Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 PM EST Tue Dec 22 2020

Valid 00Z Wed Dec 23 2020 - 00Z Sat Dec 26 2020


...Northern Plains and Upper Midwest...
Days 1-3...

A mid-level trough ejecting from the Northern Rockies will amplify
into a full latitude trough by Thursday, aiding in low-level
cyclogenesis which will deepen in response to the mid-level trough
closing off as the surface low moves across MN and into the
western Great Lakes. While the low is likely to move quickly
eastward limiting the temporal duration of strong ascent and heavy
precipitation, a period of moderate to heavy snow is likely NW of
the system in response to intensifying deformation and a
developing TROWAL leading to enhanced comma head precipitation,
especially across the Arrowhead of MN. Additional heavy snow
enhancement is expected south of Lake Superior due to LES. While
guidance continues to show some spread in the exact track of the
low and placement of the best comma precip, the ECMWF remained a
favored solution along with the ECENS/GEFS means. This suggests
the heaviest snow will occur along an axis from far SW MN into far
NE MN, with an additional maxima in NW WI due to LES. WPC
probabilities are high for 4 inches in this region, with isolated
maxima over 10" possible in the Arrowhead or NW WI. Additionally,
very strong winds in excess of 35kts are likely at times as shown
by high probabilities in the CIPS analog guidance. This could lead
to periods of blizzard conditions even during light snow, or after
the snow winds down, due to the high SLR/fluffiness of the snow in
the very cold atmosphere.

As the storm pulls away Thursday into Friday, LES will ramp up
across the U.P. of MI and along the eastern lakeshore of Lake
Michigan. WPC probabilities for 4 inches are generally 10-20% on
Day 3.


...Ohio Valley/Central Appalachians/Great Lakes...
Days 2-3...

An area of low pressure lifting across the Great Lakes and into
Canada will drape an impressive arctic cold front across the
Midwest, racing eastward through the Appalachians and towards the
east coast by Christmas Day. Along this front, secondary low
pressure development is likely across the Southeast/Mid-Atlantic,
with this low lifting quickly northward along the baroclinic zone
into New York by Friday. This low develop and strengthen in
response to PVA/height falls and an increasing upper level
diffluence acting upon the sharp low-level baroclinic zone. There
remains considerable spread in the timing and evolution of this
front/low, and this forecast relied heavily on the
ECMWF/ECENS/GEFS.

The airmass ahead of the front/low is much too warm for snow, and
much of the precipitation will fall as heavy rain. However, as the
low lifts NE the front will shift east behind it, rapidly cooling
the column to cause a p-type transition from rain to snow. Exactly
how quickly this occurs will make all the difference with snow
amounts. Usually, cold air chasing moisture is not favorable for
heavy snow. However, as the front races east, the trough becomes
negatively tilted coincident with at least the modest development
of a TROWAL and sharpening low-level fgen. Persistent mid-level
and upper level divergence for ascent combined with a weak WCB
could throw enough moisture behind the front to allow for several
inches of snowfall to accumulate. This will be aided by what could
be briefly intense rates as the fgen sharpens, so despite hostile
antecedent conditions due to warmth and rain, snow may rapidly
accumulate. This is most likely in the eastern OH VLY due to
enhanced fgen, south of Lake Erie due to LES, or in the central
Appalachians in response to upslope forcing. WPC probabilities are
high for 4 inches across these areas, with locally in excess of 6"
in the WV terrain, or 8" south of Lake Erie, possible.


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

Weiss