Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
749 PM EST Mon Dec 8 2025
Day 3
Valid 12Z Wed Dec 10 2025 - 12Z Thu Dec 11 2025

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

20z Update: There has been a consistent northward trend in the
atmospheric river/moisture plume by Wednesday. Still looking at
additional heavy rainfall totals over western WA, but we were able
to trim back some of the southern extent of the risk area. Portions
of the WA Cascades are within the higher end of the Slight risk
probability range, as the increase in IVT over what should be
saturated ground conditions is likely to result in an increase in
the coverage and magnitude of flood impacts on Wednesday. There
remains some potential for an eventual MDT risk upgrade, however
the steady northward trend seen in the models does somewhat lower
confidence on exactly where the max rainfall axis will end up,
which precludes any upgrade at this time.

The higher terrain over northern ID and northwest MT still looks
on track to see 1 to 3 inches of rainfall, which combined with
snow melt, is likely to lead to at least some flooding impacts.
The aforementioned northward trend in the QPF axis applies here as
well, which introduces enough uncertainty to prevent a Slight risk
upgrade at the moment. Although if the current forecast holds
steady we very well could be looking at the need for a Slight risk
upgrade as the event nears.

Chenard


...Previous Discussion...

...Western Washington and Oregon...

The Slight Risk across portions of the Pacific Northwest remains
largely unchanged with this update. A northwest to southeast
oriented jet stream will remain in place from southern British
Columbia through to the northern Plains. This will put the Pacific
Northwest in the favorable right entrance region of the jet. A
series of upper level shortwaves will zip down the jet, which in
turn will keep the atmospheric river (A.R.) plume in place and
barely moving through the entire period. Due to the flow
orientation changing to out of the west-northwest. The moisture
will flow largely unimpeded through the Strait of Juan de Fuca and
past the coastal/Olympic ranges and into the Cascades. Thus, the
rainfall footprint is much more heavily skewed towards the Cascades
getting much heavier rainfall than the windward side of the coastal
ranges. That said, it will hardly be dry along the coast. Expect an
additional 3-5 inches of rain in the foothills of all of the
Washington Cascades through the period, while the coastal ranges
can expect just 1-3 inches of new rainfall. Given this setup, it
appears the most likely day where a future upgrade may be needed to
the ERO risk areas will be on Wednesday. This of course will be in
no small part due to the antecedent rainfall in nearly all areas
from both Monday and Tuesday, compounding high water concerns day
by day.

...Northern Idaho and Adjacent Areas...

A secondary peak of heavy precipitation into northern Idaho is
expected on Wednesday. The same A.R. impacting areas further west
will continue into the interior of the Pacific Northwest on
Wednesday as well. Renewed upslope flow into the west facing slopes
of the Rockies will re-enhance precipitation rates, as well as
raise snow levels after having crashed due to a brief cold air
intrusion on Tuesday. Thus, expect another round of snowmelt to go
along with the heavier rainfall rates. Just as further west, here
too the forecast will need to be monitored for a potential Slight
Risk upgrade with future updates. A large contributing factor to
the need for upgrades in both areas will be seeing what impacts
there are from the heavy rainfall expected especially today, but
possibly also again on Tuesday.

Wegman

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