Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Thu Oct 15 2020

Valid 12Z Thu Oct 15 2020 - 12Z Sun Oct 18 2020


...Northern Rockies and Northern Plains to Lake Superior...
Days 2-3...
The next shortwave packet to round a deep low centered over
northern Manitoba is currently a low over northern Alaska and a
trough approaching the Gulf of Alaska. The Gulf of Alaska wave is
expected to dart ahead of the northern Alaska low and track down
the Canadian Rockies, reaching western MT late Friday. Surface low
pressure then tracks east across the northern Plains to Lake
Superior through Saturday. Wrap around flow behind the low,
combined with anti-cyclonic flow ahead of a strong high diving
down the lee of the Canadian Rockies combine to form upslope flow,
along with low-to-mid level frontogenesis and upper jet support
will support the potential for locally significant accumulations
along the western Montana to the northern Wyoming ranges east of
the Divide Friday night through Saturday night. Day 2.5 and 3 snow
probs are moderate to high for six or more inches over only the
highest eastern Rockies in MT and the Absaroka/Big Horns in
northern WY.

Farther east, the surface low tracks across northern SD Friday
night and reaches Lake Superior Saturday evening. This is a very
similar track and strength of low to what crossed this area on
Wednesday though with much colder conditions in place and behind
the system. This should enable a swath of moderate to potentially
locally heavy snow north of the low track over ND/northern MN.
There are now 20 to 30 percent probabilities for 4 or more inches
over northeastern ND on Day 2.5 and over northern MN/the Arrowhead
on Day 3.

...Northeast...
Day 3...
A shortwave trough currently over the northern Rockies amplifies
the broad-scale trough attendant to the anchored and deep closed
low over Manitoba takes on a negative tilt as it moves across the
Mid-Atlantic Friday night. This system will continue to amplify as
it lifts across the Northeast late Saturday. Surface low pressure
is forecast to develop as it tracks north along a tight baroclinic
zone in the Gulf of Maine on Saturday. Guidance continues to
tighten the precip swath over New England and eastern NY Saturday.
Enough cold air shifts east to bring some mountain snows to the
highest Adirondacks, Greens, and Whites. Day 2.5 snow probs are 10
to 20 percent for four or more inches along the higher White
Mountains from northern NY into Maine. Snow levels drop to around
3000ft after the low passage Saturday with diminishing wrap around
snows for the New England and Adirondack terrain.

For Days 1-3, the probability of significant icing is less than 10
percent.

Jackson