Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 AM EST Sun Nov 15 2020

Valid 12Z Sun Nov 15 2020 - 12Z Wed Nov 18 2020

...Great Lakes and Northeast...
Days 1-3...

A strong and becoming negatively tilted upper trough with a
rapidly developing surface low currently pushing into the Midwest
will push northeast across northern Michigan this afternoon. An
intensifying wrap around band of precip will continue to
experience dynamic cooling and should turn into a band of moderate
to locally heavy snow over eastern WI this morning and across
western and northern MI this afternoon. The orientation of the
band with motion of the system could cause multiple hours of
accumulating snow, particularly over the northern LP of MI. This
is a powerful storm with very strong westerly flow also adding
lake enhancement to western MI this afternoon. Day 1 snow
probabilities are low for 4 or more inches and contained in higher
elevations of the northern LP of MI from the WPC Super Ensemble,
but once the band switches to snow, elevation won't matter much
and accumulating snow is possible down to lake level.

As the associated surface front moves across the Northeast, a
secondary low surface low developing on the triple point is
expected to help hold enough cold air in place to maintain wintry
precipitation across northern New Hampshire and western/northern
Maine tonight with Day 1 snow probabilities for four or more
inches low for the Presidential Range in NH and far northern
Maine. 

Deep cyclonic, westerly flow and cold air advection in the wake of
the system will support lake effect snow bands in the lee of lakes
Superior and Michigan particularly tonight and then lakes Erie and
Ontario late tonight through Tuesday with the low level wind
veering from westerly to northwesterly through that time slowly
shifting the snow zones southeast.  Day 2 snow probabilities for
four or more inches are moderate in the Tug Hill east of Lake
Ontario and low over the Chautauqua and far northwest PA areas on
Day 3.


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

An strengthening zonal jet into WA/OR today will focus a stream of
enhanced precip along a baroclinic zone over OR into southern ID
into tonight with snow levels generally 5000 to 6000ft. This
moisture axis shifts north through WA late tonight through Monday
as a trough digs offshore causing the onshore flow to back to
southerly. This trough pushes into the WA/OR/northern CA coasts
Tuesday with broad onshore flow and snow levels decreasing from
around 7000ft to around 5000ft into Tuesday night. Day 1 snow
probabilities for 6 or more inches are high for the Sawtooth Mtns
of central ID, and the Tetons/Wind River Range of western WY as
well as the WA Cascades. There are moderate probabilities for 6 or
more inches in the northern WA Cascades on Day 2 then moderate for
the higher Cascades and northern Sierra Nevada on Day 3.


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