Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Wed Dec 23 2020

Valid 00Z Thu Dec 24 2020 - 00Z Sun Dec 27 2020

...Upper Midwest...
Day 1...

A deepening surface low moving across Minnesota and Wisconsin
tonight will lift away into Ontario during Thursday. This low will
be driven by a longwave mid-level trough and upper jet dynamics to
strengthen as it drifts slowly eastward the next 24 hours. This
low will be moving along a cold front, which will advect more
rapidly to the east as the low pulls away, leaving a sharp
baroclinic zone in its wake. Along this baroclinicity, robust
low-level fgen will pinch across MN/WI, coincident with an axis of
deformation owing to mid-level trough interaction and causing a
rapidly cooling column to also deepen the DGZ. Model soundings
indicate robust forcing will persist into the DGZ as it deepens
which is indicative of intensifying snow rates in mesoscale banded
structures. While the most intense bands should laterally
translate eastward with time, several hours of snowfall rates
which may approach 2"/hr at times 00Z-04Z according to the WPC
snowband probabilities will lead to rapid accumulation, highest
from MSP NE into NW WI and the Arrowhead of MN. Along this axis,
WPC probabilities for 4 inches are high, with isolated amounts
above 6" possible. Even higher amounts are likely in far NW WI
including the Bayfield Peninsula where lake enhancement off Lake
Superior could produce isolated amounts approaching 12 inches.

...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 before moving towards the
East Coast on Christmas. A wave of low pressure is likely to
develop along this front and lift northeast from the TN VLY into
Upstate NY and then Ontario by the weekend. The timing, placement,
and strength of this low will be important for snow amounts as the
developing TROWAL/theta-e advection north of the system will help
toss precipitation back atop the front to drive heavy snowfall.

The key to the snowfall forecast is timing of the front and
associated strong CAA in its wake. The GFS continues to be
progressive compared to consensus, and this forecast used heavily
the ECMWF/NAM/ECENS for timing of changeover. Ahead of the front,
the precipitation will be all rain as it will be anomalously warm.
However, rapid temperature drop behind it will allow p-type to
transition quickly to snow. Typically, cold air chasing moisture
does not produce much snow. However, in this case, pinching fgen
collocated with upper divergence and the periphery of the
strengthening theta-e ridge should drive heavy snowfall from the
Central Appalachians northward towards the Eastern Great Lakes.
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
have increased, and now show a greater than 50 percent chance for
6 inches, highest along the Lake Shore where in excess of 12
inches is locally possible.

Behind the front, strong CAA will cause 850mb temps to crash
leading to a period of heavy LES as cold air traverses the Great
Lakes producing instability and rising ELs. Heavy snow is likely
day 1-2 east of Lake Michigan where WPC probabilities indicate a
high chance for multi bands producing 6" of snowfall. Some Lake
enhancement is likely SE of Lake Erie D2 embedded within the
synoptic snows. By D3, heavy LES is likely to be dominant SE of
Lake Erie and east of Ontario where fetch becomes ideal and
instability maximizes. Pockets of greater than 6 inches of
snowfall are likely, with locally double those amounts possible,
especially into the Tug Hill Plateau.

...The West...
Day 3...

A ridge positioned across the West Days 1-2 will gradually yield
to the east as a deep trough approaches from the Pacific. This
trough will be accompanied by a 130kt jet streak with embedded
shortwaves, and the combination of height falls and LFQ diffluence
will produce a wave of low pressure moving along the west coast
and up into western WA on D3. A surface front will move onshore
coincident with the core of the jet streak, and this additional
ascent in the presence of increased PWs will produce widespread
precipitation across the West. Heavy snow is likely in the terrain
by D3 stretching from the Olympics of WA southward along the
Cascades, into the ranges of NW CA, and as far south as the
southern Sierra Nevada. Snow is also expected as far east as the
Sawtooth and Northern Rockies by the end of the forecast period.
The heaviest amounts are likely in the Sierra and Shastas which
will benefit from more orthogonal mid-level flow and better moist
advection. WPC probabilities are moderate for 12 inches across
these ranges. Elsewhere, snowfall is likely to range more in the
4-8" range according to the highest WPC probabilities.


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

Weiss