Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
635 PM EST Tue Dec 16 2025

Day 1
Valid 01Z Wed Dec 17 2025 - 12Z Wed Dec 17 2025

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

Both the 18z HREF and 12z REFS paint a similar QPF picture through
12z.  The threat of heavy rainfall across portions of northern
California continues through the period, while a renewed surge of
heavy rains impacting portions of WA early on shifts south across
western OR with time. IVT values are well above normal due to the
strength of deep layer flow, peaking near 900 kg/m/s. An additional
1-2" for the lower coastal ranges are expected with local amounts
to 4" across portions of the Olympics, Cascades, and terrain of
northern CA by 12z. Only cosmetic changes were made to the broad
Marginal Risk of Excessive Rainfall.

Roth


Day 2
Valid 12Z Wed Dec 17 2025 - 12Z Thu Dec 18 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann/Kleebauer


Day 3
Valid 12Z Thu Dec 18 2025 - 12Z Fri Dec 19 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF
WESTERN OREGON AND PORTIONS OF ADJOINING STATES...

20Z Update... Changes were minimal with the forecast remaining
steadfast through the 12z NWP suite. The threat for significant
rainfall remains elevated across the western half of Oregon with a
focal point centered over the southern Cascades and the adjacent
valleys on either side. The key in this threat is the projected
"spillover" of heavier precip to the leeward side of the Cascades
due to a pronounced 700mb jet protrusion inland from the Pacific
leading to significant Pacific moisture breaching the terrain and
allowing for heavier precip to make it downwind of the mountains.
This is the area where ARI forecasts are hovering between the
25-100 year exceedance intervals, matching the previous output from
overnight's forecast. This allowed for general continuity in the
SLGT risk area with modifications only made to match to the latest
QPF footprint with emphasis on where 2+" will fall in the D3 time
frame, especially away from the coastal plain.

Kleebauer

...Previous Forecast...

The next in a series of Atmospheric Rivers will arrive early
Thursday with its axis of greatest moisture transport being
directed primarily along the western coast of Oregon...with maximum
IVT values in excess of 800 kg per meter per second largely being
directed perpendicular into the coastal ranges early in the period.
Global models continue to advertise the possibility of 6 inches or
more of rainfall during the period...and comparing the WPC 24hr
QPF to ARI yielded a broad swath exceeding the 2-5 year return
intervals. It was also worth noting that the highest ARIs were just
to the east of the crest of the Cascades where an axis of 25-100
year exceedance intervals was depicted. This does seem like a
scenario where more spillover into the normally drier downslope
areas is plausible, so could be an impactful event even into some
of those areas. Consequently, the Day 3 excessive rainfall outlook
expanded the Slight Risk eastward in Oregon to cover areas with QPF
in excess of 5 inches and the highest ARI values mentioned above
given the concern for spillover. Snowfall within this area should
be confined to the higher elevations.

Bann


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt