050
FOUS30 KWBC 210719
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
319 AM EDT Thu Aug 21 2025

Day 1
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. 00z HREF
neighborhood probs for >1" are running between 30-45% across the
central and eastern Mogollon Rim, and across the southern extent of
the Sangre de Cristos in NM. These are the areas with the best
flash flood potential given the anticipated convective pattern and
topographic challenges that accompany these locales. The previous
MRGL forecast was maintained with little to no adjustments.

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

Our slowly progressing front is currently analyzed across the
Lower Mississippi Valley back into the southern Ohio Valley with a
general south to southeast progression over the past 24 hrs. Cold
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 with an
elongated surface trough running parallel to the immediate Gulf
coast from TX to FL/GA. 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 to perhaps scattered when
assessing the HREF probs for hourly rates and FFG exceedance probs
in the 12-00z window Thu/Fri. The strongest signature for heavy
rain concerns lies within the ridges and eastern slopes of the
Southern Appalachians across western NC down into the far western
SC escarpment. 3hr and 6hr FFG exceedance probabilities between
35-50% exist over the above area with the time frame of interest
falling in that afternoon and early evening window where diurnal
instability maximum and convective temp breach overlap.

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, aligning well with the modest >3" probs
and low-end >5" probs within the latest HREF iteration. Despite the
totals in question, the flash flood threat will likely be more
comprised of 3 and 6hr FFG exceedance compared to lone hourly
rates, however some instantaneous rates upwards of 2-3"/hr in the
terrain could spur some flash flood hazards, especially as storm
motions remain on the weaker side and generally parallel to the
topography. The previous MRGL risk was maintained given the
signatures across the South and Appalachian front. Higher FFG`s
across the above regions were the main deterrent for a higher
risk, but will be monitoring the threat closely to see if a
targeted upgrade is necessary.

...Northern Plains and Upper Midwest...

An energetic upper-level shortwave is forecast to move east across
the northern Plains and Upper Midwest later this 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. Trends
of an areal maximum across eastern SD have gained traction within
the latest CAMs and matches the global ensemble means and EC AIFS
Ensemble probs being highest in-of that region. This correlates
with the stronger mid-level vorticity advection entering the area
to couple with the best low-level instability axis along the
southern edge of the front. The previous MRGL risk was generally
maintained with some minor trimming on the eastern edge to account
for lower potential downstream given timing of convective
advancement among the hi-res.

Kleebauer


Day 2
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...

...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 areal average of ~2-3" of precip currently forecast
across southern SC through much of southeastern GA.

The most robust signal was within the probabilities for FFG
exceedance just based off the 12-00z window on Fri/Sat indicating
>60% chance of both 3hr and 6hr FFG exceedance probabilities
centered from Charleston, SC down through the SC Low Country into
far southeast GA. This includes the Savannah, GA area, a place more
prone to flash flooding due to its urbanization factors. This will
be a common theme in the flash flood prospects with this setup as
heavy rainfall will develop further inland in proximity to the
front leading to even inland areas of GA seeing a greater threat
for localized flooding than usual. Rates will be pretty stout
considering the environmental factors in place when you couple
1500-2500 J/kg SBCAPE with deep layer moisture being forecast from
the sfc to near the tropopause. This is a 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 if the PWATs in a given area get up towards
that 2.5" marker. Even with the higher FFG`s in place, the signal
for enhanced convective rainfall output and flash flood concerns
are worthy of a higher risk compared to the previous forecast. In
coordination with local WFOs across SC and GA, a SLGT risk upgrade
is now in effect for the period beginning Friday AM, likely
carrying into early Saturday AM.

...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" PWATs 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. 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.
Consistent signal in the means and modest probabilities within the
beginning of the convective window on the end of the 00z HREF were
satisfactory for maintaining continuity in the current MRGL risk
outlined. Thus, little change was necessitated from the previous
forecast.

Kleebauer


Day 3
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 PWATs will linger across the Southeastern CONUS through
the first half of the weekend as the stalled frontal positioning
will lead to sustained moisture pooling within the confines of the
boundary. Increasing meridional component upstream over the
Midwest/Ohio Valley will lead to a modest surge in moisture along
the Appalachian front from GA up through WV 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 if the conditions are
truly compromised considering the plethora of rain forecast in the
D2. The trend is 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...

A relative repeat in the setup from D2 will yield widespread
convection across southern and eastern CA 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. thanks to forecasted vorticity maxima sliding down the
eastern flank of the ridge and providing focused ascent over the
aforementioned terrain. Areas in the more prone slot canyon, burn
scar remnants, and dry washes will once again be the target of
interest for flash flood prospects allowing for a broad MRGL risk
placement to account for the threat. Pending evolution of guidance
and perhaps more focused areas of concern in the range of CAMs, an
upgrade could very well be plausible in future updates.

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