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FOUS30 KWBC 220841
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 AM EDT Fri Aug 22 2025

Day 1
Valid 12Z Fri Aug 22 2025 - 12Z Sat Aug 23 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF
SOUTHEAST GEORGIA AND SOUTHERN SOUTH CAROLINA...

...Upper Midwest...
On-going shower and thunderstorms with locally heavy rainfall rates
mainly over eastern South Dakota preceding the start of the Day 1
period at 22/12Z should persist beyond 12Z. The expectation is that
the risk of excessive rainfall should diminish within a couple of
hours. Until then...locally heavy rainfall rates in excess of 1.5
inches per hour could result in flash flooding. Additional details
in WPC Mesoscale Precipitation Discussion 0972.

...Southeast...

A slow-moving front which becomes a quasi-stationary front upon
reaching the Gulf later today will be a focus for heavy to
potentially excessive rainfall. Precipitable water values with
areal average 2.25-2.5" encompassing a majority of the Gulf coast
from LA over towards the Southeast coastal plain. This...combined
with 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. Latest
HREF/RRFS runs still key in the Georgia/South Carolina area
(especially coastal areas) due to stronger sfc-850mb convergence.
Areal average rainfall of ~2-3" of rainfall remained in the latest
deterministic QPF.

The most robust signal was within the probabilities for FFG
exceedance just based off the 22/12Z to 23/00Z window still
indicated a greater than >60% chance of both 3hr and 6hr FFG
exceedance probabilities...generally from Charleston, SC down
through the SC Low Country into far southeast GA. Rainfall rates
will be pretty stout considering the environmental factors...with
classic tropical convective scheme that is capable of producing
2-3"/hr rates more regularly in the strongest cell cores, even
seeing 4-5"/hr instantaneous rates.

...Texas and Louisiana Gulf Coast...

Pooling moisture along a shallow quasi-stationary front along the
Gulf coast will amplify environmental conditions to favor isolated
to scattered convective development capable of localized flash
flooding thanks to deep layer moisture between 2-2.25" precipitable
water values and slower mean cell motions. The signal is not as
prolific as areas downstream, but the threat is still being
forecast based off the latest CAMs with some localized totals of
2-4" being forecast along and south of I-10, enough to maintain
general MRGL risk continuity from previous forecast.

...Southwest to Central Rockies...

Little change was necessary given run to run continuity on the
next widespread convective pattern centered over the Southwestern
CONUS to Central Rockies. Western ridge will shift towards the
Great Basin/Four Corners by the end of the week with the monsoonal
moisture pattern also migrating westward to include more of the
Great Basin and interior Colorado river basin. The amount of dry
air in the sub-cloud region of area sounding allowed for a bit of
area to be trimmed off the western and northern portion of the
Marginal risk area....but maintained it in the area of terrain.
Precipitable water anomalies on the order of +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 Colorado.

Overall...the current Excessive Rainfall Outlook changed little
provide the previous one.

Bann

Day 2
Valid 12Z Sat Aug 23 2025 - 12Z Sun Aug 24 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL AND SOUTHERN APPALACHIANS, SOUTHEAST AND SOUTHWEST U.S...

...Southeast into the Appalachians...

Elevated precipitable water values will linger across the
Southeastern CONUS through the first half of the weekend as a
stalled front leads to sustained pooling due to prolonged surface
moisture flux convergence. A modest surge in moisture along the
Appalachian front from Georgia up through West Virginia on
Saturday afternoon and beyond allowing for more scattered
convective signatures to materialize with the aid of diurnal
destabilization. Totals across the Southeast and Appalachians are
not as robust compared to previous periods to warrant anything
greater than a MRGL, however antecedent soil conditions over
portions of the Carolina`s and Georgia may necessitate a targeted
upgrade. Given the trend for at least some modest convective
coverage through the morning and afternoon time frames on Saturday,
so there`s a non-zero possibility of warranting a greater risk.
For now, kept the previous forecast over the Southeast with some
tweaks on the northern edge of the MRGL in the Appalachian front.

...West...

Little change in the large-scale pattern resulted in few changes
needed to the previous Day 3 outlook as it propagates into the Day
2 period. Numerous convection across southern and eastern
California and across the interior west with the eastern extent
likely back over the Central Rockies. Ridging over the western
CONUS will remain stationary with a recycled vorticity pattern
rotating under the guide of the ridge with elevated moisture pretty
much encompassing the Southwestern U.S and interior portions of
the west like NV/UT/CO. Heaviest QPF footprint is currently
situated over the Sangre de Cristos and San Juan Mtns given
embedded shortwave energy and orographics providing areas of focused
ascent. The more prone slot canyon, burn scar remnants, and dry
washes will once again be the target of interest for flash flood
prospects..

Bann

Day 3
Valid 12Z Sun Aug 24 2025 - 12Z Mon Aug 25 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER THE
SOUTHWEST U.S. TO THE CENTRAL PLAINS AND ACROSS PARTS OF THE
APPALACHIANS AND MID-ATLATNIC...

...West...

Little change in the large-scale pattern spells another day of
scattered to numerous late day and evening showers and
thunderstorms for the areas generally west of the Rockies. A shift
in the mid-level flow may allow for increasing amounts of moisture
to start a slow northward expansion across parts of the
Intermountain region...resulting in an expansion to the Marginal
Risk area there. Otherwise...the driving forces will continue to be
weak shortwave energy embedded within fast flow...locally
encountering/interacting with orographics during the time of
maximum heating/instability. Once again...slot canyons, burn scar remnants,
and dry washes will once again be the target of interest for flash
flood prospects..

Father east...the risk of excessive rainfall starts to spread out
away from the central Rockies Front Range and out over the adjacent
plains where any shortwave energy encounters lowering mid-level
heights. Flow aloft is forecast to be northwesterly at this point
but models do show a period of low level upslope flow. Spaghetti
plots do show some potential for 2 to 3 inch rainfall amounts
extending as far east as western Kansas by the end of the period.
Given the precipitable water values forecast...the most active
convection could drop 1 to 2 inch rainfall amounts in an hour.

...Appalachians to Mid-Atlantic and Pennsylvania/New York...
A deepening area of low pressure north of the border will force a
cold front to sweep eastward from the Great Lakes into the eastern
third of the country on Sunday...driven in part by a 75 to 100 kt
upper level jet providing support via divergence aloft. With the
renewed push of colder air...the low level boundary will act to
focus shower and thunderstorm activity capable of locally heavy
rainfall rates that could challenge flash flood guidance-
especially in areas where flash flood guidance has been suppressed
recently.

Bann


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