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

Valid 12Z Wed Dec 23 2020 - 12Z Sat Dec 26 2020


Day 1...
...Central Plains to the Upper Midwest...
After several runs of struggling to reach a consensus, the 00Z
model runs have moved into better agreement -- offering a strong
signal for moderate to heavy snows developing from eastern
Nebraska and southeastern South Dakota to northwestern Minnesota,
northwestern Wisconsin and the western U.P. of Michigan.  A
powerful surface low centered over South Dakota this morning is
forecast to move into the upper Mississippi valley later this
morning.  As a well-defined shortwave moving through the base of
the associated upper trough begins to lift northeast from the
central Plains into the upper Mississippi valley, a developing
comma head of moderate to locally heavy snow is expected to
develop on the west side of the low.  Low-to-mid level
frontogenesis is expected to help support banded precipitation,
with the 00Z HREF showing several members suggesting snowfall
rates of 0.5-1 in/hr within these bands as they move east across
the region on Wednesday.  This snowfall along with strong, gusty
winds is likely to result in reduced visibilities and blizzard
conditions across portions of the region.  Latest WPC PWPF
indicates high probabilities for accumulations of 4-inches or more
extending from northeastern Nebraska and eastern South Dakota to
northwestern Minnesota and the western U.P.  Within this area,
high probabilities for 8-inches or more extend from south-central
Minnesota to northwestern Minnesota and the western U.P.  Some of
the highest probabilities center along the Wisconsin and western
U.P. shores of Lake Superior, where in addition to the
synoptically driven snows, northerly flow on the backside of the
system may provide some lake-enhanced totals.

Day 2...
...Ohio Valley/Central Appalachians/Great Lakes...
As the previously noted low moves north of the Upper Great Lakes,
a sharp trailing cold front is forecast to move east across the
Ohio and Tennessee valleys on Thursday.  Rain changing to snow
before precipitation ends is expected to produce mainly light snow
accumulations across the lower Ohio and Tennessee valleys through
Thursday.  Then by Thursday afternoon, as a well-defined shortwave
begins to move through the base of the associated upper trough,
surface low pressure is forecast to develop and move north along
the front into the central Appalachians -- raising the potential
for significant snows to develop back in the cold air.  While
there is some uncertainty in the details given the timing of the
cold air, significant accumulations look likely across portions of
the southern and central Appalachians and Ohio valley Thursday
night into early Friday.  As a new upper low begins to develop
over the Ohio valley, surface low pressure will continue to
intensify as it moves north through the upper Ohio valley and
Lower Great Lakes region early Friday.  As the low moves north of
the Lakes, northerly flow behind the departing system will support
lake effect snow bands developing off of Lake Erie into
northwestern Pennsylvania and northeastern Ohio Friday morning.

Meanwhile, northwesterly flow across the Upper Great Lakes will
support developing lake effect snow bands, with locally heavy
accumulations likely across portions of the western and central
U.P. of Michigan and along the eastern shores of Lake Michigan.

Day 3...
...California and the Pacific Northwest...
Following a few days of dry weather, precipitation is expected to
return to the region late Friday into early Saturday.  Models show
a deepening surface low tracking northeast toward Vancouver Island
with an occluded front moving inland across the Northwest and
Northern California Saturday morning.  A well-defined mid level
shortwave/upper jet moving through the base of the associated
trough is expected to support organized heavier precipitation
across Northern California, including significant snows from the
Klamath Mountains to the Sierra.  

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

Pereira