938
FOUS30 KWBC 181555
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Mon Aug 18 2025

Day 1
Valid 16Z Mon Aug 18 2025 - 12Z Tue Aug 19 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER MIDWEST AND SOUTHERN MID-ATLANTIC...

...Upper Midwest/Great Lakes...

A frontal system marching east through the Upper Mississippi Valley
is supporting a large cluster of thunderstorms across eastern IA and
southern WI that will soon head for northern IL later today. As
the warm front inches closer to Chicagoland, SWrly low-level flow
will intersect the front and continue to develop additional storms
this afternoon. The environment will grow increasingly unstable,
with the 12Z DVN sounding showing an impressive >4,000 J/kg MUCAPE
profile that will be advected east into northern IL and southern
WI. Area-averaged HRRR soundings across northern IL/southern WI
this afternoon show as much as 2.0" PWATs and marginal low-level
helicity/shear to help organize developing convection.

12Z HREF probabilities show moderate probabilities (40-60%) for
rainfall totals >3" from water-logged southeast MN and northeast IA
on south and east to northern IL and northwest IN. The inherited
Slight Risk was adjusted to the HREF`s probabilities. Should
guidance trend wetter tonight, there could be locally significant
flash flooding within the Slight Risk, which does include the
Chicago metro area. The Chicago metro area is particularly
vulnerable given the high concentration of hydrophobic surfaces and
0-40cm soil saturation around the Chicagoland area are topping the
90th percentile according to NASA SPoRT-LIS. In summary, flash
flooding is a concern once again in parts of the Midwest with
locally significant flash flooding possible once again this
afternoon and evening.

...Southern Mid-Atlantic & Appalachians...

In collaboration with AKQ and MHX, decided to introduce a Slight
Risk for parts of eastern NC and southeast VA. Latest 12Z HREF
probabilistic guidance shows moderate-to-high chances (50-70%) for
rainfall totals >3" within the Slight Risk area, with some
individual CAM members showing >3" totals as far north as the more
populated areas of the southeast VA Tidewater. Some CAMs also show
isolated amounts topping 5" in eastern NC with 12Z HREF
probabilities showing 20-30% chances for >5". The atmosphere is
primed with tropical PWATs topping 2.2" in some areas, MUCAPE
around 1,000 J/kg, and warm cloud layers at least 15,000ft deep.
This could support rainfall rates up to 3"/hr and 30-minute rates
over 1"/hr. Given these factors, the Slight Risk was introduced
this forecast cycle.

In addition, there is some concern for flash flooding in the NC
Appalachians where latest 12Z HREF guidance shows moderate-to-high
chance probabilities (50-70%) for rainfall totals >3" through 12Z
Tues. Soils are also sensitive with some of the Blue Ridge and
Smokeys sporting near 90th percentile saturation per 0-40cm soil
moisture on NASA SPoRT-LIS. There is the potential for flash
flooding along complex terrain and along streams that historically
tend to rise quickly in short-duration downpours.

Mullinax

---Previous Discussion---

...Front Range through Central Plains...

Series of convective complexes will migrate across the region
extending from the CO Front Range across the KS/NE line until it
reaches towards the Missouri River valley near northwestern MO.
Models are a bit all over the place on exactly how the setup
evolves downstream, but the consensus on initiation across the
Front Range and along the KS/NE border was enough to warrant
coverage with a MRGL risk as cells could produce rates >2"/hr when
assessing hourly prob fields from the HREF and individual CAM
outputs. Highest probs for >2" of rainfall are across northeast CO,
northwest KS, and the MO/KS/NE border with some of the CAMs
indicating upwards of 4" in either location. If consensus grows in
either location for >3", there`s a chance for a targeted upgrade in
future updates. For now, the MRGL risk was sufficient.

...Southwest into Southern Plains...

Monsoonal moisture trapped under the ridge across NM/Southern
Plains will continue the persistent threat of scattered to
widespread convective development from southeast AZ up through
much of NM, west TX, and western OK. The scattered nature of the
convection and no real organized zone of precip will limit the
higher end potential for any risk, however the threat is certainly
within the bounds of a MRGL risk considering the threat for 2-3"
locally in any one location is forecast. The greatest threat will
likely situate over the Sacramento`s thanks to the remnant burn
scar presence that enhances flash flood and debris flow concerns
locally in southern NM.

...Central Gulf Coast...

Very little change in the forecast for Monday. An easterly wave is
expected to traverse areas of central Gulf coast, leading to heavy
rainfall later this afternoon. PWATs generally >2" coupled with
sufficient buoyancy/instability running between 2500-4000 J/kg
MUCAPE will be plentiful for heavy convective cores capable of
rates between 2-4"/hr in the strongest cells. HREF neighborhood
probs for >3" are running between 30-45% near and south of I-10
with the best threats likely in the urban corridors from New
Orleans across into Lafayette to Lake Charles. The progressive
nature of the precip should alleviate the worst flash flood
concerns, but still remains within the lower end of the MRGL
threshold for the threat. Maintained general continuity from
previous forecast with some minor trimming of the eastern extent of
the risk area to remove southern MS.

...Florida...

