FOUS30 KWBC 260808
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Fri Sep 26 2025

Day 1
Valid 12Z Fri Sep 26 2025 - 12Z Sat Sep 27 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST & IN THE VICINITY OF THE CAROLINAS...

...Southwest...
Numerous showers and thunderstorms are expected across the 
Southwest, especially for much of southeastern Arizona and parts of
southwestern New Mexico, which should occur primarily during the
afternoon and evening hours. An upper low should slow to a crawl 
across southeast California. Precipitable water values should 
remain as high as 1.25-1.5" in spots. ML CAPE values of 2000-3000 
J/kg are possible as available moisture interacts with the cold 
pool aloft. A Slight Risk was maintained across parts of Arizona 
and New Mexico, but it could be higher end Slight Risk. Hourly rain
amounts to 2" with local totals to 4" are possible where cells 
merge and/or train.


...Southeast...
A boundary will remain in place as a cold low forms near the base
of an upper level trough, a feature that lingers for a while near
the Southern Appalachians. Farther inland across the Piedmont, some
degree of upslope flow should bring some of the higher moisture
back in from the southeast. Moisture and instability lifting ahead
of the closing system will support periods of showers and storms. 
Precipitable water values of up to 2" with MU CAPE of 1000+ J/kg 
across the Piedmont and 2000-3000 J/kg in coastal areas are 
expected, which could yield hourly rain amounts to 2.5" and local 
totals to 4". As the 00z HREF and 18z REFS had their strongest 
signal for 3"+ and 5"+ across far southwest VA and the western 
Carolinas, a Slight Risk was added in this update, which was 
coordinated with the local offices. For the time being, a Slight 
Risk was left out across the coastal plain of the Carolinas as 
recent dryness could make a higher incidence of flash flooding 
harder to come by, which was coordinated with the coastal Carolina 
offices.

Roth


Day 2
Valid 12Z Sat Sep 27 2025 - 12Z Sun Sep 28 2025


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF FAR
WEST TEXAS & NEW MEXICO...

...Southwest/Southern Rockies...
An upper low remains fixed across the Lower Colorado River while 
Gulf Moisture streams into the TX Big Bend and New Mexico, moving 
up the Rio Grande. Precipitable water values of 1.25-1.5" are 
forecast, which when combined with 1000+ J/kg of ML CAPE could 
yield hourly amounts to 2.5" and local totals to 4" where cells 
train and/or merge. Believe the risk level from continuity (Slight)
conveys the risk appropriately at this time for portions of far 
west TX and NM. Closer to the upper level center across the 
Southwest, slow-moving convection is possible under the cold pool 
aloft, with ML CAPE of 1000-2000 J/kg of instability and
precipitable water values in the Lower Desert of 1-1.25" expected.
Based on the decline of the 1000-500 hPa thickness across UT 
towards 5670 meters, limited the northern extent of the Marginal 
Risk to southern UT which is about the northern extent of the 0.75"
precipitable water value.


...Carolinas/Southern Mid-Atlantic States...
Divergence aloft around the eastern periphery of a cold low near 
the Southern Appalachians and what is now a tropical disturbance
moving through the Bahamas as a possible tropical cyclone on 
Saturday team up to maintain precipitable water values of up to
2.25" across the coastal plain, which should also see the greatest
instability, 1000-2000 J/kg. The Piedmont should see times when the
MU CAPE rises above 500 J/kg. This is a PRE-like set up for the 
region, so the heavy rainfall potential has a higher upside than 
usual. There has been some convergence in the model guidance QPF- 
wise, and both the 00z HREF and 18z REFS had a cohesive and 
agreeable enough heavy rain threat of 3"+ for a new Slight Risk 
area from roughly Richmond VA south on both sides of the fall line 
into eastern SC. Hourly rain amounts to 3" and local totals to 6" 
are possible, which is potentially problematic anywhere within the 
Slight Risk area. A higher risk level can't be ruled out in future 
updates.

Roth



Day 3
Valid 12Z Sun Sep 28 2025 - 12Z Mon Sep 29 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF FAR
WEST TEXAS, NEW MEXICO, & THE CAROLINAS...

Southwest/Southern Rockies...
The combination of an upper low moving to near the Four Corners, a
kicker upper level trough moving into CA, and moisture feed from 
the Gulf into NM persist into Sunday. Precipitable water values
remain above 1" locally in the Desert Southwest and 1-1.25" in the
Southern High Plains. Pockets of 1000+ J/kg of ML/MU CAPE are
forecast to develop and/or persist.  A Slight Risk still appears 
reasonable across portions of far west TX and NM, with a broader 
Marginal Risk surrounding that. The guidance appears to be trending
somewhat drier across the Great Basin, so limited the northern 
extent of the Marginal Risk area when compared to continuity.


Florida northward into the Southern Mid-Atlantic States...
A system now considered a tropical disturbance over Hispaniola
should lie somewhere north of the Bahamas and east of Florida as a
possible tropical or hybrid cyclone on Sunday. The upper level 
environment doesn't look kind for whatever emerges, and there are 
two scenarios -- a more convective system that remains offshore 
during the medium range period or a more sheared system that 
attempts to occlude when it approaches landfall early next week. 
The guidance is not ideal in differentiating which is correct 
during these scenarios, historically. 

Regardless, moisture will not be a problem regionally --
precipitable water values locally 2"+. Coastal areas will see 
1000+ J/kg of ML CAPE while 250+ J/kg of MU CAPE invades portions 
of VA. The guidance has slowed down the approach of the tropical 
low since yesterday, but the other players in the flow pattern 
remain -- the upper low near the southern Appalachians/southern 
Cumberland Plateau and the front near the East Coast. A PRE- like 
set up appears to be in the cards, with moisture streaming into the
front from the south. The best signal for heavy rainfall on Sunday
into early Monday appears to be in the vicinity of Myrtle Beach, 
and considering previous days of rain, thought a new Slight Risk in
that area would be prudent. In theory, hourly rain amounts to 3" 
should be possible, which would be most problematic should 
thunderstorms merge or train.

Roth
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