Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
950 AM EST Sun Dec 7 2025

Day 1
Valid 16Z Sun Dec 07 2025 - 12Z Mon Dec 08 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Cook/Wegman

Day 2
Valid 12Z Mon Dec 08 2025 - 12Z Tue Dec 09 2025

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

The first in a series of lows is expected to slam into the coast of
the Pacific Northwest on Monday. While the low centers will track
well north of the area, the trailing cold fronts will be steered
right into the coast and will be the primary forcing for heavy
rain. The deep Pacific and tropical moisture will originate well
west of Hawaii, then track ENE to NE along the northwestern
periphery of a strong, stationary subtropical high parked off the
central California coast. This will act as the "train track"
directing moisture and any front associated with lows well to the
north into the same stretch of coast (Washington and Oregon) for
the next several days. The air mass will feature PWATs of up to
1.25 inches, which is above the 95th percentile of atmospheric
moisture for this part of the world in December. This is also 3
sigma above normal. In other words, there will be plentiful
moisture for the coastal mountains and the fronts to wring out into
the foothills and adjacent valleys in the form of rainfall. For
just this Day 2 period, expect a widespread 3-5 inches of rain in
the Slight Risk area, with the highest amounts into the highest
terrain. For portions of the northern Washington Cascades, where
most of the precipitation falls as snow, it will be measured in
feet. Note all of these statistics are for just this Day 2 period.
Expect several more rounds, albeit slightly weaker than this one,
to impact these same areas over the coming days.

Contrasting the heightened flooding threat will be that the area is
highly resistant to flooding, at least flash flooding, as the area
topographically is quite efficient at moving rainfall to the local
rivers and the ocean. Thus, the highest flooding threat is in the
urban areas such as Portland, Olympia and the eastern suburbs of
Seattle. Elsewhere flood-sensitive low lying areas will also be at
greater risk for localized flash flooding. Meanwhile, the rivers
may take a couple days of heavy rainfall to really respond and
potentially threaten bankside communities.

The Slight Risk area was largely unchanged with this update. It
should be noted there was a small northward shift in the latest 00Z
guidance, but so as to not flip-flop the forecast along the
Canadian border and the Juan de Fuca Strait, the Marginal Risk
there was left unchanged so as to allow for another cycle of model
guidance to determine whether the flood risk should be returned to
the Slight category in these areas. For the vast majority of the
area however, the flooding risk remains steady, with no major
changes to the overall rainfall forecast, just where some of the
heavier rainfall amounts may occur.

Wegman

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 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt