Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
223 PM EST Wed Dec 10 2025
Day 2
Valid 12Z Thu Dec 11 2025 - 12Z Fri Dec 12 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
NORTHERN IDAHO...

...Northern Idaho...

Thursday will be the final day of the atmospheric river across all
of the Pacific Northwest, including into Idaho. A fast moving
shortwave trough diving southeast in the jet streak will
reinvigorate the heavy rain across northern Idaho and western
Montana during the day Thursday. This shortwave will also usher in
an Arctic front to the east of the Rockies across much of the rest
of Montana. This much colder and drier air will finally allow
conditions to dry out as upper level ridging shoves what's left of
the A.R. northward into Canada, and a large low well out over the
Pacific cuts off the tropical feed as it moves towards Alaska. The
ceasing of tropical moisture will effectively end the heavy rain
threat everywhere, though lingering flooding is likely to continue
in this area for multiple days to come.

Heavy rain will likely be ongoing across northern Idaho at the
start of the period on Thursday morning. A surface low will form
along the advancing Arctic air, which will draw the Arctic front
south across much of Montana Thursday night, turning what moisture
is left over to snow east of the range, while putting northern
Idaho into the "dry slot", ending the heavy rainfall. While rain
rates may stay elevated for much of the day Thursday across
northern Idaho, the Slight Risk is at least equally for the ongoing
impacts going into the period from the heavy rain that is likely to
occur tonight. Rock and mudslides will remain a threat until
conditions begin to dry out Thursday night, as well as a
continuation of both ongoing flooding and potentially some new
flooding as the rain ends across the area.

...Western Washington...

A Marginal Risk remains in place across portions of Western
Washington, specifically the foothills of the Cascades on Thursday.
The Marginal Risk reflects the end of the heavy rainfall from the
A.R. from Wednesday and Wednesday night...i.e. diminishing rainfall
rates...and the likely much higher impacts that will be ongoing by
then along all of the Washington Cascades. Thus, expect Slight
level impacts over this corridor to continue, but the heavy rain
ending should spell the end of new or worsening impacts going
forward. The Marginal attempts to strike a balance between these
two competing concerns.

Meteorologically, the A.R. will still be in full swing at the start
of the period 12Z Thursday, but as the forcings from inland lows,
rapidly weakening westerly flow orthogonal to the ranges, and
surface high pressure builds in from the south, rain rates will
rapidly diminish through the morning, ending entirely for many
areas of western Washington by Thursday night. Since the tropical
flow responsible for the A.R. will be cut off by Thursday night,
this will be the end of the event for the area, and a much needed
and "mostly" dry period of weather will follow across most of
western Washington, and those areas that do see additional rain
into the weekend will find it will be a fraction of the intensity
that this A.R. produced, and should be essentially non-impactful.

Wegman

Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt