Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 AM EST Thu Dec 24 2020

Valid 12Z Thu Dec 24 2020 - 12Z Sun Dec 27 2020



...Great Lakes/Ohio Valley/Central Appalachians..
Days 1-3...

An amplifying longwave trough will drive an arctic front across
the Ohio Valley and Central Appalachians into the Mid Atlantic. A
wave of low pressure is likely to develop along this front and
lift northeast across the WV panhandle, PA, NY, and into the St
Lawrence Valley or the nearby Ontario/Quebec border by 12z Fri.
The development of the wave allows a cross-frontal circulation to
advect moisture into the cold sector of the low and change rain to
snow across
the central Appalachians to western PA, eastern Ohio, and
southwest NY.  Some enhancement in snow is also likely due to LES
south of Lake Erie and on the slopes of WV due to upslope. WPC
probabilities continue to show a greater than 50 percent chance
for 6 inches along the Lake Shore where in excess of 12 inches is
locally possible.

The key to the snowfall forecast is timing of the duration of snow
before the frontal wave departs. The high res runs show less than
the global models runs, with good agreement among the
GFS/ECMWF/Canadian global of enhanced snow in the mid level
deformation zone across WV, eastern Ohio, and western PA.

Behind the front, strong CAA will lead to a period of heavy LES as
cold air traverses the Great Lakes producing instability and lee
shore convergence to help produce ascent. Snow is likely day 1
downstream of Lake Superior and Lake Michigan in Michigan where
WPC probabilities indicate a high chance for multi bands producing
6" of snowfall. Some Lake enhancement and then lake effect snow is
likely SE of Lake Erie, and then develops Fri into early Saturday
east of Lake Ontario where fetch becomes ideal and instability
maximizes.  The snow belts are expected to receive a much as a
foot of snow in the lee of Lake Erie and east of Lake Ontario,
including in the NY Tug Hill Plateau.

...Pacific Northwest/Northern CA/Northern Rockies...
Days 2/3...

A deep trough approaches from the Pacific, which produces moisture
advection onshore into northern CA and the Pacific northwest
Friday during the day, continuing in the northwest to the northern
Rockies Fri night.  A burst of strong ascent as the 700 mb wave
approaches in the ranges of northwest CA leads to moderate to
heavy snow there.  The heaviest amounts are likely in the
Shastas/Siskiyous/CA-Southern OR Cascades which will benefit from
the strong mid level jet and greater moisture advection. Snow
tapers on Day 3 as the wave progresses inland and ascent wanes.

Heavy snow is likely in the terrain stretching from the Olympics
of WA southward along the Cascades. Snow is also expected in the
Boise Mountains and Sawtooth and Clearwater ranges of ID into
northwest MT, where moisture and warm advection are accompanied by
ascent centered in the dendritic growth zone, favoring an extended
period of snow, with a foot of snow possible.  As the wave moves
further east, several inches of snow is forecast in the Tetons of
northwest WY Sat.  

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Petersen