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FOUS30 KWBC 100056
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
756 PM EST Tue Dec 9 2025

Day 1
Valid 01Z Wed Dec 10 2025 - 12Z Wed Dec 10 2025

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

...Western Oregon and Washington...

The atmospheric river event will continue across the Pacific
Northwest tonight as the next surge of heavy rain reaches
Washington. As of 00Z, steady light to moderate with occasional
heavy rain was ongoing across northern Oregon, but rates have
tapered off compared to earlier Tuesday morning. Ahead of an
approaching cold front, a plume of IVT with magnitudes in the
500-750 kg/m/s range along the northern Oregon coast will translate
north and increase in magnitude into the 800-1000 kg/m/s range
along coastal Washington into the southern portions of the Puget
Sound through Wednesday morning.

The probability of hourly rainfall in excess of 0.5 inches will
increase overnight, especially into the 06-12Z time frame across
the Olympics into the northern Cascades. 850-700 mb mean layer
winds of 50 to 60+ kt are expected across western Washington,
coupled with precipitable water values of roughly 1.2 to 1.4
inches. Snow levels will also increase with the surge of warmer air
lifting north, with all rain below elevations of 9,000 to 10,000
feet.

Peak 12 hour rainfall totals through 12Z Wednesday are forecast to
range between 2-4 inches from the Olympics into the northern
Cascades. The forecast rain rates and additional rainfall through
12Z carry an increased flood threat on area rivers/streams due to
roughly 5 to 10 inches of rain which have fallen over the past 3
days, most over the past 24-36 hours. In addition to potential
flooding of low-lying and urban locations, the potential will also
exist for landslides and debris flows due to the increase in
rainfall intensity overnight.

...Northern Idaho and Adjacent Areas...

The Marginal Risk across northern Idaho into neighboring western
Montana, northeastern Oregon and southeastern Washington remains
unchanged with this update. Similar to coastal locations of the
Pacific Northwest, heavier rainfall intensities are expected in the
06-12Z time frame tonight. Hourly rainfall potential will increase
up to and beyond 12Z Wednesday with peak values generally under
0.3 inches, but the Blue Mountains of northeastern Oregon will have
potential for peak hourly rainfall of 0.3 to 0.4+ inches
overnight. Total peak 12 hour rainfall of 1 to 1.5 inches is
expected (close to 2 inches for the Blues) through 12Z with
snowmelt contributing to rising streams and rivers. Given increased
soil moisture due to antecedent rainfall, localized impacts of
heavy rain will remain possible.

Otto


Day 2
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...

2100 UTC update: No significant changes made to the previous
outlook areas across the coastal Pac NW and Northern Rockies. Edits
to the previous areas were mostly to better fit latest qpf and
snowfall forecasts.

Oravec

...Previous discussion...

...Western Washington...

The wettest day of the next 3 will be on Wednesday across much of
western Washington. The continuous westerly flow of 40-50 kt at 850
will continue off the Pacific through the day. A vigorous jet
streak to 150 kt will remain just north of Washington before diving
southeastward across the upper high Plains. This will keep
Washington in the favorable right entrance region. A series of lows
will barrel into the British Columbia coast, each one keeping the
A.R. plume nearly stationary and highly energetic. Thus, widespread
4-6 inch rainfall totals are expected for the Olympics and the west
facing foothills of the Washington Cascades. As with most A.R.s,
very high snow levels will keep any snow confined to the highest
peaks of the far northern Cascades. This will keep the lion`s share
of the expected precipitation as heavy rain for most of the
Cascades and certainly all of the coastal ranges as well. This
being the third day of more-or-less steady rainfall across much of
Washington, especially south of the Puget Sound area means the
soils are well past saturated, and all of the precipitation should
convert to runoff. Further, the high snow levels will promote
snowmelt as well, which will only further raise river levels.
Finally, the lows moving into the British Columbia coast will turn
the surface flow out of the WNW for a time, especially Wednesday
night. This will direct the flow straight down the strait of Juan
the Fuca and put the low-level flow just slightly off from
orthogonal to the mountains, favoring a slight northerly flow of
moisture down the foothills and the Puget Sound Region. This should
temper the downsloping side east of the mountains just a bit,
allowing for more rainfall into Seattle, Olympia, and the Puget
Sound region broadly. Thus, Wednesday and Wednesday night will also
be the wettest day in the urban cities as well.

