Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EST Sat Dec 17 2022

Valid 00Z Sun Dec 18 2022 - 00Z Wed Dec 21 2022

...Northeast...
Day 1...
A deep, slow-moving, closed low remains centered over the upper
Great Lakes.  East of the low, a well-defined shortwave and its
associated surface low are lifting north across the Gulf of Maine.
 The cross country storm that impacted a large portion of the U.S.
beginning last week is expected to finally wind down this period,
but not before producing some additional locally heavy snow across
portions of central and northern Maine tonight into Sunday. 
Favorable upper forcing in additional to enhanced low level
convergence along the a trough extending west from the surface low
back toward the low over the Great Lakes is expected to help
support locally heavy amounts across the region.  For the Day 1
period (ending 00Z Mon), the WPC PWPF shows widespread high
probabilities for additional accumulations of 4 inches or more,
with embedded higher probabilities for 8 inches in the higher
terrain. 

...Great Lakes...
Days 1-3...
The previously noted noted closed low over the Great Lakes is
forecast to gradually shift east and elongate, allowing cold air
to spread across the eastern Great Lakes.  This will set the stage
for the continued development of lake effect, with locally heavy
accumulations expected east of lakes Erie and Ontario.  Heavy
accumulations are expected both east of Lake Erie in western New
York and east of Lake Ontario in the Tug Hill.  For tonight and
tomorrow, local amounts of a foot or more are possible, with the
Tug Hill likely to see the highest amounts.  Snows are expected to
continue into early Monday, with additional locally heavy totals
possible, especially in the Tug Hill, before winds begin to veer
with a passing shortwave ridge.  Two day totals of 2 feet or more
can expected over the Tug Hill.

...Pacific Northwest & Northern Rockies...
Days 1-3...
A shortwave diving south across British Columbia is expected to
amplify a trough over the Pacific Northwest, pushing colder air
farther south and lowering snow levels across the region on
Sunday.  While widespread heavy snows are not expected, there will
likely be some heavy totals across the higher elevations of the
northern Cascades and the northern Idaho and western Montana
ranges on Sunday.  Mixed precipitation, including some
accumulating ice, can also be expected at the lower elevations,
impacting travel through the Cascade passes tonight into Sunday. 
Precipitation is expected to diminish from west to east Sunday
night into Monday before the next system begins to impact the
region Monday night into Tuesday.  Energy moving through the
broader scale trough is expected to bring additional precipitation
into Washington, Oregon, the northern Rockies, and possibly
farther south into northern California.  Significant model spread
continues to limit confidence in the details of the forecast,
however the WPC PWPF indicates that locally heavier snow
accumulations are possible for portions of the Cascades, Blue
Mountains, and the Idaho, western Montana, and western Wyoming
ranges.  

Pereira