Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 PM EST Mon Nov 15 2021

Valid 00Z Tue Nov 16 2021 - 00Z Fri Nov 19 2021


...Pacific Northwest to the Northern Rockies...
Days 1-2...
The ongoing atmospheric river event will continue to ease today as
an upper-level trough and associated surface cold front come
ashore Washington. Snow levels will begin to drop later this
afternoon as the cold front ushers in much cooler air, lowering
snow levels to below 2000 ft over the Cascades as precipitation
begins to taper off. WPC probabilities of at least 6 inches of
snow at some pass levels near 3000-4000ft, including the
Snoqualmie and Stevens Passes, are around 70-90%. Farther east,
progression of the front will bring some snow and gusty winds to
portions of the northern Rockies overnight into Tuesday. Areas
near the Montana/Idaho border including the Bitterroots and the
Lewis Range will likely receive the most snow east of the
Cascades, as WPC probabilities of at least 4 inches of snow meet
or exceed 50% in those areas. Additional light accumulations can
be expected throughout the Northern Rockies and north-central
Colorado ranges into Wednesday morning with the passage of the
upper-level trough and further inland/southward penetration of the
draping cold front.

...Great Lakes and Northeast...
Day 1...
Snow is forecast to continue through Tuesday afternoon over the
Great Lakes and the Northeast behind the clipper system exiting
into southeast Canada later this evening. Northwesterly winds
generated by this system will be poised to pass over warm lake
waters and interact with surface shortwave instability and a
moderately favorable atmospheric thermodynamic profile to generate
more lake effect snow. While light accumulations are expected for
the U.P. and northern lower Michigan, areas downwind of Erie and
Ontario will likely recieve more moderate totals. WPC
probabilities of at least 4 inches of snow are 20-40% near
Syracuse, NY, dropping to 10-30% over portions of the Southern
Tier. Meanwhile, remnants of the clipper will contribute to an
additional several inches of snow further east over the Green and
White mountains.

Day 3...
A rapidly intensifying low pressure system is forecast to traverse
southern Canada and approach the northeastern US Wednesday
evening. Cold air and low to mid-level ascent will allow for light
snow accumulations from northern Vermont through interior Maine.
Most notably, passage of the associated warm front through the
Northeast will cause a rapid warming of the air as precipitation
continues through the region. This abrupt temperature change will
promote the development of freezing rain throughout the interior
Northeast from northeast Vermont through north-central Maine. At
present, WPC probabilities for freezing rain accumulations greater
than 0.1 inch are 20-30% for northern New Hampshire and western
Maine. On the western side of the low pressure system,
northwesterly winds passing over Lake Huron and Lake Michigan will
allow for light lake effect snow accumulations along the coastal
U.P. and northwestern lower Michigan.

For Days 1-3, the probability of significant icing is less than 10
percent.

Zavadoff/Fracasso