Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 PM EST Fri Jan 24 2020

Valid 00Z Sat Jan 25 2020 - 00Z Tue Jan 28 2020

...Northwestern CONUS...
Days 1-2...

The wave train south of a southern Alaska low continues to push
into the Pacific Northwest (with associated fronts reaching the
north/central Rockies) into at least the middle of next week.
However, since cold air is contained to the low to the
north/Alaska, higher snow levels will persist.

A trough crosses the WA/OR/far northern CA coast tonight with
second and third waves moving into western WA Saturday night and
Sunday. Snow levels rise ahead of the trough tonight to roughly
5000ft in western WA, 6000 to 7000ft in OR, and over 8000ft in CA.
Therefore Day 1 snow probs for 6 or more inches are the lowest of
the three days, generally moderate for the highest WA/OR Cascades
and western MT Rockies. Snow levels drop a bit with the Saturday
night wave, to generally around 5000ft from western WA to northern
CA. Day 2 snow probs for 6 or more inches are moderate to high for
the WA/OR/CA Cascades, low for the northern Sierra Nevada and
moderate to high for the northern Rockies of eastern OR/ID.

Day 3...
Yet another plume of Pacific moisture of tropical origin pushes
into the Pac NW Monday. This raises snow levels over OR/CA back to
around 6000ft with levels dipping in WA to 3000 to 4000ft. Day 3
snow probs for 6 or more inches are higher for the OR/WA Cascades
and Olympics than day 2 and moderate for the northern Rockies of
ID/MT.

The trough that pushes into the northwest Saturday night
progresses to the central Rockies Monday with associated snow
expected for the Tetons of NW WY, Wasatch in UT, and higher peaks
near the CO/WY border which have moderate probs for 6 or more
inches of snow.



...Midwest/Great Lakes/Mid-Atlantic/Northeast...

Days 1-2...
The close low drifting northeast over IL today will drift across
southern MI Saturday. Snow in the mid-level deformation zone over
from southeast MN across IA into northern MO will shift east to
WI/northern IL tonight and persist there through Saturday morning.
By this time the wind will have shifted over Lake Superior with
lake enhanced snow for the UP and northern lower MI into Sunday.

A secondary low at the end of the occlusion will develop over the
southern Mid-Atlantic in the lee of the Appalachians tonight and
take an inland track up the Eastern Seaboard through Sunday. The
inland track leads to strong low-mid level warm advection across
eastern NY and New England. This is turn causes a wintry mix with
rapid changes from snow to sleet and freezing rain expected.

With the warm air aloft overrunning the low-level cold air,
freezing rain is expected, WPC probabilities are moderate for a
tenth inch of ice on Day 1.5 from northeast PA/the Catskills and
Adirondacks of eastern NY and the Berkshires/Green Mtns/White
Mtns/ranges of western Maine. Day 2 ice probs for a tenth inch or
more are moderately high across northern Maine. Day 2 probs are 10
to 20 percent for a quarter inch of ice from the White Mtns to
northern Maine.

Day 3...
In the lee of the Great Lakes, cyclonic flow brings an expected
period of westerly boundary layer flow across both Lake Erie and
Lake Ontario. This results in lee shore convergence and lift that
is aided by topography east of Lake Ontario in the Tug Hill. Where
moisture and loft coincide, several inches of snow is expected in
the lee of both Lakes Erie in southwest New York and Lake Ontario
in upstate New York from late Sunday into Monday.

Jackson