Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 PM EDT Tue Oct 25 2022

Valid 00Z Wed Oct 26 2022 - 00Z Sat Oct 29 2022

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

The focus for heavy snow continues to reside in the higher
elevations of the Intermountain West and Pacific Northwest. The
primary weather maker is a soon-to-be amplifying upper trough
diving through the Pacific Northwest tonight. The trough will be
responsible for periods of snow, falling heavily at times, along
the higher peaks of the Northwest. This includes mountain ranges
such as the Cascades, the Blue Mountains, the Sawtooth, and
Bitterroots. Latest WCP PWPF contains probabilities for >8" of
snowfall >70% in the tallest peaks of the Cascades tonight into
Wednesday. The trough will continue to amplify over the
Intermountain West Wednesday into Wednesday night, taking on a
sharper positive-tilt and resulting in 500mb height falls across
the central Rockies, forcing snow levels to crash as the upper
trough approaches. Strong 300-500mb PVA over the central Rockies
allows for strong vertical ascent atop the atmosphere while an
influx of 700-500mb moisture provides sufficient saturation aloft
to cause periods of snow over the region. The combination of
strong vertical velocities driven from both synoptic-scale ascent
and upslope flow along topographically-favored terrain likely
results in periods of heavy snowfall Wednesday night.

The wildcard in the forecast going into Day 2 (Thursday) is the
magnitude and speed of the developing upper low. The southern
Rockies could see a modest easterly component in the 850-700mb
layer over the central High Plains, leading to upslope enhancement
as far south as the Sangre De Cristo Range where the core of the
500-700mb low tracks just south of on Thursday. Latest WPC PWPF
highlights the central Colorado Rockies with up to ~60%
probabilities of snowfall >6" and up to a 30-40% chance for
snowfall >8". There are ~10% probabilities for >6" of snow in the
Sangre De Cristos of northern New Mexico and far southern Colorado
at this time. Farther northwest, another longwave trough in the
northeast Pacific will funnel additional Pacific moisture at the
northern Cascades, resulting in additional snowfall (>4" of
snowfall probabilities ~30% on Day 3) in elevated areas >6,000 ft.


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

Mullinax