FOUS30 KWBC 300816
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Tue Sep 30 2025

Day 1
Valid 01Z Tue Sep 30 2025 - 12Z Tue Sep 30 2025

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

...Great Basin/Northern Rockies...
A deep upper low off the British Columbia Coast will send 
shortwave energy through the Northwest today. A progressive cold 
front will focus showers and thunderstorms over portions of the 
Great Basin from northeast Nevada to western Montana/northern 
Idaho. MUCAPE values between 100-500J/Kg and PWATs of 0.5-1in. 
could produce excessive rainfall through this afternoon. This will 
be especially true over portions of eastern Nevada into northern 
Utah where 24 hour 1 inch exceedance probabilities are between 20-45%.

...Olympics...
The occluded parent low will direct anomalous Pacific moisture at 
the Olympic coast beginning this afternoon. Some weak instability 
(<200J/Kg MUCAPE) and PWAT's around 1 inch could produce some cells
capable of generating efficient rain rates, especially within the 
windward side of the Olympics. IVT values between 500-700Kg/ms
support the excessive rainfall threat with isolated instances of
flash flooding.

Kebede


Day 2
Valid 12Z Tue Sep 30 2025 - 12Z Wed Oct 01 2025

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

The excessive rainfall threat over the Olympics bleeds into
Wednesday as the upper low arrives off the coast of the Pacific
Northwest. A jetstreak aloft with MUCAPE around 150J/Kg Wednesday
morning could support some weak convection into the Olympics.
Unlike on day 1, maximum IVT values will remain offshore with 1 
inch PWATs arriving early Thursday morning. Canadian and Euro
ensemble 2 inch exceedance probabilities are between 40-60% while 
the GEFS 1 inch probabilities are around an inch. 

Kebede


Day 3
Valid 12Z Wed Oct 01 2025 - 12Z Thu Oct 02 2025

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

...Great Basin...
The upper low is expected to dig into the West Coast on Thursday
directing diffluent flow and left exit region jet dynamics into the
Great Basin. Weak instability (100-200J/Kg MUCAPE) along a
developing cold front could generate thunderstorms capable of 
producing some isolated flash flooding, especially over parts of 
the Great Basin that received rainfall on Tuesday.

...Eastern Florida...
An upper trough in the Southeast will direct embedded vorticity 
into the Florida peninsula on Thursday. An upper ridge centered 
over the West Atlantic will advect tropical moisture (PWATs 
between 1.5-2in.) into eastern Florida, where it'll interact with 
an inverted trough along the coast. Multiple rounds of short fused
convection will proliferate from the central Gulf across the 
Florida Peninsula throughout the day with onshore flow into eastern
Florida producing isolated instances of flash flooding. 
Probabilities of 24 hour qpf exceeding 1 inch is between 15-30% 
across eastern Florida. Urban areas are especially susceptible to 
flash flooding.

Kebede
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