FOUS30 KWBC 290830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Mon Sep 29 2025

Day 1
Valid 01Z Mon Sep 29 2025 - 12Z Mon Sep 29 2025

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF 
NORTHERN CALIFORNIA, SOUTHWESTERN OREGON, THE GREAT BASIN, SOUTH-
CENTRAL NEW MEXICO AND THE SOUTHEAST...

...Carolinas...
Moisture from tropical storm Imelda is expected to continue 
streaming into the Southeast Coast today. A surface front draped 
off the coast should provide forcing for thunderstorm development 
beginning this morning. Convective bands may work their way into a 
moisture rich environment in the Carolinas, where PWATs will be 
between 1.5-2.25in.. MUCAPE between 250-500 J/Kg will spread 
inland throughout the day, but the greatest convective potential 
will be within the bands that propagate inland through North 
Carolina this morning and afternoon. Neighborhood probabilities of 
exceeding 2 inches of rainfall in 24 hours are between 15-30% over 
parts of western and central North Carolina today.

Despite the presence of deep tropical moisture, much of the
convection and heaviest rainfall associated with Imelda should stay
offshore closer to Imelda and the draped front.

...Northern California/Great Basin...
A deep upper low centered over the Gulf of Alaska will send waves
of low pressure into the West Coast over the next couple of days. 
A strong cold front may bring heavy rainfall to portions of 
Northern California up into southern Oregon and the Great Basin 
today. A weak instability gradient will develop ahead of the front 
this afternoon and during peak heating. The 00z HREF suggest up to 
0.25in./hr. rates across the coastal ranges, southern Cascades and 
northern Sierra, with potential for upslope enhancement across 
windward facing slopes. 

EAS 24 hour 1 inch exceedance probabilities are over 45% along the
coast and southern Cascades, while a more modest 5-15% chance 
remains over the Great Basin. There's a steep drop off in 
probabilities once you get into the 2 inch exceedance range.

...South-central New Mexico...
There continues to be a signal for heavy rainfall over much of the
Sacramento Mountains, where instability (500-1000 J/Kg MLCAPE)
could support thunderstorm development and propagation into the
mountains early this evening. Much of the state has already
received over a half inch of rainfall in the past 24 hours with
pockets of 2 inch amounts present including within the Sacramento
Mountains. This may contribute to susceptibility of soils to runoff
from additional rain. Burn Scars such as the Ft. Stanton Piles
prescribed burn area are especially susceptible to flash flooding.

Kebede


Day 2
Valid 12Z Mon Sep 29 2025 - 12Z Tue Sep 30 2025

...THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GREAT BASIN AND NORTHERN ROCKIES...

The marginal risk area inherited from the previous cycle was mostly
maintained with an expansion south into central Utah due to trends
in the guidance. The cold front responsible for heavy rainfall
potential in northern California on Monday will propagate through
the Great Basin and Northern Rockies on Tuesday, focusing
additional instability and moisture along its convergence zone.
Ensemble exceedance probabilities of 0.5 inches in 24 hours are 
between 5-25% over parts of northeastern Nevada, northern Utah,
southeastern Idaho and western Wyoming. The progressive nature of
the front should mitigate flash flooding concerns.

Kebede


Day 3
Valid 12Z Tue Sep 30 2025 - 12Z Wed Oct 01 2025

...THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OLYMPIC PENINSULA...

A shortwave propagating through the southern periphery of the Gulf
of Alaska low will phase with it and move just off of the British
Columbia Coast by Wednesday. This new system will send a cold 
front into the Pacific Northwest. The proximity of the upper low
and associated vorticity lobes should destabilize the environment 
(100-300 J/Kg MUCAPE) enough to support isolated thunderstorm 
activity. Favorable RER dynamics and mid-level moisture 
advection should support efficient rain rates especially over the 
Olympic Mountains. ECENS 24 hour exceedance probabilities of 2 
inches is between 20-40% for much of the area.

Kebede
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