045 
FOUS30 KWBC 151559
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1059 AM EST Mon Dec 15 2025

Day 1
Valid 16Z Mon Dec 15 2025 - 12Z Tue Dec 16 2025

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

...16Z update...

IVT values approaching 1000 kg/m/s were found offshore and to the
coastline of southern Washington/northern Oregon between 12-15Z 
this morning with 60-70 kt of flow in the 850-700 mb layer. Peak 
3-hr rainfall of 1.0 to 1.3 inches was observed for the Olympics 
and isolated spots within the northern Washington Cascades ending
15Z. IVT values will drop off across western Washington/Oregon 
through the day today as the moisture axis pushes east and south,
out ahead of a cold front. Snow levels will also fall behind the
cold front to about 6000 ft by 00Z and 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 2
to 3 inches of rain through 12Z (maximum values), although rain
rates will be much lower than this morning with occasional 0.25+
in/hr rates and perhaps an isolated spot with 0.5 in/hr where weak
instability combines with strong left-exit jet stream forcing and
continued onshore flow.

Elsewhere, the forecast remains on track with a surge of heavier
rainfall likely to impact portions of the northern Rockies over the
next 3-6 hours followed by locally high rain rates in excess of
0.5 in/hr for the coast of Oregon into northern California where 
the Marginal Risk was maintained.

Otto

...previous discussion follows...


Overall...the changes were fairly minor as yesterday's Day 2
Excessive Rainfall Outlook propagated into today's Day 1 outlook.
There were a few run to run differences in the deterministic and
probabilistic runs...especially over northeast Washington- northern
Idaho and northwest Montana. The broad forecast reasoning did not
fundamentally shift.

The areal coverage of rainfall expands and rain rates increase
early today as a well defined atmospheric river continues to
direct abundant moisture into the Washington coastal ranges and
Cascades. IVT values in excess of 700 kg per meter per second early
today will be spreading inland but weaken as the axis makes its way
inland and as the axis of best moisture transport gets directed
southward into Oregon. Given antecedent conditions...the forecast
of another 3 or 4 inches in the terrain raises concerns of worsening
any on-going flooding and result in new areas. In
addition...considering how water-logged the soils became in the
recent heavy to excessive rainfall event...the potential is there
for additional landslides or mudslides. Latest guidance still
showed the nose of the moisture plume getting as far east as parts
of eastern Washington....northern Idaho and northwest Montana where
a second and more focused Marginal Risk area remained. The focus
of the heavy rainfall should gradually shift southward into an area
that did not receive as much rainfall from the previous atmospheric
river and has had a chance to drain off some of that water.

Bann

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.

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.

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