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Message   Mike Powell    All   HVYRAIN: Excessive Rainfa   August 6, 2025
 9:19 AM *  

FOUS30 KWBC 060817
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Wed Aug 6 2025

Day 1
Valid 12Z Wed Aug 06 2025 - 12Z Thu Aug 07 2025

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

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

Relatively high coverage of convection is expected once again
today, as a stalled frontal boundary remains draped across the
region. While storms should generally remain relatively short-lived
over any one location with shear being the main limiting factor, 
more significant convective organization is possible in the
vicinity of the NC/SC Coastal Plain border region where deep layer
shear is progged to be closer to 20-30 kts (versus less than 20 kts
from surroundings). This is due to the slow eastward progress of
the blocked upper trough to the west, and a vorticity maxima may
also contribute to convective proliferation later today via DPVA
and enhanced upper divergence via right-entrance region of ~80 kt
jet streak over the Northeast. While the 00z HREF 5" exceedance
probs are quite high (50%+ over portions of the Coastal Plain),
uncertainty remains high with regard to the extent of resulting
flash flooding, as the QPF spread between the HREF members remains
quite large (with the NAM-nest and FV3 being the wetter outliers,
while the ARW and ARW2 are the drier outliers). Maintained the
inherited Slight Risk, and a targeted upgraded Moderate Risk could
be possible later today should models come into better agreement. 

...Central Plains into Middle and Upper Mississippi Valley...

A mature MCS is diving southward early this morning from SD/MN into
IA/NE (see MPD #877 for more information) along with anomalously 
high tropospheric moisture (PWs nearing 2.0", or around the 90th 
percentile). Maintained a Marginal Risk for lingering convection 
associated with the MCS this morning, along with the potential 
sustained or redeveloped convection in association with the remnant
MCV later today. 

Churchill


Day 2
Valid 12Z Thu Aug 07 2025 - 12Z Fri Aug 08 2025

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

...Northern Plains...

Models continue to indicate a potent upper-level shortwave digging
southward into the PacNW by Day 2, and have maintained a Marginal 
Risk centered on ND where convection is anticipated to initiate in
the afternoon. The approach of the trough will result in another 
round of low-level return flow into the Northern High Plains, 
sending PWs to near (or above) the 90th percentile once again. 
Moderate to strong instability with daytime heating should yield
organized convection by late afternoon/evening, and a targeted
Slight Risk upgrade may be necessary with subsequent outlooks. 

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

Confidence in the rainfall forecast is relatively low going into 
Thursday, as much depends on how convection ultimately evolves
today. That said, the recent trends in QPF have been downward with
most models showing anomalously high tropospheric moisture and
low-level convergence shifting to the immediate coastline. Have
maintained an inherited Marginal Risk for this cycle, which may be
further reduced in coverage should these model trends continue. 

Churchill


Day 3
Valid 12Z Fri Aug 08 2025 - 12Z Sat Aug 09 2025

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

...Northern Plains into Upper and Middle Mississippi Valley...

Inherited Marginal Risk was expanded to include a relatively large
portion of the north-central portion of the CONUS, a continuation
of the threat from Day 2 from a potent upper trough/closed low.
Convective details remain quite fuzzy at this range, but guidance
suggests the potential for localized 3-5" totals (high spatial
spread in these depictions) which may necessitate future targeted
upgrades. 

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

Low confidence forecast continuing into Day 3, maintaining a
Marginal Risk area that is largely focused on the same area. At
this juncture most models depict heavy rainfall remaining largely
offshore, but given recent heavy rains and the potential for
significant amounts today have opted to keep the Marginal Risk. 

Churchill
$$
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