climb closer to +3/+4 deviations as me move into the
late-morning afternoon hours over the Northern Mid Atlantic into
New England. Assessment of local bufr soundings across Southern New
England from Central LI up through CT/MA/NH/ME signal a deep, uni-
directional wind field with modest theta_E advection occurring
during the morning and afternoon time frame as the warm-conveyor
points directly through the area with a well-defined warm front
bisecting the area by 18z.

The time frame between 18-00z will be the most significant time
frame with regards to heavy rainfall potential across much of New
England as the cold front begins racing eastward and the warm front
tilts southwest to northeast from Western CT up through Southern
ME. This alignment will become a focus of a strong, cold rain band
that is textbook in these evolving inland synoptic pressure falls
leading to a period where rainfall rates will be a solid 0.5-1"/hr
extending within that convergence zone. The line will advance
eastward eventually as the cold front approaches and kicks the
convergence axis downstream leading to the heavy rain threat
shifting more towards the coastal regions with sights on Eastern
MA/NH/ME towards the 00-03z time frame. Backed flow off the
Atlantic will aid in maintaining the convergent field along and
ahead of the advancing front with heavy rains lingering until the
front back finally pass through and the winds shift west to
northwesterly ushering in colder air and stabilizing the mid-levels
enough to limit the heavier precip. Prior to this occurring, a
solid 2-3" with locally as high as 4" of rainfall are anticipated
in the zone encompassing Central and Eastern LI, much of MA/NH and
the western half of ME. Rapid snow melt will be a problem for
areas located in the Northern Berkshires and Worcester Hills
towards the MA/NH border extending northward as NOHRSC snow depth
analysis pins a solid 2-4+" on the ground currently within those
zones with over a foot in parts of NH/VT/Western ME. This will
allow for potential small stream cresting and ice jam concerns in
the interior that would exacerbate flood concerns during the
heaviest of the precip time frame later this afternoon.

An expanse of Flood Watches are positioned across much of the
above areas, including all of VT as complex terrain and higher snow
depths in the Green Mountains and surrounds leading to higher than
normal flood potential, despite the heaviest rain footprint likely
a bit further east. Considering the above variables and in
coordination with local WFOs, the inherited SLGT risk was
relatively unchanged with the only shift being an extension a bit
further southwest into CT to cover for urban flood concerns within
the axis of expected heavy rainfall during the cold frontal band
being forecasted across much of the 00z CAMs suite. The MRGL risk
still extends around the SLGT and further south into the Central
Mid Atlantic and Carolinas with the primary concerns likely
isolated urban flood potential, especially in the larger metro
corridors like Raleigh, DC/Balt, and Philadelphia.

Kleebauer


Day 2
Valid 12Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 3
Valid 12Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA AND SOUTHWESTERN OREGON...

Fairly robust disturbance over the Pacific will translate
southeastward towards the North-Central CA coast by the end of the
week with a persistent IVT pulse oriented southwest to northeast
into the coastal plain over the course of the forecast period
leading to waves of moderate precipitation to impact areas north of
the Bay area. Current progs indicate precip totals on the order of
2-4" with locally higher embedded within the coastal ranges
spanning north of Santa Rosa. Current rates anticipated to be
modest with upwards of 0.5-0.75"/hr at peak intensity lending to a
lower end threat for flash flooding, however totals are significant
enough to warrant a low-end threshold MRGL, mainly for the coastal
urban areas of Northwest CA up through Southwestern OR where
totals will be on the order of 2-4", especially into the windward areas
of the Klamath Range. MRGL was generally maintained from previous
forecast with only some minor modifications to align with the
latest guidance.

Kleebauer
$$
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