Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
222 AM EST Sun Dec 7 2025
Day 3
Valid 12Z Tue Dec 09 2025 - 12Z Wed Dec 10 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
PACIFIC NORTHWEST...

The atmospheric river (A.R.) directing a plume of Pacific moisture
into the coast will shift a bit southward through much of the day
and the first part of Tuesday night. This will focus the heaviest
rainfall into Oregon, with a brief break from the steadiest heavy
rain across northern portions of Washington. While this will allow
for a break in the heaviest rainfall for northern Washington, it
will keep the flooding threat elevated for much of western Oregon,
including portions of southwestern Oregon, which will be largely
missed by Monday's rainfall. Rainfall amounts come down a touch
compared to Monday, with a broad 2-4 inches of new rain expected
into the Oregon coastal ranges as well as the Oregon and far
southern Washington Cascades. Given the rain that is likely to have
impacted this area on Monday, by Tuesday the slightly lesser amount
of new rain will be well offset by the higher stream, creek, and
river levels from the coast to the Western Cascades. Thus, the
inherited Slight Risk for this area remains on track. Due to the
aforementioned southward shift in location, the Slight was expanded
south further into west-central Oregon. However, the risk areas
were unchanged into Washington as the 1-3 inches of rain expected
there should still cause isolated to widely scattered instances of
flash flooding since it closely follows Monday's heavy rains over
much of the western half of the state. 50 kt orthogonal flow of
PWATs at or even a bit above 1.25 inches will still be nearly ideal
for producing upslope heavy rain on the west facing slopes of the
Oregon coastal ranges and Cascades, which will continue to support
the heavy rain threat.

The large subtropical high centered well off the California coast
will continue to be the steering mechanism for the atmospheric
river. This high will be nudged south to allow for the heavy rain
to primary impact Oregon on Tuesday, but this shift will be short-
lived as the high begins to advance back to the north as soon as
Tuesday night, which will gradually shift the atmospheric river
back into Washington.

Wegman

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt