Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
437 PM EST Thu Feb 04 2021

Valid 00Z Fri Feb 05 2021 - 00Z Mon Feb 08 2021


...Great Lakes...
Days 1-3...
A negatively-tilted upper trough and associated strong surface low
will continue to move north along with a swath of light to
moderate synoptically driven snows across the upper Great Lakes
Thursday evening.  Northwesterly to westerly flow on the backside
of the system will begin to support lake-enhanced snows along the
western U.P. of Michigan, including the Keweenaw Peninsula this
evening, before spreading east into the eastern snow belts as the
low tracks north of the Great Lakes overnight.  Westerly flow will
also foster the development of lake effect snow showers along the
eastern shores of Lake Michigan as well.  Deep cyclonic flow
persisting through the period, with a deep low redeveloping north
of the Great Lakes, will support periods of lake effect snow in
the lee of the Upper Lakes through the weekend.  Three day totals
of a foot or more are likely along the Keweenaw Peninsula and
along the portions of the eastern U.P. snow belts.  Locally heavy
accumulations are also likely along the northeastern shores of
Lake Michigan.  Meanwhile, cold air spreading east across the
lower Great Lakes, along with strong southwesterly to westerly
winds will support single band lake effect snow development, with
locally heavy amounts likely on Saturday.  Amounts of a foot or
more are likely across the Buffalo region, as well as the Tug Hill.

...Northern Cascades to the Central Rockies...
Days 1-3...
A low-amplitude, but well-defined shortwave is expected to drop
through British Columbia into the Pacific Northwest, supporting
organized heavy snowfall across the northern Cascades Thursday
night into Friday.  Then this energy, along with a strong cold
front sagging southwest will support periods of heavy snow moving
southeast across the northern into the central Rockies.  By late
Friday, locally heavy totals of a foot or more can be expected
across portions of the northeastern Oregon mountains and along the
Rockies from northern Idaho to northern Utah and northern
Colorado.  Strong northwesterly flow will continue to carry a
string of shortwaves across the region, with additional periods of
heavy snow continuing through the weekend.  It is expected that
portions of the northern Cascades, as well as the northern Rockies
from northern Idaho to western and central Montana, will see storm
totals well in excess of two feet.

...Southern Appalachians to the Northeast...
Day 3...
Shortwave moving through the Northwest on Friday is expected to
translate through the base of the broader scale trough, moving
through the lower Mississippi into the Tennessee valley Saturday
evening.  After downplaying the scenario for several runs, the
general consensus of the 06Z and 12Z deterministic models,
including the GFS, ECMWF and NAM was toward a more amplified
shortwave moving through the Tennessee valley into the southern
Mid-Atlantic, with surface low pressure tracking closer to the
Mid-Atlantic and Northeast coasts on Sunday.  While confidence in
the details is limited given the significant run-to-run variance,
measurable snow is appearing more likely now from the southern
Appalachians northeastward to the northern Mid-Atlantic coast and
then along the Northeast coast on Sunday.

Pereira