535
FOUS30 KWBC 200814
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Wed Aug 20 2025

Day 1
Valid 12Z Wed Aug 20 2025 - 12Z Thu Aug 21 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN EFFECT FROM
SOUTHERN NEW ENGLAND DOWN THE APPALACHIANS, THE MID-SOUTH AND TEXAS,
AS WELL AS EASTERN ARIZONA...

...Northeast...

Combination of a transient shortwave out of Canada and a flux of
moisture from the Atlantic will generate a period of heavy rainfall
potential in-of the interior Northeast across Southern New England.
Reasonable consensus in the handling of the shortwave progression
among models lends credence to the MRGL risk forecast over NY state
down into northern PA, but still some questions exist on the threat
of heavier precipitation into parts of Southern New England. The
greatest probs from NBM and HREF indicate the area from the
Berkshires over into the Hudson Valley as the primary target for
heavy rainfall that could exhibit threats for flash flooding,
mainly in more urbanized settings given very dry antecedent
conditions. Over into Southern New England to the east of the
Berkshires, there`s discrepancy on the handling of the moisture
advection pattern and potential convergence along the northern edge
of Hurricane Erin in the Western Atlantic and the surface ridge in
place over maritime Canada. HREF means tend to limit the scope of
the threat for heavy rainfall to more isolated east of the Hudson
and Berkshire areas with some guidance indicating a stripe of heavy
rainfall bisecting portions of MA/CT/RI before fizzling out the
back end of the forecast. There`s still some deterministic hanging
on the idea of more prolific rainfall across LI up into Southeast
MA and neighboring CT/RI, but that seems to have less agreement
among even the individual ensemble members as the timing of the IVT
pulse and connection with the transient shortwave might just miss
the potential for a more textbook Predecessor Rainfall Event
(PRE).

HREF EAS probs for >1" remain elevated (50-70%) across the Hudson
and Catskill areas of NY state over into the western slopes of the
Berkshires. EAS probs for >2" drop off significantly with less
consensus on the higher end threat in the same zone with even less
signal further east where a PRE would be most prominent if one
evolved. EC AIFS ENS has been steady with the projection of the PRE
just barely missing with the 850-700mb moisture flux converging off
the LI/SNE coast and moving out to sea with maybe places like
Nantucket and eastern Suffolk the beneficiaries of the heavier
precip potential from a PRE organization. This is still the less
likely among the ensemble members and deterministic suite, so
generally stayed away from any higher risk implementation unless we
see greater consensus.

The MRGL risk from the previous forecast was unchanged in this
forecast with a threat of convectively driven rainfall most aligned
from I-80 and points south with more dynamical heavy rain support
for areas north where the shortwave will traverse. Pockets of
convection exhibiting 1-2"/hr will be plausible in-of the Mid
Atlantic urban corridor with the best threat likely between I-70
and I-80 across MD/PA/NJ to the east of the Blue Ridge. Some
guidance have been indicating a localized maxima over Southern NJ
near the Philadelphia metro, likely attributed to some convergence
from the advection pattern to the south and mid-level perturbation
migrating across Southern PA. Something to monitor over the course
of CAMs output.

...Southern Plains through the Appalachians...

Slow migration of a cold front through the central and eastern
CONUS will continue into Wednesday as the frontal progression will
lead to yet another focus of convection firing near and along the
front as it moves into a more favorable environment of elevated
moisture and instability. There are two areas of interest for today
with regards to more regional maxima in QPF; one over east TX and
the second over the Central Appalachians in WV/southwest VA. Both
are amplified in the threat due to the approach of mid-level energy
that will enhance convective development during peak diurnal
destabilization. WV into VA will have the assistance of terrain to
help with the evolving convergence pattern as the front migrates
into the Appalachian front later today. Some of the CAMs output has
been relatively robust for the region in question with HREF
neighborhood probs for >2" running between 60-80% from the Smoky
Mountains, northeast through the WV Highlands and Southern Laurels
in PA. Environment favors this area considering the pronounced
theta_E tongue situated over the above locations, a signature that
exemplifies the best proxy for greatest instability/thermodynamic
support. Mean flow remains on the lighter side as well with the
expectation for storms to move slowly and drift more east to
southeast with the steering pattern. This sets up well for locally
heavy rainfall in those complex terrain areas of the Appalachians
leading to more of a risk of flash flooding in adjacent valley
towns. Considering some of the drier antecedent conditions and more
localized flash flood prospects, the MRGL risk was maintained from
the previous forecast, but if guidance trends upward in the
potential, would not be shocked to see a targeted upgrade in the
next update.

The threat over TX is more simplified with an abundance of surface
based instability as SBCAPE is forecast to be between 2000-3500
J/kg across east TX with the frontal approach. PWATs pooling to
between 2-2.25" based on areal average forecast from the HREF along
with the instability maxima leads to a threat of 2-3"/hr rates with
higher instantaneous rates that could easily subject areas from LA
over into east TX to see localized flash flood concerns. There`s
not a particularly organized zone of convection being forecast with
more of a scattered signature among the CAMs, so the threat is
truly localized and falls well within the MRGL risk threshold.
Thus, little changes were necessary from the previous forecast.

...Southwest...

Monsoonal moisture and strong diurnal destabilization will yield
another day of widely scattered heavy rain prospects across the
higher terrain of eastern AZ into far western NM. The main areas of
concern will continue to be the Huachucas in Cochise county AZ and
across the eastern Mogollon Rim to the north. There`s no real
organized signature to point out that could yield a higher risk
area, so the MRGL risk was generally maintained with little to no
adjustment.

...Eastern North Carolina...

