231
FOUS30 KWBC 160031
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
731 PM EST Mon Dec 15 2025

Day 1
Valid 01Z Tue Dec 16 2025 - 12Z Tue Dec 16 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
WASHINGTON AND OREGON TODAY AND TONIGHT...

While IVT values continue to drop off across western
Washington/Oregon as the moisture axis pushes east and south, a
renewed surge of heavy rainfall is possible around the coast of WA
at the end of the period early Tuesday morning. Snow levels will
lower to near 3000 ft for the northern Cascades by 12Z Tuesday.

The Slight Risk was maintained given the wet antecedent conditions
and ongoing rainfall. The potential will exist for an additional 1
to 2 inches of rain through 12Z (maximum values), with hourly rain
amounts maximizing in the 0.25-0.50" range where weak instability
combines with strong left- exit jet stream forcing and continued
onshore flow. The main adjustment to the ERO areas was to remove
portions of their southern areas in OR where the 18z HREF no
longer supports heavy rains.

A surge of heavier rainfall impacting portions of the northern
Rockies early on with hourly rain amounts of 0.5"+ should weaken
overnight.

Roth


Day 2
Valid 12Z Tue Dec 16 2025 - 12Z Wed Dec 17 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE PACIFIC NORTHWEST...

Maintained the previously issued Marginal Risk area from
Washington southward along the Cascades and the coastal ranges into
northwest California on Tuesday and early Wednesday. Latest
deterministic and ensemble guidance showed the highest QPF in the
complex terrain...rivaling the QPF on Day 1 in some places in
Washington. The main reason for holding with a Marginal Risk area
was due to the lowering snow-levels given post-frontal lowering of
heights/thickness values that result in snow. Even so...did expand
the eastern boundary of the Marginal Risk area a bit to the east in
Washington and a portion of nearby Oregon. While this resulted in
some additional overlap with snow areas...the idea was to capture
some of the higher QPF amounts from regions where (presumably) the
precipitation started as rain and then transitioned to snow. Tried
to avoid overlap with the Winter Weather Desk snow amounts of 6
inches or greater.

The overall synoptic pattern continued to show the deepest moisture
and broadest coverage of rainfall arriving in association with the
surface cold front around 17/00Z and the diminishing QPF as the
area gets whisked eastward.

Cook/Bann

Day 3
Valid 12Z Wed Dec 17 2025 - 12Z Thu Dec 18 2025

...THE MARGINAL RISK OF EXCESSIVE RAINFALL AREA LINGERS OVER PARTS
OF THE PACIFIC NORTHWEST. ON WEDNESDAY...

There should be a relative lull in activity across parts of
Washington...Oregon and northern California in the wake of a system
on Tuesday into early Wednesday. Even though there may still be
some rainfall that lingers into Day 3...the Marginal Risk area was
maintained in deference to the approach of the next and seemingly
stronger AR on Day 4 and to account for the possibility that the
guidance could be too slow with the onset of rainfall with this
system. Refer to guidance from the WPC Medium Range Desk for
additional info. Even if the rainfall associated with the next
system does not arrive prior to 12Z Thursday as shown by the
models...any lingering rainfall still has the potential for be
impactful in the most hydrologically sensitive areas.

Cook/Bann


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