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FOUS30 KWBC 101542
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1042 AM EST Wed Dec 10 2025

Day 1
Valid 12Z Wed Dec 10 2025 - 12Z Thu Dec 11 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF WESTERN
WASHINGTON AS WELL AS FOR PORTIONS OF NORTHERN IDAHO AND FAR
WESTERN MONTANA...


16z update...
Prior discussion/thinking remains on track with respect to
magnitude, timing and placement to the rainfall forecast over
western Washington and downstream into the northern Rockies of
ID/W MT. The core/peak of the AR is currently ongoing and is
expected through 00z with 800-900 kg/m/s of fairly orthogonal
unidirectional flow through depth. After 00z, values will decrease
to 500-600 J/kg but persist through the remainder of the forecast
period, 11.12z.


Major to Record riverine flooding is expected/ongoing across the
western slopes of the Washington Cascades, yet rates remain between
.25-.5" with only very highest orthogonal (westward) facing peaks
having the potential of an hour or two reaching 2/3rds to .75"/hr.
Additional 3-5" for the foothills to peaks (perhaps 6"+) of the
Olympics/Washington Cascades over the next few hours. Recent NASA
SPoRT LIS 0-40cm soil saturations are over 90% and well into the
80-90th percentiles which is very impressive given the time of year
is naturally high. As such, the additional rainfall is almost
completely going to run- off further swelling the flooded river and
potential for landslides continues to increase.

Downstream, the IVT values are steadily increasing from 250
currently toward 500 kg/m/s after 00z, while also shifting
northward from the Bitterroot/Clearwater Range centering just south
of the Canadian border toward the end of the forecast. With less
overall moisture mainly filtering through the Columbia river and
Plateau of E WA, rates from orographic ascent will be lighter
reaching .10-.25" with peaks near .33". As such, additional totals
of 2-3" are possible. Soil saturation are also well above normal
(99th percentile) and over 70-80% suggesting solid run-off and
rivine flooding concerns to continue here as well too.

Given the higher confidence in placement/timing, only small
adjustments were required to the current Day 1 categorical areas.

Gallina


~~~~Prior Discussion~~~~

...Western Washington...

An atmospheric river is currently impacting much of Washington
State this morning. The narrow but intense plume of deep tropical
moisture originating from near Hawaii is being drawn northeastward
by a strong subtropical high that is parked off the California
coast. This area of high pressure is forming the southeast side the
atmospheric river. Meanwhile, polar high pressure over the
Aleutians is sending copious Arctic air well to the south through
the North Pacific. This cold air has set up a strong baroclinic
boundary as it runs into the warm air associated with the
subtropical high. Finally, a strong area of low pressure, fed in
part by this same baroclinicity, will move into the British
Columbia coast today. The low and it`s trailing cold front moving
into the coast is adding substantial lift to the atmosphere where
the A.R. is located in western Washington. The result of these
various forcings is a "hose" of deep tropical moisture aligned
right into Washington State, then continuing inland through Idaho
and Montana. The low and its front are locally enhancing the rain
rates, especially on the upwind/west-facing slopes of the coastal
ranges and the Cascades. This latest round of heavy rain is coming
after 2+ days of nearly steady rainfall into this same region.
Saturated soils, full rivers, and abundant additional rainfall are
all contributing to flooding concerns through this area.

Heavy rain is likely to continue for much of the day, gradually
diminishing in intensity tonight, but unlikely to stop completely
most anywhere in western Washington. Since the forecast is little
changed, particularly with expected rainfall amounts, the Slight
Risk area is also little changed. A higher end Slight remains in
effect for portions of the western slopes of the Cascades. Very
high snow levels between 8,500 and 9,000 ft are keeping all but the
northernmost Cascades as rain. A wedge of colder air trapped in the
northern Cascades are locally keeping snow levels there down closer
to 6,500-7,000 ft. The higher end Slight includes much of the I-90
corridor between Seattle and Snoqualmie Pass. Land and rockslides
remain a concern, and numerous Flood Warnings remain in effect for
much of the western slopes of the Cascades.

...Northern Idaho and Far Western Montana...

The Slight Risk of excessive rainfall also remains in effect for
much of the northern Idaho Panhandle. This includes the I-90
corridor roughly between Coeur d`Alene on the west side to Missoula
on the east side. The strong atmospheric river moving in from the
Pacific will drop the lion`s share of its moisture into the coastal
ranges and Cascades. However, plentiful leftover moisture will then
continue east and be re-lifted into the Bitterroots of Idaho and
Montana. While rainfall amounts will be tempered greatly compared
to areas further west, they will still be significantly impactful
for this region, especially when considering it`s been raining
off-and-on in this region from this same atmospheric river for the
past 48-72 hours. Thus, soils are saturated and rivers are already
running high in this area. All additional rainfall will convert to
runoff, both worsening and prolonging any resultant flooding
impacts. The area south of I-90 and east of Lewiston with west-
facing slopes will be hardest hit. It is in this area that a higher
end Slight remains in effect.

The heaviest rain across this region will be through the day today,
with some modest diminishing of rainfall rates through tonight.
Regardless, most upslope areas should see steady rainfall
throughout this period.

Wegman

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 3
Valid 12Z Fri Dec 12 2025 - 12Z Sat Dec 13 2025

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

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