FOUS11 KWBC 250818

QPFHSD



Probabilistic Heavy Snow and Icing Discussion

NWS Weather Prediction Center College Park MD

418 AM EDT Sat Oct 25 2025



Valid 12Z Sat Oct 25 2025 - 12Z Tue Oct 28 2025





...Pacific Northwest through the Central Rockies...

Days 1-3...



The core of the atmospheric river (AR) which has been progged to

lift into the Northwest for several days now will be advecting

onshore this morning and dropping south into CA and the Great Basin

through the day. IVT exceeding 500 kg/m/s is likely (>70% chance)

which will also be above the the 99th percentile according to NAEFS

within a narrow channel focused from CA into the Northern Rockies.



Into this moistening column, ascent will maximize downstream of the

primary upper low which will track northeast into British Columbia

tonight. South of the core of this low, impressively confluent

mid-level flow will help persist moisture advection such that much

of the region will experience above normal PWs leading to periods

of moderate to heavy precipitation as height falls, PVA, and

diffluence combine atop the area. This will yield two round of

heavy precipitation, the first today, and the second Sunday aftn

and Sunday night in response to secondary shortwave energy lifting

onshore near Oregon. Although there continue to be intensity

differences among the various global guidance, the end result in a

long period of heavy precipitation, with snow becoming more

widespread as snow levels crash.



Initially, snow levels will remain high, generally around 7000 ft

within the core of the AR, but falling to 4000-5000 ft behind the

accompanying cold front and in response to the associated height

falls. Snow levels will then continue to drop on Sunday with the

secondary shortwave energy, becoming as low as 3500 ft in the

Cascades and 4500 ft farther south and east. However, with steep

lapse rates and periods of impressive omega, cold air could be

dragged down by heavy precipitation rates (snowfall rates above

1"/hr at times) such that accumulating snow may occur as low as

2500-3000 ft, or near the NBM 10th percentile. This will enhance

the areal footprint of snowfall, and also lead to more widespread

pass level impacts Saturday night and Sunday.



The heaviest and most widespread snowfall this period is expected

today through Sunday. For today, WPC probabilities are high (>70%)

for 6+ inches of snow across the spine of the Cascades of OR and

WA, as well as into the higher terrain of the Olympics and Sawtooth

ranges. Locally, 1-2 feet of snow is likely in the highest terrain.

During D2, the snow intensity begins to wane across the Pacific and

Interior Northwest, although WPC probabilities suggest a moderate

to high chance (40-70%) for an additional 6+ inches in many of 

these same areas. However, in general the heaviest snowfall is 

expected to shift into the area around Yellowstone NP including the

Wind River and Teton ranges where WPC probabilities for 8+ inches 

of snow on D2 peak above 70%. With snow levels falling late D1 and 

through D2, pass-level impacts are likely with significant snowfall

accumulations probable at both Stevens and Santiam Passes.



The probability of significant ice across the CONUS is less than 

10 percent.





Weiss





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