Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Fri Jan 10 2020

Valid 12Z Fri Jan 10 2020 - 12Z Mon Jan 13 2020

Days 1-3...

...Pacific Northwest to the Northern Rockies...
An active period will continue with additional heavy snows for the
mountains of the Northwest and northern Rockies with snow levels
lowering through the period.

The next feature approaching the Pacific Northwest in a multi-week
active pattern for the northeastern Pacific is a powerful cold
core low weakening as it drops southeast from Vancouver Island to
Washington state late today. Snow levels will rise to around
3000ft in western WA today in the moisture plume ahead of the low
before dropping below 2000ft tonight under the remaining trough.
2-4 inches of liquid for Day 1 in the Olympics and WA/OR Cascades
will result in heavy accumulations of 1 to 2 feet with 3+ feet for
the highest Cascades. A brief lull between waves is expected late
Saturday before a weak shortwave trough pushes into western WA
Saturday night, immediately followed by a more organized system
Sunday, supporting additional heavy snows.

WPC guidance indicates 3-day total snow accumulations are likely
to be 3 to 5 feet for the Olympics and Cascades and only slightly
less for northeastern Oregon and far southeastern Washington
mountains, as well as the northern Rockies from northern Idaho and
northwestern Montana to northwestern Wyoming. Snow levels drop
below 500ft across WA and northern OR (including the Seattle and
Portland metro areas) Sunday night in the wake of the organized
system, but reduced moisture and forcing behind the system looks
to limit the potential for valley floor heavy snow.
   

...Southern Plains to the Great Lakes...
A large winter storm will bring snow and ice across portions of
the southern and central Great Plains, the Great Lakes and
northern New England today into Sunday. 

A cold front will slow to a creep from the Great Lakes to the
southern Plains today. Temperature contrast across the boundary is
expected to increase as an upstream northern wave moves colder
into the central CONUS today. Meanwhile, an amplifying southern
stream wave over the Desert Southwest will shift to the southern
High Plains today. Significant to potentially damaging ice is
expected as increasing southerly winds begin to push deeper
moisture across the shallow cold air north of the front. A stripe
of notably accreting freezing rain is expected from the southern
KS/MO border to northern Illinois and across southern Lower
Michigan. Canadian high pressure sliding northeast from the
central U.S. into Ontario will keep the low level cold air in
place, raising the potential for significant ice accumulations.
The Day 1.5 WPC probabilities are 50 to 70 percent for storm total
ice amounts of a 0.25 inch or more over MO/IL and over 80 percent
for the southern LP of MI. Day 1.5 probabilities for 0.5 inches of
ice are moderate across this swath from MO to MI as well.

As the low to the south strengthens Saturday, comma head snow
bands are expected to develop from the OK/TX border northeast and
continue through eastern WI and the central LP of MI through
Saturday night. As the upper trough takes on a negative tilt the
surface low turns more northerly over the Midwest Saturday night
with potent bands moderate to heavy snows from southern Wisconsin
to northern Michigan Saturday night. Day 2 WPC probabilities are
moderately high for snow accumulations of 8-inches or more from
southern Wisconsin to northern Lower Michigan and the eastern
Upper Peninsula. 

...Northeast...
The surface front moving east through eastern Canada will sag
southeast through the St. Lawrence Valley into the Northeast late
Saturday. Deep southerly flow ahead of the low lifting into the
western Great Lakes will continue to draw moisture into the
shallow cold air north of the front, extending the freezing rain
threat farther east into northern New York and New England. With
high pressure remaining in place across eastern Canada, the low
slides to the east from Lower Great Lakes through the Northeast on
Sunday. The shallow cold air is expected to support a stripe of
significant ice accumulations from northern New England to eastern
Maine where there are moderate probabilities for a half inch of
ice. Meanwhile, deeper within cold air there are moderate Day 2.5
probabilities for 8 or more inches across far northern Maine.

Jackson