Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 AM EDT Fri Oct 24 2025

Valid 12Z Fri Oct 24 2025 - 12Z Mon Oct 27 2025


...Pacific Northwest through the Central Rockies...
Days 2-3...

Complex mid-level evolution will result in active weather expanding
across the Northwest and eventually into the Central Rockies
through the weekend.

The primary driver of this development will be an anomalous and
persistent 500mb low over the Northeast Pacific that will gradually
advect east to come onshore near the Olympic Peninsula Sunday
morning. Downstream of this feature and before the onshore
movement, impressive and pinched SW flow will funnel an atmospheric
river (AR) into the Northeast characterized by IVT which may exceed
750 kg/m/s (50-70% chance). This will rapidly moisten the
atmospheric column, and as forcing for ascent increases through
height falls, PVA, and the LFQ of an accompanying Pacific jet
streak, periods of moderate to heavy rainfall will result from
Friday night through Sunday night. There are modest discrepancies
among the various deterministic models and accompanying ensembles,
but in general the consensus is good for significant rainfall
across much of the Pacific Northwest, with moisture spilling well
eastward Saturday night, with a second round possible Sunday night
as a secondary, more zonally oriented and weaker, AR pivots
eastward.

Initially, the pronounced SW flow will warm the column such that
snow levels are above most passes (6000-8000 ft). However, a cold
front accompanying the first wave embedded within the AR (or just
behind it) will combine with the height falls to crash snow levels
to as low as 4000 ft by Saturday morning in the Pacific Northwest,
with the advection of this cold front eastward driving snow levels
down to 4000-4500 ft in the Northern Rockies by Sunday morning. The
secondary surge of moisture and continued cooling as a low pressure
moves into British Columbia will help drive snow levels down even
further late D2 into D3, with the NBM featuring mean snow levels as
low as 3000 ft late Sunday in the Pacific Northwest (rising to
around 5000 ft in WY). However, despite forcing weakening during
this time, steep lapse rates may allow precipitation to drag cold
air even further down towards the surface, so once again the
NBM10th percentile for snow level, around 2500 ft in WA to around
3500 ft in MT/WY may be more realistic as the level for at least
minor accumulating snow and accompanying transportation impacts.

This will be a long-lasting event that will occur in waves, so
impacts will be drawn out, and the heaviest snow is likely where
upslope flow is most pronounced. Day 1 /Friday and Friday night/ is
likely to be the quietest of the period and WPC probabilities for
4+ inches are modest (just 10-20%) and confined to the highest WA
Cascades. By D2, however, precipitation and snowfall become much
more expansive, and WPC probabilities become high (>70%) for 6+
inches along the spine of the WA and OR Cascades, and spill over
into the Sawtooth/Salmon River range of ID, primarily above 5000
ft. During D3 as the snow levels fall more considerably, WPC
probabilities for an additional 6+ inches continue above 70% in
much of these same areas, and expand into the NW WY ranges as well.
With snow levels falling, impacts to the passes become more notable
late D2 into D3 as well, with moderate impacts likely across the
Cascades including at Santiam and Stevens Passes.


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

Weiss