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

Valid 00Z Fri Oct 24 2025 - 00Z Mon Oct 27 2025

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

A compact upper low traversing the Central Rockies this afternoon
and overnight will result in height falls and modest mid-level
divergence over Colorado. The increased vertical velocities within
the atmospheric column will work to moisten low-levels as SW flow
pushes PWs above the 90th percentile (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 moderate-to-high (40-70%) 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 develops due to an impressive mid and upper level trough 
amplifying over the Northeast Pacific ocean that becomes shunt
southeast, reaching the Pacific Northwest coast by Sunday morning.
Both the ECENS and GEFS members depict an IVT approaching 750
kg/m/s pushing onshore on Friday, suggesting a strong AR event is
likely to impact the region. As the 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 >6" are
above 50% only in the highest peaks of the Cascades. However,
during D3 as moisture expands and snow levels crash, WPC
probabilities for 6"+ exceed 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/Mullinax