Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 AM EST Tue Nov 20 2018

Valid 12Z Tue Nov 20 2018 - 12Z Fri Nov 23 2018

...New York and New England...
Days 1-2...

A northern stream shortwave trough will continue to swing from a
parent low centered over Hudson Bay and cross New England through
tonight. A surface low currently near NYC will rapidly develop as
it crosses the southern New England coast this morning and the
Gulf of Maine into this evening. Snow is expected just north of
the surface low track where thermal profiles are cold enough.
Looking at beyond 12Z today, a band of robust 925-850mb
frontogenesis will drive a swath of moderate snow across eastern
New England north from the MA border. Coastal Maine has moderately
high probabilities for four inches and some low probabilities for
six.

Moisture and forcing are best close to the low, so northern
interior New England will have less snow.

A reinforcing shortwave trough swings east across Quebec Wednesday
with the associated surface low crossing just north of the
Adirondacks and Maine late Wednesday. This clipper-like system is
limited in moisture by a continental airmass and is quite
progressive, so probabilities are in the two inches range.
However, a polar front will accompany this low and there is a risk
of a snow squall with the frontal passage where snow rates could
be briefly heavy with blustery conditions.


...Great Lakes...
Days 1-2...

Periods of heavy lake effect snow are likely at times through
Wednesday on the E/SE side of all of the Great Lakes (except
southern Lake Michigan). A shortwave trough pushes east from the
eastern Great Lakes this morning with a favorable LES setup for
the east side of Lakes Ontario and Erie in the wake this
afternoon. A more potent shortwave trough crosses to the north of
the Great Lakes tonight into Wednesday. A brief round of
synoptically forced snow is expected with the low passage with LES
continuing until the surface high builds across the lakes from the
northeast Wednesday night. Upstate NY in the lee of Lake Ontario
has moderate to high risks for four inches both days.


...The West...
Days 2-3...

A deep trough that spans the west coast and surface cold front
will push across the west coast Wednesday night. This front will
be preceded by a warm conveyor belt of subtropical Pacific
moisture with precipitation onset in the Coast Ranges Wednesday
morning. The south flow will keep snow levels initially high;
6-7kft Wednesday, but decrease through Wednesday night under the
trough. Heavy snowfall is likely from the higher Cascades through
the Sierra Nevada, with a high probability of 8 inches for the
high Sierra for Day 2.

The west is particularly active for snow on Day 3. The initial
deep trough crosses the Rockies Thursday night with heavy snows
for the high terrain of the Intermountain West and Rockies. Of
particular note is moderate probabilities for 8 inches for the
northern Wasatch near SLC.

Also on Day 3, an atmospheric river (AR) is directed into far
northern CA on the south end of a reinforcing cold front from a
low off BC. Snow levels rise south of the front in the AR and
lower north of the front in OR/WA. High moisture content will
result in heavy precipitation over the Pacific NW with high
portions of the Cascades and Olympics with moderately high
probabilities for 8 inches.




The probability of significant ice (0.25-inch or more) is less
than 10 percent.

Weiss