FOUS11 KWBC 230705
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 AM EDT Thu Oct 23 2025

Valid 12Z Thu Oct 23 2025 - 12Z Sun Oct 26 2025

...Colorado Rockies...
Day 1...

A compact upper low will traverse the Four Corners today, pushing
height falls and accompanying modest mid-level diverge into
Colorado through tonight. This ascent will work into moistening
low-levels as SW flow pushes PWs to above the 90th percentile (in
some places above the 95th percentile) of the CFSR climatology. The
combination of synoptic lift and periodic upslope flow into this
saturating column will drive periods of moderate snowfall into the
terrain of the Colorado Rockies. The combination of high snow
levels (generally 10,000 - 11,000 ft) with modest total ascent and
peak precipitation occurring during daytime hours should limit
overall snowfall. However, WPC probabilities are high (>70%) for at
least 4 additional inches of snow in the higher terrain, especially
across the Sawatch and Mosquito Ranges where locally 12" of snow is
possible (10-30% chance).


...Pacific and Interior Northwest...
Days 2-3...

An active pattern begins in the Northwest starting late Friday and
persists through the weekend as an atmospheric river (AR) surges
into the Pacific Northwest and then spreads moisture inland.

This AR will be created by an impressive mid and upper level trough
which will amplify over the Northeast Pacific ocean and then shunt
southeast, reaching the coast by Sunday morning. There is very good
agreement amongst both the ECENS and GEFS members for resulting IVT
approaching 750 kg/m/s pushing onshore D2, suggesting a strong AR
event for the region. As this AR pushes southeast in response to
the approach of the trough, forcing for ascent will additionally
increase through resultant height falls/divergence downstream of
the primary trough, and thanks to an accelerating upper jet streak
placing favorable LFQ diffluence into the Northwest on Saturday. At
the same time, this evolution will drive a frontal wave and
associated cold front eastward into the Northern Rockies by
Saturday morning, with a secondary impulse traversing the region
during D3. Together, this will produce a long period of moderate to
robust ascent, with lowering snow levels.

Initially, snow levels during D2 will be generally 6000-7000 ft, so
above all but the highest passes. However, a rapid fall in snow
levels behind the first front will combine with steepening lapse
rates beneath the upper trough and impressive mid-level confluence,
suggesting intense ascent into favorable upslope terrain. This
should allow for snow levels to crash, and the NBM indicates snow
levels falling to as low as 3500 ft in the Cascades and 4500 ft in
the Northern Rockies by 12Z/Sunday. It is possible with the strong
ascent and steep lapse rates, snow levels will be even lower than
that, reflected by NBM 10th percentile snow levels falling to 2500
ft/4000 ft, respectively. This will be accompanied by climbing
SLRs, with early season, abov-climo SLR likely the latter half of
the event.

With the incoming AR, snowfall probabilities gradually increase and
expand across the region. For D2, WPC probabilities for more than 4
inches are above 50% only in the highest peaks of the Cascades.
However, during D3 as moisture expands and snow levels crash, WPC
probabilities for 4+ inches reach above 70% across much of the WA
and OR Cascades above 4000 ft, and extend into portions of the
Salmon River and Sawtooth Ranges as well. Additionally, as snow
levels crash, several inches of snowfall will create hazardous
travel at many of the Cascade Passes including Stevens, Washington,
and Santiam Passes


The probability of significant ice across the CONUS is less than
10 percent.

Weiss


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