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