Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
455 PM EST Thu Jan 09 2020

Valid 00Z Fri Jan 10 2020 - 00Z Mon Jan 13 2020

Days 1-3...

...Pacific Northwest to the Northern Rockies...
An active period is forecast to continue with additional heavy
snows likely for the mountains of the Northwest and northern
Rockies.

An upper ridge shifting east across the West will offer a brief
dry period before quickly giving way to the next shortwave trough
dropping southeast into the Pacific Northwest on Friday.  This is
expected to produce heavy accumulations across portions of the
Olympics,  northern Cascades and the northern Rockies before
sliding farther southeast of the region on Saturday.  Persistent
onshore flow with embedded mid level energy will continue to fuel
snow showers before another organized system drops southeast into
the region on Sunday, supporting additional heavy snows.  WPC
guidance indicates 3-day total snow accumulations are likely to
exceed 2-feet across portions of the Olympics, Cascades,
northeastern Oregon and far southeastern Washington mountains, as
well as the northern Rockies from northern Idaho and northwestern
Montana to northwestern Wyoming.
   

...Southern Plains to the Great Lakes...
A large winter storm will bring snow and ice across portions of
the southern Plains, Midwest and Great Lakes Friday and Saturday. 

A frontal boundary trailing a northern stream wave moving from
central into eastern Canada will remain in place from the Great
Lakes back into the southern Plains Friday.  Temperature contrast
across the boundary is expected to increase as an upstream
northern wave moves colder into the central U.S. on Friday. 
Meanwhile, an amplifying southern stream wave will move from the
Southwest into the southern High Plains Friday evening.  Models
are still presenting a strong signal for significant to
potentially damaging ice as amplifying southerly winds begin to
push deeper moisture across the shallow cold air north of the
front.  Heaviest ice accumulations are expected to occur from
portions of northeastern Missouri to northern Illinois and
southern Lower Michigan.  High pressure sliding northeast from the
central U.S. into Ontario will help to keep the low level cold air
in place, raising the potential for significant ice accumulations.
 Latest WPC probabilities exceed 50 percent for storm total ice
amounts of a 0.25 inch or more across these areas.  There is the
potential for some areas to receive over a half-inch, especially
portions of southern Lower Michigan.

As the low to the south strengthens, deepening cold air will
promote a changeover to snow northwest of the center, especially
as the low lifts through the mid Mississippi valley into the Upper
Great Lakes on Sunday.  Models show the upper trough assuming a
negative tilt with low to mid level frontogenesis, along with
strong upper forcing supporting moderate to heavy snows from
southern Wisconsin to northern Michigan Saturday night.  Latest
WPC guidance shows high probabilities for snow accumulations of
8-inches or more from southern Wisconsin to northern Lower
Michigan and the eastern Upper Peninsula. 

...Northeast...
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 New York and New England.  With high
pressure remaining in place across eastern Canada, models show the
low sliding 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.  Meanwhile, deeper within cold air there
is the potential for heavy snow accumulations across far northern
Maine.

Pereira