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FOUS30 KWBC 080815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Mon Dec 8 2025

Day 1
Valid 12Z Mon Dec 08 2025 - 12Z Tue Dec 09 2025

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

...Western Washington and Oregon...

The inherited Slight Risk across Western Washington and Oregon
remains little changed this morning. A hauling 150+ kt upper level
jet will bring a vigorous shortwave trough into British Columbia
north of the Canadian border Monday afternoon and evening. The
trailing front associated with the surface low that is being
strengthened by that shortwave will plow into the coast Monday
afternoon. Behind that disturbance, mid-level 850 mb flow will
rapidly increase out of the due west at at least 50 kt, while PWATs
also rapidly increase from 0.75 inches to 1.25 inches as the head
of the incoming atmospheric river (A.R.) makes its way ashore. On
the synoptic scale, the overall rainfall footprint will be greatest
today as compared with the expected rainfall over the next few
days. This in turn will set the stage for multiple days of flooding
concerns as the rain continues through much of the upcoming
workweek. For 24 hour rainfall totals, expect a widespread 4-6
inches of rain for this period for much of the upslope areas of the
Olympics/Coastal Ranges/Cascades. The typical downslope areas in
due westerly flow, which includes metro Seattle and Portland,
should be spared the heaviest rainfall, with totals in the 1-2 inch
range in those metros likely to be more common. Soils in these
areas are already near saturation, which represents an average
amount of soil moisture going into today`s big rainfall event, so
at least the second half of the rainfall should largely convert to
runoff, leading to significant rises in streams, creeks, and
eventually the rivers that drain the Willamette and Puget Sound
areas.

The abundance of Pacific moisture that will be moving into the
Pacific Northwest with the atmospheric river is originating from
the tropics well west of Hawaii. This plume of moisture is moving
over multiple thousands of miles of Pacific Ocean before it reaches
the coast, which is further enhancing the amount of moisture
available to fall as rain once that moisture plume runs into the
wall of mountains that is the coastal ranges and the Cascades.
Strong subtropical high pressure centered off the California coast
is directing the moisture plume`s south side around its
northwestern periphery, while strong Pacific lows are drawing the
moisture northeastward on the plumes north side. This is acting to
focus locally enhance the rainfall within the moisture
plume/atmospheric river, which is why the heavy rain is
concentrated into western Washington and Oregon.

Expect that there will be a bit more "slop-over" of rainfall onto
the immediate downwind side of the Cascades and Olympics, so a few
minor expansions of the Marginal were made to include some of the
east-facing slopes of the Cascades, and a Slight Risk area
expansion was made on the east side of the Olympics.

...Northern Idaho and Adjacent Areas...

The inherited Marginal Risk was adjusted slightly to account for
small changes in the expected rainfall across interior areas of the
Pacific Northwest. The Marginal was trimmed from the Glacier N.P.
area due to most of the precipitation expected to fall as snow, but
expanded into far southeastern Washington and far northeastern
Oregon, where upslope potential will be increased in the terrain of
those areas. The A.R. will feature warm conditions, which will
raise snow levels to several thousand feet above ground level,
assuring that except in the highest elevations, the precipitation
type will be all rain, especially west of the Continental Divide.
While today should also bring the most rain to the area compared to
the upcoming several days, all of the soils in this area are nearly
saturated. In fact, recent colder weather and lower snow levels may
locally enhance the flash flooding threat once the heavier rain and
warmer temperatures move in as snowmelt becomes a contributing
factor to rising stream and river levels.

Wegman

Day 2
Valid 12Z Tue Dec 09 2025 - 12Z Wed Dec 10 2025

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

...Western Washington and Oregon...

The atmospheric river (A.R.) plume on Tuesday morning will be
aimed at the northwestern quadrant of Oregon, after the plume
shifts south down the coast Monday night. The A.R. will remain
across northwestern Oregon for much of the day Tuesday. Expect an
additional 3-5 inches of rain across the west facing slopes of both
the coastal ranges and the Oregon and far southern Washington
Cascades during the day Tuesday. This will have followed the
roughly 3-5 inches of rain expected Monday night. Thus, expect high
creek, stream, and river levels by Tuesday night after all that
rainfall, as well as low-lying and urban flooding impacts. The
Slight Risk area remains largely unchanged.

By Tuesday night, a much weaker low than Monday`s will track
northeast into British Columbia. This low will draw the A.R. back
towards the north before its trailing cold front plows into the
coast. This will reignite the heavy rainfall across much of western
Washington State, while rain rates trail off across west-central
Oregon. Unfortunately the heavy rain will continue into the coastal
ranges and far northern Oregon Cascades through the period. Expect
another 1-3 inches of rain along the Columbia River Tuesday night.
Rain rates will be notably lower than on Monday due to the weaker
low and a slightly weaker onshore flow at 850 (40-50 kts). Due to
the return of steady rain across the Olympics and central Cascades
Tuesday night, the inherited Slight Risk area was expanded north to
include portions of those ranges and their foothills. Meanwhile,
the highest peaks of the Cascades should still remain cold enough
for mostly snow.

...Northern Idaho and Adjacent Areas...

Weakening, but continuous 20-40 kt westerly flow at 850 across the
plains of eastern Washington will allow for locally heavy upslope
rain to continue into much of northern Idaho and northeastern
Oregon. The inherited Marginal is little changed, and mostly
reflects carving out the highest peaks expected to remain all or
mostly snow, while expanding into low lying areas where forecast
rainfall has increased. Regardless, snow levels will remain plenty
high enough to ensure all populated lower lying areas remain all
rain, and thus continue to pose a flooding threat.

Wegman

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...

...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 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