Seasonal PWATs and convergence of the sea breeze anticipated across
central FL will lead to a period of heavy rainfall along the I-75
corridor, including threats to the Orlando metro area. CAMs are
consistent on a zone of 3-6" locally the evolution across the above
area. This is a generally localized concern as is customary with
these setups, so the previous MRGL risk was kept with removal of
the eastern side of the state as the convergence zone will likely
occur too far west to generate much of a significant rainfall risk
in the I-95 corridor.

Kleebauer


Day 2
Valid 12Z Tue Aug 19 2025 - 12Z Wed Aug 20 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTH-CENTRAL PLAINS TO GREAT LAKES, SOUTHERN APPALACHIANS, AND
SOUTHWESTERN U.S...

...Southern Plains to Great Lakes...

Mid and upper level progression will undergo a wave break pattern
with the eastern flank of the ridge being compressed with an
overall shift westward in the ridge axis leading to energy dropping
southeast out of the Great Lakes area. Widespread +1 to +2
deviation PWATs will exist in proximity of the Missouri Valley up
into the Great Lakes with a cold frontal progression moving
southeast out of the Northern Plains into the Central U.S. by
Tuesday afternoon. Coupled low and mid-level ascent provided by
migrating shortwaves and the aforementioned front will interact
with the above normal moisture environment in place allowing for
widespread convective development across a large chunk of the
Plains up through the Ohio Valley and Great Lakes creating a broad
threat for localized flash flooding. Greatest potential will likely
exist over the Mid-Mississippi Valley into the western Ohio Valley
given the strongest low-level convergence signature and mid-level
shortwave focus in tandem for Tuesday afternoon and evening. Areal
average QPF output still isn`t as robust as one would look for to
entice an upgraded risk, however there are some early indications
from the end of the CAMs window where perhaps a targeted upgrade
could occur if the trends remain favorable for the outlined area.
Locally as much as 3" are indicated in that zone with some 2-3"
totals littered across the Central U.S in proxy to the advancing
cold front. First Guess Fields remain consistent on just a broad
MRGL risk area which was general continuity from previous forecast.
As a result, maintained that posture, but will monitor for the
upgrade potential in the succession of updates.

...Southern Appalachians...

Elevated moisture and narrow tongue of instability in the confines
of the Southern Appalachians will allow for a threat of focused
heavy rainfall across the western Carolinas down into GA. A quick
moving mid-level perturbation dropping south out of the Ohio Valley
will enhance the potential providing just enough forcing and weak
shear to warrant some heavier convective outputs capable of
dropping 2-3" in a short time that would generate some flash flood
prospects in the vicinity of the complex terrain. Previous forecast
was still on target with the latest model trends leading to
relative continuity in the MRGL in place.

...Southwest...

Little to no change in the expected pattern evolution across
portions of the Southwest U.S. with regards to convective
opportunity. Remnant moisture with PWAT anomalies around 1 standard
deviation above normal will be situated over eastern AZ/much of NM
on Tuesday. Considering the moist environment in place, hourly
amounts between 1-2" are plausible where cells stall, backbuild,
merge, or train. An isolated terrain driven flash flood risk is
possible in the diurnal convection, especially over the Sangre de
Cristos, Sacramento`s, and eastern Mogollon Rim. Considering run to
run variance being limited over the past 24 hrs., there was no
reason to deviate much from the inherited forecast. The MRGL risk
was generally maintained with some minor adjustments on the
northern periphery to account for QPF enhancement in-of the Sangre
de Cristos.

Kleebauer


Day 3
Valid 12Z Wed Aug 20 2025 - 12Z Thu Aug 21 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN EFFECT FROM
THE SOUTHERN PLAINS TO CENTRAL AND SOUTHERN APPALACHIANS, AND
SOUTHWEST U.S...

...Southern Plains to Appalachians...

Slow migrating cold front will become the focal point for
convective development over the course of Wednesday into early
Thursday leading to widely scattered thunderstorms capable of
localized flash flooding. Signal is really diffuse overall, but
environmental conditions are still relevant for pockets of heavy
rain that could cause some flash flood concerns in more urbanized
corridors of the Southern Plains to the southern half of the
Appalachian front. The best potential likely exists in the
mountains where complex terrain and convergent flow will aid in
upslope forcing capable of slow-moving cell motions and heavy
rainfall over smaller foci (mountain ridge tops). A low-end MRGL
exists over a broad area with potential for a more concrete area of
interest pending how the broad synoptic pattern evolves and any
focal points within the mesoscale (remnant outflows).

...Southwest...

Monsoonal moisture presence and general diurnal instability will
aid in traditional afternoon and evening convective output across
east and southeast AZ into west-central NM on Wednesday. Signals
for heavy precip are very isolated leading to the threat lying just
inside the MRGL risk threshold. There is a chance the areal
coverage shrinks or the threat goes away altogether, but model
output maintains the prospects at this juncture, so maintained
continuity from previous forecast.

...Northern Mid Atlantic...

As of this update, there is no risk area in place across Central PA
into New York State, however will be monitoring the pattern
progression of a shortwave ejecting out of Ontario by the end of D2
into D3 that could entice a targeted risk in these two specific
zones. Models are truly split on the threat, and with antecedent
conditions running very dry over the areas in question, it will
take guidance amping up the precip output to consider any upgrades.
This has some definition within the ML guidance and EC ENS, so
wanted to provide some insight on the threat despite a lack of a
risk area.

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