Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
416 AM EST Tue Nov 17 2020

Valid 12Z Tue Nov 17 2020 - 12Z Fri Nov 20 2020


...Western U.S....
Days 1-3...

An upper-level trough along with a series of shortwave impulses
rounding the base of the trough and embedded within moist deep
layer southwest flow out ahead of it will be moving inland across
the West Coast by tonight, and then continuing through the Pacific
Northwest and northern Rockies going through Wednesday and
Thursday. The influx of deeper layer Pacific moisture in
conjunction with orographic ascent, and at least moderately strong
upper-level divergent flow will favor moderate to heavy snowfall
over the higher terrain of the northern Sierra-Nevada and Cascades
initially, but then overspreading areas downstream over the
northern Rockies including the Sawtooth, Bitterroots, Tetons and
Wind River ranges. As the larger scale trough and series of
shortwave impulses move inland, the snow levels should gradually
lower with time.

The heaviest snowfall will be over the northern Sierra-Nevada and
the Cascades where 3-day snowfall totals of 1 to 2 feet are
expected, with isolated totals in the northern WA Cascades
potentially approaching 30 inches. Farther inland across the
northern Rockies, snowfall totals generally around a foot can be
expected, with locally heavier totals over the southwest facing
slopes of the Sawtooth that may approach 2 feet.


...Lower Great Lakes...
Day 1...

Deep layer cold cyclonic flow over the Great Lakes will be
reinforced early this morning by the passage of a shortwave
impulse and associated weak area of low pressure. This energy
along with a cold front will sweep quickly across the northern
Mid-Atlantic region and in the wake of this will be plenty of
lake-effect snow shower activity, with a few organized bands of
heavier squalls by midday and through the afternoon that will
likely impact areas of northwest PA and western NY. Some
accumulating snow is likely to make it south down into the central
Appalachians including southwest PA, far western MD and northern
WV, but the overall heaviest totals are likely to be across
northwest PA downwind of Lake Erie as the deeper layer cyclonic
flow, strong cold-air advection and sufficient mid-level moisture
fosters a set-up for a multi-lake fetch involving both Lake Huron
and Lake Erie. This will support the potential for a particularly
robust and organized band of heavy snowfall. In fact, the latest
WPC snowfall probabilities indicate a moderate to high risk (40 to
70%) of snowfall totals exceeding 4 inches across parts of
northwest PA. By later tonight, the low-level flow will begin to
weaken over the lower Great Lakes as high pressure advances east
across the OH Valley, and this will allow the lake-effect snow
shower activity to begin to taper down, and then gradually
diminish on Wednesday.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent. 

Orrison