Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 PM EDT Mon Oct 24 2022

Valid 00Z Tue Oct 25 2022 - 00Z Fri Oct 28 2022

...Pacific Northwest into the Northern & Central Rockies...
Days 1-3...

A pair of shortwave troughs are set to produce periods of snow
across the more mountainous terrain of the Pacific Northwest, the
northern Rockies, and the central Rockies. The first, and both
less amplified and more progressive of the shortwave troughs, will
traverse the northwestern U.S. tonight. Weak PVA, modest 700mb
moisture advection, and orographic enhancement will prompt period
of snow, falling heavily at times where upslope flow along
topographically-favored terrain is present. WPC PWPF for >6" of
snowfall is as high as ~60-70% in parts of the northern Cascades,
the Idaho Bitterroots, Montana's Lewis Range, and the Tetons of
western Wyoming. The experimental pWSSI shows a 20-40% chance of
"Moderate Impacts" in the Bitterroots, which suggests the
potential for hazardous travel conditions and possible
road/infrastructure closures closures. By Tuesday afternoon, a
second and more vigorous upper level disturbance will reach the
Pacific Northwest coast, providing yet another surge in 500-700mb
moisture to the region. The tallest peaks of the Washington
Cascades feature as much as an 80% chance for >8" of snowfall
according to the latest WPC PWPF.

From Day 2 (Wednesday) into Day 3 (Thursday), the upper trough is
expected to continue digging southeast through the Northwest on
Wednesday and into the central Rockies by Thursday. Wednesday's
highest PWPF becomes focused in the Tetons, Uintas, and central
Rockies. Probabilities for >4" of snow range between 40-70% along
these ranges. There are subtle differences in model guidance
members in the handling of the upper trough as it deepens into an
upper low on Thursday over western Colorado with some members
slower/stronger and other a tad faster/weaker. This can lead
subtle differences in totals using deterministic guidance. It is
worth noting the ECMWF EFI's Shift of Tails does contain locations
as far south and west as northern Arizona and New Mexico,
suggesting there are a handful of ensemble members to bring
anomalous snowfall to these regions. That said, most ensemble
guidance is keying in on the Colorado Rockies having the best odds
of seeing >6" snowfall totals. Probabilities from WPC's
experimental pWSSI are ~20-40% for seeing snowfall impacts
reaching "Moderate" criteria on Day 3 (Thursday).


The probability of significant icing is less than 10 percent
nationwide.

Mullinax