Given all of the above, the inherited Slight Risk was expanded with
this update to include the Seattle metro and the surrounding areas
around southern Puget Sound with this update. A higher end Slight
was expanded south down the Cascades to near the Columbia
River/Oregon state line. Once the impacts from all the rainfall
seen across Western Washington become more clear, it will be easier
to ascertain what the impacts from this round of heavy rain on
Wednesday are likely to be. A Moderate Risk upgrade may be needed
with future updates or all or a portion of the Cascades and western
foothills with future updates.

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

The atmospheric river (A.R.) impacting Western Washington on
Wednesday will continue inland and impact the Rockies of northern
Idaho and western Montana as well on Wednesday. Just like further
west, the A.R. will cause snow levels to rise to as high as 8,000
ft. This will introduce a significant snowmelt component to the
resulting runoff from the upslope rainfall. Over the last 24 hours,
the area has picked up between 1-2 inches of rain, causing soils
to become fully saturated. Today, expect even more rain, with 2-3
inch totals expected to be common. The steady rain, snowmelt, and
abnormally warm conditions will increase the risk for avalanches in
the prone areas, especially where a heavy wet snow falls on top of
much weaker snow layers. Also expect an increase in rock and
landslides, as well as rising streams, creeks, rivers, and
localized urban flooding. Given all these expected impacts, a large
portion of northern Idaho and far western Montana were upgraded to
a Slight Risk with this update, and a smaller portion of northern
Idaho along the Montana state line from I-90 south is in a higher
end Slight Risk, as this area is expected to see the highest
rainfall amounts to around 3 inches through the period, almost all
of which falling as rain.

Wegman

Day 3
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...

2100 UTC update: The previous slight risk area over northern ID was
decreased in size to better fit the latest day 3 qpf. The previous
marginal risk area was removed along the WA coast, into far
northern OR coast based on the latest light day 3 qpf across these
areas. The marginal risk was confined to the upslope area of the WA
Cascades.

Oravec

...Previous discussion...


...Northern Idaho and Western Montana...

A double upgrade from no risk area to a Slight Risk was introduced
for a portion of northern Idaho, generally south of I-90 and along
the Montana state line with this update. This upgrade was
introduced largely as a continuation of heavy rain from Wednesday
night across this region. Rainfall rates should be gradually
diminishing through the day Thursday, and as far as the heaviest
rain and immediate impacts, the rain on Wednesday should be more
impactful in this region. However, given the steady rain continues
into the day Thursday, the impacts resulting therefrom should also
continue. Meanwhile further east into Montana, a strong Arctic
front pushing south down the Plains should turn all of the
precipitation over central Montana over to snow. Thus, the flooding
threat is largely confined to northern Idaho and far western
Washington where the warm air associated with the A.R. will remain
in place on Thursday. Rock slides, avalanches, low-lying flooding,
sporadic urban impacts, and very high river levels will all be
ongoing Thursday morning over this area, supporting the Slight Risk
upgrade.

...Western Washington...

A Marginal Risk was expanded mainly for the Thursday morning time
period across much of western Washington with this update. This
expanded Marginal Risk includes the Seattle metro and the Puget
Sound area more broadly. As with further east, it will be a
continuation of the steady rain from Wednesday night into Thursday
across the area. It`s likely a Slight Risk upgrade will be needed,
as the rain looks to take its time until Thursday afternoon to
significantly diminish in intensity. The A.R. event broadly will be
diminishing through this period. A large low well out over the
Pacific will take a more northward track, as compared to its
predecessors, as the pattern amplifies. This will effectively cut
off the A.R. as the moisture plume gets directed north towards the
Gulf of Alaska, and therefore, the flow with the A.R. shifts north
into British Columbia and substantially diminishes.

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