Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
725 PM EST Wed Dec 17 2025
Day 2
Valid 12Z Thu Dec 18 2025 - 12Z Fri Dec 19 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY OVER WESTERN
OREGON EXTENDING INTO A SMALL PORTION OF WASHINGTON AND
CALIFORNIA...

2000 UTC Update: Have added a 25% area -- which is an enhanced or
'higher- end' Slight Risk -- across eastern portions of the
Willamette Valley into the western slopes of the Cascades, while
also clipping the far eastern reaches of Portland metro along with
the western Columbia Gorge into far southern WA. The 12Z guidance
came in even wetter, owing to the strong AR which given better
sampling, the intensity of which is coming into better view now
within 24-36 hours of the main event. CW3E global ensemble guidance
(GEFS and ECENS) show peak IVT values ~1000 kg/m-s, while the
Western Region WRF mean peaks closer to 1200. Guidance QPFs show
widespread 4-8" amounts within 24hrs, with higher totals depicted
from the CAM (particularly in the 'enhanced' Slight Risk area). 12Z
HREF probabilities of QPF exceeding 5" within the Day 2 period are
90% or higher withing the enhanced Slight Risk area, while within
the same area, probabilities of exceeding 8" peak between 60-80+
percent. Both the HREF and RRFS show high probabilities of
0.50"+/hr rainfall rates between 16Z Wed through 12Z Thu (probs
over 50%; in many areas over 70-80%), with quite a bit of overlap
within the same areas through ~03Z Thu.

Therefore, the potential for small stream and/or river flooding
continues to increase with this event (per collaboration with the
National Water Center). However the potential for flash flooding
(more short-fused inundation), especially over burn scars, is also
increasing within the enhanced or 'higher-end' Slight Risk area.
Next shift will evaluate the potential need for a Moderate Risk.

Hurley

Previous Discussion below...

The next in a series of atmospheric rivers will arrive along the
west coast of North America early on Thursday. The threat for
significant rainfall remains elevated across the western half of
Oregon with a focal point for maximum precipitation centered over
the southern Cascades and the adjacent valleys on either
side...although a subtle northward shift was noted in the 17/00Z
guidance. 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 from the Pacific leading to significant
Pacific moisture being drawn inland. This is the area where ARI
forecasts are hovering between the 25-100 year exceedance
intervals, matching the output from the past several model runs and
deterministic QPF from WPC. As a result, Day 2 excessive rainfall
outlook was expanded the Slight Risk northward across portions of
the Washington Cascades in response to the subtle northward shift
noted earlier...with little reason to expand any farther eastward.

Bann


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