Outer periphery of Hurricane Erin will lead to an uptick in
regional moisture as signaled by a consistent PWAT increase to
above 2" across Cape Hatteras and neighboring areas within the
Pamlico sound. The threat for >2" remains low with the primary
concerns likely the combination of locally heavy rain coupling with
coastal flooding to exacerbate areal flood concerns across the
Outer Banks. Tropical Storm Warnings remain in place with locally
heavy rainfall wording in the forecast from outer bands, so to
remain consistent with messaging, elected to maintain continuity
from previous forecast as the threat has not wavered the past 24
hrs.

Kleebauer


Day 2
Valid 12Z Thu Aug 21 2025 - 12Z Fri Aug 22 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST, SOUTH-CENTRAL TEXAS, SOUTHERN APPALACHIANS, EASTERN
NORTH CAROLINA, UPPER MIDWEST, AND SOUTHWEST U.S...

...Southwest...

Elevated monsoonal moisture lingering under the western ridge will
lead to yet another afternoon of scattered convective development
across much of the desert Southwest and accompanying terrain.
PWATs running slightly above normal across the region will provide
just enough deep layer moisture to couple with relevant diurnal
buoyancy to account for periods of heavy rainfall in some of the
stronger convective cores that materialize Thursday afternoon into
the evening. Totals generally will fall <1" over the Southwest when
looking at areal averages, however some 1-2" totals are certainly
plausible in the setup with the best threat for impacts likely in
those slot canyon areas, drainage across complex terrain into
adjacent valleys, and remnant burn scar locations. The previous
MRGL forecast was generally maintained with only minor adjustments
to the edges of the risk.

...Texas through the Southeast and Southern Appalachians...

Our slowly progressing front will continue to wander further south
with yet another day of convective firing near and along the front
as it migrates through south-central TX and towards the Gulf
coast. The front will arc back into the Southern Appalachians where
scattered thunderstorm activity is likely to occur for at least one
more day in the terrain with the northern extent extending up into
Southern WV and Southwest VA. The signal is still scattered in
terms of convective coverage and flash flood prospects leaning
towards isolated when assessing the HREF probs for hourly rates
and FFG exceedance probs in the 12-00z window Thu/Fri. Convection
will likely dissipate slowly after sunset with the loss of diurnal
heating and surface decoupling putting a cap on the threat. Totals
between 3-5" are most common among some of the stronger cores in
guidance, a target that characteristically covers a MRGL risk with
some targeted SLGT risk opportunities if the signal becomes more
favorable within a favored area for flash flooding (IE: urbanized
zones and terrain). For now, the MRGL risk from previous forecast
was generally maintained with minor adjustments based on 00z QPF
footprint.

...Northern Plains and Upper Midwest...

An energetic upper-level shortwave is forecast to move east across
the northern Plains and Upper Midwest Thursday afternoon through
the overnight. A strong cold front trailing a strengthening surface
low over southern Canada will aid in surface based forcing to
couple with the improving mid and upper dynamics as the trough
migrates along the Canadian border. Sufficient buoyancy and
moisture anomalies of 1-2 deviations above normal will exist along
and ahead of the front coinciding with a strong, yet progressive
line of thunderstorms that will be capable of isolated flash flood
prospects thanks to hourly rates between 1-2"/hr expected. Guidance
remains steadfast in its interpretation of the setup lending
confidence in maintaining continuity from previous forecast with
only small adjustments on the western flank of the risk area to
account for timing adjustments.

Kleebauer


Day 3
Valid 12Z Fri Aug 22 2025 - 12Z Sat Aug 23 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, SOUTHEAST, AND CENTRAL ROCKIES ...

...Southwest to Central Rockies...

Western ridge will shift to the west by the end of the week with
the monsoonal moisture pattern also migrating westward to include
more of the Great Basin and areas around the Colorado river basin.
PWAT anomalies ~ +1 deviation and strong surface destabilization
will yield a fairly widespread convective regime across the
Southwestern part of the CONUS with heavy rain signatures littered
among the activity. Rates up to 1-1.5"/hr in the strongest cores
will likely promote flash flood concerns in those more prone dry
washes, slot canyons, and burn scars residing over the region. The
threat extends into the Central Rockies as shortwaves round the
eastern flank of the ridge and aid in convective initiation over
the terrain of Central CO. Signal currently runs within the MRGL
threshold, but will assess for prospects of targeted upgrades if
any specific area stands out as we get closer to the event with
CAMs input.

...Southeast...

The slow-moving front that will plague the South through the week
will finally hit a road block once it reaches the Gulf coast
becoming a focal quasi-stationary front by Friday morning.
Hurricane Erin will be exiting off into the northwest Atlantic by
this point, but the prominent in-flow trailing the storm will
continue across the Western Atlantic leading to pooling moisture
situated along the confines of the boundary. PWATs will surge with
areal average 2.25-2.5" encompassing a majority of the Gulf coast
from LA over towards the Southeast coastal plain between
Wilmington, NC down into northern FL. The combination of elevated
deep layer moisture, instability, and enhanced surface convergence
along the front will yield some fairly prolific rainfall makers in
any convective scheme during the period. The best signature for
heavy rain is more likely to occur over the SC/GA coast where
there`s a stronger sfc-850mb convergence signature near the coast
leading to an area of ~2" of precip currently forecast in the area.
This is even prior to hi-res model forecasts, a testament to a
growing signature for heavy rain prospects that could very well
necessitate an upgrade in future forecasts, especially as we add
more CAMs input. For now, a broad MRGL exists across a good portion
of the Southeastern U.S.

Kleebauer


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt