Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
441 AM EDT Fri Oct 16 2020

Valid 12Z Fri Oct 16 2020 - 12Z Mon Oct 19 2020

...Northern Rockies...
Days 1-3...

A shortwave trough and surface low currently shifting into BC will
reach MT today before swinging east across the northern plains as
it rounds a deep and sprawling low centered over northern
Manitoba. A reinforcing jet/trough will then persist through
Saturday night over the Canadian and northern Rockies as a 1030mb
surface high builds down the lee of the Rockies. As the low pushes
into north-central MT late this afternoon, the trailing low-to-mid
level baroclinicity and upstream upper level energy will support
periods of precipitation banking up against the eastern slopes and
ranges of west-central MT through the weekend. The heaviest
precipitation through Saturday night is expected to center along
and east of the Divide across western Montana into northern
Wyoming, with most areas changing over to snow tonight as cold air
spreads south and banks up against the Rockies. Heaviest snow
accumulations through Saturday night are expected along the
west-central Montana ranges, including the Lewis, Flathead, Swan
and northern Absarokas as well as the Big Horn Mtns of Wyoming
where snow probabilities are moderate to high for 6 or more
inches.

The next shortwave trough tracks a little farther south, crossing
the MT Rockies late Sunday. This raises snow levels to around
5000ft with moderate Day 3 snow probabilities for 6 or more inches
are limited to the higher elevations of the west-central MT ranges.


...Northern Plains to Lake Superior...
Days 1/2...

Low pressure swings east across the northern Plains tonight before
lifting northeast and reaching Lake Superior by late Saturday.
Low-to-mid level frontogenesis, in addition to favorable upper
level forcing, is expected to support a swath of wet snow with
mesoscale enhancements north of the low across North Dakota
northern Minnesota late Friday through Saturday. Sufficient cold
air ahead of the low allows lake enhanced snow to the escarpment
above the North Shore of Lake Superior. Day 1.5 snow probabilities
for 4 or more inches are pretty consistent between 40 and 50
percent from the northern MT/ND border across northern ND/northern
MN with 60 percent probs in the MN Arrowhead.


...Northeast...
Days 1/2...

Low pressure quickly develops and moves north along the Northeast
coast as a negatively tilted amplifying shortwave moves from the
Ohio valley and up the Northeast tonight through Saturday. This
system is expected to bring widespread, much-needed precipitation
to the region. A tight baroclinic zone shifts east across New
England tonight with cold side snow levels generally 3500 ft
allowing rain to change to snow for the higher Adirondacks, Greens
and Whites northern New England mountains, with locally moderate
rates for the White Mtns of NH/western Maine where Day 1.5 snow
probabilities are around 30 percent for four or more inches.


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

Jackson