Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Fri Nov 17 2023

Valid 12Z Fri Nov 17 2023 - 12Z Mon Nov 20 2023

...The West...
Days 1-3...

Upper low off the coast of California will weaken and move inland
on Saturday, spreading moisture into the Sierra with a few inches
above 8000-9000ft on day 1. Into day 2, a sharper northern stream
system will dive into the Pacific Northwest and into the Great
Basin, capitalizing on the remnant moisture over the Sierra
(precipitable water values over +1 sigma) and increased upslope
flow. WPC probabilities for at least 6 inches on day 2 over the
Sierra are at least 30% above about 7000ft. Farther north, strong
height falls and lowering snow levels will start to bring snow to
pass levels in the Washington Cascades by the end of day 2 (12Z
Sun) as snow levels plunge from 6500-7000ft down to around 4000ft
by the start of day 3. The heaviest snow over the OR Cascades is
forecast for late Saturday into Sunday, crossing over between days
2-3, as strong upslope will favor snow accumulations well over 6
inches in the higher terrain. On day 3, the sharp trough will
continue through the Great Basin with lowering snow levels and
snow for the mountains. Lingering pooling of moisture in the
region (PW values around +1 to +2 sigma) will help to squeeze out
more than 6 inches to elevations above 8000ft across UT into the
CO Rockies where probabilities are above 50% (and perhaps light
snow to some of the valley floors as well). To the north, WPC
probabilities are just a bit lower (20-40%) for at least 6 inches
in parts of the Sawtooths, Bitterroots, and Tetons.

...Northern Maine and the Northeast...
Day 2...

A coastal low will track northeastward into Nova Scotia as a cold
front passes through Maine on Saturday. Trend has been toward
keeping most of the coastal low-related precipitation east of the
state, but some die-hard western ensemble members remain which
keep the WPC probabilities of at least 4 inches around 10-20% in
northeastern Maine.

Behind the coastal system, another cold front will move across the
eastern Great Lakes with a rain/snow mix (closer to the lakes) and
snow in the higher elevations including the Tug Hill where a few
inches are possible.

For Days 1-3, the probability of significant icing (at least
0.25") is less than 10 percent.

Fracasso