Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 AM EST Tue Jan 04 2022

Valid 12Z Tue Jan 04 2022 - 12Z Fri Jan 07 2022

...Pacific Northwest to the Central Rockies...
Days 1-3...

Unsettled, active weather will continue through the forecast
period across the Pacific Northwest. A closed low well southeast
of Alaska will open up with pieces of sheared energy spreading
into the region through the day 1 period. Modest Pacific jet
energy and confluent flow in the mid-levels will drive an
impressive moisture plume into the region where PWs up to +2 sigma
are expected across OR/WA. Northward morning frontal boundary will
help lift snow levels that are initially low this morning as the
upper ridge builds/pushes north. Another shortwave trough then
approaches northwest Washington late tonight into Wednesday. As
the cold air aloft retreats, lingering cold air at the surface may
set the stage for mixed precipitation, including accumulating ice
across the valleys of Washington and northern Oregon.

Beginning late Wednesday and continuing into Thursday, an
approaching shortwave will focus the threat for heavy snow farther
north, with heavy snows likely for the Washington Cascades. 
Locally heavy snows are also likely for the Blue Mountains and
portions of the northern Rockies, especially the northern to
central Idaho ranges. 

...Northern Plains and Great Lakes...
Days 1-3...
Shortwave trough energy originating from Alberta and a piece of
sheared vorticity from the Pacific Northwest works together to
eventually carve out a closed 500 mb low over the western Great
Lakes Wednesday. A strengthening area of low pressure is then
forecast to track from eastern Montana toward MSP then the
northern Great Lakes. A corridor of frontogenetical forcing within
the very cold air will support moderate to locally heavy snowfall
across the northern Plains to Upper Midwest late tonight into
early Wednesday and then as the system passes east of the Lakes,
the very favorable wind direction and cold air will bring a
prolonged, potentially significant, lake effect snow event to the
Great Lakes. SLRs approaching 20:1 will help drive higher snow
totals in excess of a foot locally for the favored areas of Lake
Superior, Michigan, and perhaps Erie/Ontario snow belt areas. The
latest WPC probabilities for 6 inches are moderate to locally high
for Superior/Michigan snow belt areas.


...Central High Plains...
Day 2...
Increasing divergence along the left-exit region of an upper jet
extending southeast from the northern Rockies, overlapping strong
low-to-mid level frontogenesis, is expected to support the
increasing potential for heavy snow development from southeastern
Wyoming into the Nebraska Panhandle on Wednesday.  WPC
probabilities indicate at least a moderate threat for snow
accumulations of 4-inches or more across this region.  The threat
for heavy snow is less certain farther east, but a stripe of at
least light accumulations is expected across Kansas and Nebraska
into Missouri late Wednesday into Thursday. 


...Ohio Valley into Mid-Atlantic/Northeast...
Day 3...
Shortwave energy crossing the central U.S. in the quasi-zonal flow
begins to sharpen and dig as it approaches the lower Ohio Valley
Thursday. An area of low pressure over the Deep South is then
expected to strengthen as it lifts northeast through the southern
Appalachians then off the Mid-Atlantic coast. Modest moisture
being tapped from the Gulf will help drive widespread
precipitation across the lower Ohio Valley through much of the
central/southern Appalachians into the Mid-Atlantic and Northeast.
With the current forecast track, a transition zone of rain/snow
would potentially bisect the urban corridor with the current
favored area for accumulating snow to the west/interior areas but
there remains some considerable uncertainty in the eventual low
pressure track and therefore the amount/depth of cold air in
place. But there is an increasing signal for several inches of
snow for portions of eastern Kentucky into West Virginia through
the end of Day 3 and the latest probabilistic guidance has
increased subsequently. The latest WPC probabilities for 4 inches
are now high across eastern Kentucky through much of West Virginia
with a broad area of moderate probabilities around it from central
Kentucky into the Mid-Atlantic.


Taylor/Pereira