FOUS30 KWBC 250744
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
344 AM EDT Thu Sep 25 2025

Day 1
Valid 12Z Thu Sep 25 2025 - 12Z Fri Sep 26 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST
ARIZONA AND FAR SOUTHWEST NEW MEXICO...

...Southwest to Sierra Nevada...
A cold low near the central California coast is forecast to drop 
southeast into southern California by early Friday. With cold air 
aloft overlying sufficient moisture near and east of the upper 
center, locally heavy amounts are possible from the Sierra Nevada 
east into southern Nevada and southwestern Utah.

Ahead of the system, increasing southerly flow will draw deeper 
moisture into the Southwest from the Gulf of CA, fueling 
widespread shower and thunderstorm coverage, especially across 
southeastern and central Arizona into southwestern New Mexico. ML 
CAPE should rise into the 1000-2000 J/kg range. A Slight Risk was 
maintained across this area where cold air aloft interacts with 
monsoon-like moisture, with precipitable water values as high as 
1.25-1.5". Hourly rain amounts to 2" are possible where cells merge
and/or train.

...Northeast/Upper Ohio Valley...
Unsettled weather at the present time is forecast to continue into
Thursday as showers and storms stream east across the Upper Ohio
Valley, WV, and southwest PA ahead of a shortwave aloft and surface
frontal wave. While there may be a gap in heavy rainfall between 
this region and points farther east, chose to connect the areas 
and significantly broaden the inherited Marginal Risk. 
Strengthening southwesterly flow along the Mid Atlantic coast into 
the Northeast will support deepening moisture (PWs increasing to 
~2" and ML CAPE surpasses 1000 J/kg. Hourly amounts to 2.5" are 
possible in this region with local totals to 4", but coverage of
such amounts should be sparse enough for a Marginal Risk to still apply.

...Southern Appalachians into parts of the Gulf Coast...
Amplifying mid-upper trough digging into the northwest and north- 
central Gulf will slow the south and east progression of the 
surface cold front and pre-frontal buoyant airmass. ML CAPE values
should rise to 1000 J/kg or so across the southernmost 
Appalachians while areas of the Gulf Coast see 2000-3000 J/kg of ML
CAPE. With precipitable water values at or above 2", hourly rain 
amounts to 2.5" with local totals to 4" are possible wherever short
periods of training or cell mergers are able to occur. Activity 
currently across South TX should survive past 12z, so looped the 
Marginal Risk to encompass this area. At the present time, any
flash flood issues are expected to be isolated to widely scattered.

Roth


Day 2
Valid 12Z Fri Sep 26 2025 - 12Z Sat Sep 27 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST
ARIZONA AND SOUTHWEST NEW MEXICO...

...Southwest...
Numerous showers and thunderstorms are expected across the 
Southwest, especially for much of southeastern Arizona and parts of
southwestern New Mexico. 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, but the guidance shows more 
potential on the ML CAPE front when compared to Thursday, with 
values of 2000-3000 J/kg possible as available moisture interacts 
with the cold pool aloft. Widespread shower and thunderstorm 
coverage is expected, particularly during the afternoon and evening
hours. A Slight Risk, with a footprint similar to the Day 2 area, 
was maintained for Day 3 across parts of Arizona and New Mexico, 
but it could be higher end Slight Risk. Hourly rain amounts to 2" 
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. Moisture and instability lifting ahead 
of the closing system will support periods of showers and storms. 
Precipitable water values of up to 2" with ML CAPE to 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".

Roth


Day 3
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 up to 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. 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, which was coordinated with the SLC/Salt 
Lake City forecast office.

Carolinas/Southern Mid-Atlantic States...
Divergence aloft around the eastern periphery of a cold low near 
the Southern Appalachians maintains precipitable water values of 
2"+ near the coast, which should also see the greatest instability,
up to 1000 J/kg. The guidance shows a bit of dispersion here in
placement/northeast movement of the heavy rain area with the GFS 
and NAM in their usual camps (fast and slow, respectively). Hourly
rain amounts to 2.5" and local totals to 4" are expected, similar 
to previous days. Forecast uncertainty, as much as anything else,
maintained the risk level as a Marginal. Should the guidance
converge, areas of risk overlap between Friday and Saturday across
portions of the coastal plain of the Carolinas could see an upgrade
to a Slight Risk at some point as we get closer to Saturday.

Roth
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