FOUS30 KWBC 050832
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
432 AM EDT Tue Aug 5 2025

Day 1
Valid 12Z Tue Aug 05 2025 - 12Z Wed Aug 06 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST AND INTO THE MID-ATLANTIC...

...Southeast and Mid-Atlantic...

A heavy rainfall and flash flood event may be getting started at 
the beginning of the period, as convection is expected to increase
in coverage and intensity from northeast Georgia into Upstate 
South Carolina (potentially extending into far southwest North 
Carolina) this morning. A conducive overall setup for locally heavy
downpours exists, with a low-level front/convergence zone and the 
right entrance region of a strong upper-level jet streak 
superimposed over the same area (along with anomalously high 
moisture and PWs over the 90th percentile). The main limiting 
factor for organized heavy rainfall is certainly instability, 
though a modestly strengthening low-level jet (15-25 kts) will 
provide some isentropic upglide to kick things off this morning 
(with hourly totals likely limited to 1-2". Most hi-res models 
depict a break in the precipitation during the day, but another 
round may ensue later tonight over some of the same areas, as the 
frontal zone and parent trough to the west move very little. 

Further diurnally-driven development is expected within the broader 
warm sector from the central Gulf Coast to south-central Georgia, 
and the rest of South Carolina, with stronger instability building
today with daytime heating and PWs in excess of 2 inches. Hi-res 
models continue to indicate fairly high probabilities of 2"/hr 
rain rates, and a complex pattern of cell mergers and colliding 
outflow boundaries and sea breezes. This may lead to periods of 1-2
hours where convection is maximized in particular areas, and 
scattered instances of flash flooding. The inherited Slight Risk
was expanded a bit to cover coastal portions of SC/GA with the
highest probabilities for localized 5" exceedance. 

...Northern Plains and Minnesota...

There continues to be a fairly strong signal for storms to initiate
in the northern extent of an instability plume in the Plains, near
the nose of a moisture transport maximum, in the afternoon today. 
Consensus among the hi-res models is this will rapidly organize 
into an MCS and then likely propagate quickly to the southeast in 
the general direction suggested by the thickness contours. The 
potential for relatively fast forward motions may mitigate some of 
the flash flood risk. However, precipitation anomalies over the 
past 14 days indicate the pattern has been relatively wet of late 
in the Northern Plains, and precipitable water values will be 
anomalously high. Deep moisture combined with strong instability 
should support high instantaneous rain rates. Therefore, any areas 
where the duration of heavy rain could be lengthened (such as 
backbuilding along the periphery of a cold pool, cell mergers with 
antecedent convective development) may lead to flash flooding. The 
Marginal Risk was maintained and adjusted southward based on the 
latest guidance (factoring in upwind propagation favoring southern
propagation late). 

...Ohio River Valley...

An inherited Marginal Risk was maintained and adjusted slightly 
for a potential repeat of the pattern from yesterday, just 
displaced slightly to the east over S IN, C KY, SW OH. A slow 
moving mid-level wave is expected to continue to slowly shift 
east, in the presence of relatively strong instability. Therefore, 
scattered afternoon thunderstorms are expected. Although moisture 
levels are not expected to be anomalously high, storm motions 
should be very slow with mean winds in the lowest 8-km around 5 kts  
or so. This could lead to localized flash flooding by extending the
duration of heavy rain with the most intense storms. Rain rates 
should reach the 1-2"/hr range.

Churchill/Lamers


Day 2
Valid 12Z Wed Aug 06 2025 - 12Z Thu Aug 07 2025

...THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS
PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND FOR THE COASTAL 
SOUTHEAST AND MID-ATLANTIC REGION...

...Upper Mississippi Valley...

A region of anomalously high moisture, with PWs near the 90th 
percentile, should shift east during the Day 2 period, and 
generally continue to be superimposed with moderate to strong 
instability on the periphery of a ridge. Confidence in the 
convective details remain low at this time, but the overall 
environment will remain supportive of organized convection with 
high instantaneous rain rates. The pattern may still lead to a
forward-propagating MCS, which would mitigate flash flood risk 
overall, but the overall environment is favorable enough to 
continue to maintain a Marginal Risk across the region.

...Coastal Southeast and Mid-Atlantic...

Confidence remains relatively low in the placement of the heaviest
rainfall on the Day 2 period, as the ECMWF/ECENS suite continues 
to concentrate QPF farther north (mostly across North Carolina) 
whereas the bulk of the other global models and ensembles are 
focused south and east (mainly in SC and GA). Regardless of the 
precise details, a plume of anomalously high precipitable water 
values is likely to remain present across the coastal Southeast 
and Mid-Atlantic region into the Day 2 period, and the flash flood
threat areas will likely come down to mesoscale details that will
come much more into focus in subsequent outlook periods. 

Churchill/Lamers


Day 3
Valid 12Z Thu Aug 07 2025 - 12Z Fri Aug 08 2025

...THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS
PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND NORTHERN PLAINS, AS
WELL AS FOR THE COASTAL SOUTHEAST AND MID-ATLANTIC REGION...

...Upper Mississippi Valley and Northern Plains...

An inherited Marginal Risk area was maintained and adjusted across
North Dakota and surroundings, as models are in good agreement in
indicating a potent upper-level shortwave digging southward into 
the CONUS by Day 3. The approach of the trough is likely to result 
in another wave of low-level return flow into the Northern High 
Plains, sending PWs to the 90th percentile or higher once again. 
Moderate to strong instability is expected to build once again with
daytime heating on the periphery of a ridge, and organized 
convection is likely to ensue in the late afternoon and evening. 

...Coastal Southeast and Mid-Atlantic...

Confidence remains low in the placement of the heaviest rainfall
going into the Day 3 period, with substantial differences still
between the global model suites. As anomalously high tropospheric
moisture is likely to remain near the coast, maintained an
inherited Marginal Risk area across portions of the area most
likely to realize heavy precipitation with daytime heating.

Churchill
$$
--- SBBSecho 3.28-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)