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FOUS30 KWBC 091550
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1050 AM EST Tue Dec 9 2025

Day 1
Valid 16Z Tue Dec 09 2025 - 12Z Wed Dec 10 2025

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

16z Update: Forecast still looks on track so only minimal changes
needed to the inherited risk areas. Impacts should ramp up after
00z as the atmospheric river lifts back north and into western WA
and IVT magnitudes increase.

Chenard

...Previous Discussion...

...Western Oregon and Washington...

The atmospheric river (A.R.) event is well underway across the
Pacific Northwest this morning. The plume of anomalous moisture
associated with the A.R. is currently aligned across northwestern
Oregon. Radar indicates rainfall rates are between 1/4 inch and 1/2
inch per hour. As is fairly typical of most A.R.s, the impact from
the rain isn`t necessarily from the instantaneous rates from heavy
rain, but rather the long duration of steady rain, which can be
heavy at times on the windward (west-facing) sides of the coastal
ranges and the Cascades. A strong and vertically stacked
subtropical high is parked well off the California coast.
Meanwhile, frequent lows are tracking northeast across the
northeastern Pacific and slamming into the Canadian coast. The
trailing cold fronts are tapping into the plume of moisture that is
riding the gradient between the strong high to the south and the
lows to the north.

A Pacific low moved into the British Columbia coast earlier this
evening. This shoved the A.R. to its current position into
northwestern Oregon. As the next low tracks a little further north,
the subtropical high will begin ridging towards the north in
response. This will push the A.R. north with time. A moving A.R. is
usually a very good thing, as it reduces the time it`s heavily
raining in any one area. Unfortunately this time, it will simply
move the A.R. back to the already hard hit areas of western
Washington that were hard hit yesterday. This northward shift is
expected this evening as the low moves ashore near Haida Gwaii, or
just south of the southern Alaska Panhandle. As the trailing front
follows into the Washington coast tonight, rainfall rates will
increase significantly as the A.R. aligns into Washington. This
will allow the rain rates to diminish substantially in most of
Oregon, though light rain near the Columbia River is likely to
continue to some much lesser degree. The A.R. is characterized by
PWATs to 1.25 inches, which is between 2 and 3 sigma above normal
for this time of year. Orthogonal westerly flow into the mountains
at 40-50 kt, which is plenty strong enough for substantial upslope
enhancement.

In Oregon, expect the heavy rain, ongoing at present, to continue
for much of the day, while gradually shifting north and weakening.
In Washington the heaviest rain will be overnight tonight. The
Slight Risk area was expanded north with this update to include all
of the Olympic Peninsula as well as for a portion of the foothills
of the Cascades east of Seattle. The surrounding Marginal was
trimmed on the southern end in central Oregon as the trend
throughout the period will be a northward readjustment of the A.R.

...Northern Idaho and Adjacent Areas...

The Marginal Risk into northern Idaho, a small sliver of far
western Montana, and the northeast corner of Oregon is largely
unchanged with this update. Over the last 24 hours, the area has
seen anywhere from 1-1.5 inches of rain. At least that much is
expected over many of the same areas of the Sawtooths, Bitterroots,
and west into northeastern Oregon. Since the A.R. is originating
from the tropics, in addition to the anomalous moisture, much
warmer air will also accompany the rain, causing snow levels to
rise. Expect a significant snowmelt contribution to the rising
streams and rivers as a result. A Slight Risk upgrade may become
necessary if rainfall forecasts increase further through the day.
NASA Sport soil moisture analyses show that soils all across
northern Idaho are at saturation, so all new rainfall today will
convert to runoff. The impact of this runoff is unclear, hence the
possibility of a Slight Risk upgrade.

Wegman

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...

...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...

...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