Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 PM EST Thu Jan 23 2020

Valid 00Z Fri Jan 24 2020 - 00Z Mon Jan 27 2020

...Pacific Northwest...
Days 1-3...

Snow levels decrease to generally around 4000ft through Friday as
a cold front and mid-level trough shift east through the region.
The next tropically sourced moisture plume then pushes into the
northern CA/OR coast Friday night with snow levels sharply rising
above 7000ft, though precip rates do not get heavy until the next
cold front arrives Saturday night. Heavy snow is limited to the
highest elevations of the WA/OR Cascades and northern Rockies into
western MT on Day 1, are very limited on Day 2, and expand over
the WA/OR/CA Cascades and northern Sierra Nevada...along with the
eastern OR and ID northern Rockies on Day 3.

Snow melt and heavy rain are the greater threats in the Pacific
Northwest, so please see the excessive rainfall discussions
(QPFERD) for further information.


...Great Plains/Midwest/Great Lakes/Mid-Atlantic/Northeast...
Days 1-2...

The closed low over southwest MO this evening will swing northeast
to lower MI through Friday night as weak surface low development
occurs under the occluded low. A secondary low at the end of the
occlusion will develop over the southern Mid-Atlantic in the lee
of the Appalachians late Friday and take an inland track up the
Eastern Seaboard through Sunday. The lagging shortwave will help
snow snow through the Upper Midwest/northern Great Lakes, while
the secondary feature will become predominant and drive stronger
dynamics over the Mid-Atlantic and Northeast/New England.

Forcing through Friday is generally modest in the vicinity of the
slow moving upper low. WAA east of the primary low will spread
moisture northward and then westward over the northern Mississippi
Valley, with some mesoscale enhanced ascent in the TROWAL expected
immediately north of the surface low. Lake enhanced snow is
expected over Lake Superior on an easterly wind. WPC probabilities
for 4 inches on Day 1/1.5 are over northern MO through northwest
IL and the north shore of MN (Arrowhead) as a result.

Days 2-3...
The secondary low will intensify as it moves across the
Mid-Atlantic Friday night into Saturday, with moisture on
increasing warm air advection spreading northward. Ascent will
increase through height falls, WAA, and increasing left exit jet
diffluence to produce a swath of snow and mixed precipitation over
the interior Northeast. The heaviest snow is likely in the terrain
of Northern New England and Upstate New York where WPC Day 3
probabilities are moderate to high for 4 inches.

With the WAA overrunning the low-level cold air, freezing rain is
likely in the transition zone as well, and Day 2 WPC probabilities
are moderate for a tenth inch of ice from the Allegheny Plateau in
WV/western MD into PA, across northern PA/eastern Upstate NY and
the Berkshires/southern Green Mtns. Day 3 probabilities for a
tenth inch of ice are low from northern NH across most of interior
Maine.

Jackson