Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Mon Dec 19 2022

Valid 12Z Mon Dec 19 2022 - 12Z Thu Dec 22 2022


...Great Lakes...
Days 1...
A shearing out upper-level low that will slide eastward today over
New England combined with strong westerly winds allows for
additional moderate to heavy lake effect snow downwind of the
Lower Great Lakes today. Most of the localized heavy snowfall
rates are likely this morning and afternoon east of Lake Erie and
Lake Ontario before the pressure gradient and upper level forcing
weakens tonight. WPC PWPF for additional snowfall amounts greater
than 4 inches are between 40 and 70 percent across the Tug Hill
Plateau.

...Pacific Northwest, Northern Rockies, and Northern High Plains...
Days 1-3...
A deep closed-low is currently centered over western Canada with a
broad upper trough extending from the Pacific Northwest into the
northern High Plains.  A well-defined shortwave is forecast to
move through the base of the trough today. This will drive an
arctic airmass farther south, with accumulating snow expected for
portions of the northern Cascades, Blues, and northern Rockies.
While widespread heavy totals are not expected, some locally heavy
totals are possible for the higher peaks. 

A more amplified and wetter system will begin to impact the region
on Tuesday. While the heaviest amounts will likely fall in the
mountains, cold air in place is expected to support accumulating
snows across the lowlands of western Washington by early Tuesday.
In addition to favorable upper forcing, strong low-to-mid level
frontogenesis will support heavy mountain snow spreading east from
the Pacific Northwest into the northern and north-central Rockies
on Wednesday. By late Wednesday the areas most likely impacted
with heavy accumulations are to include the northern Cascades, the
northern Idaho and the western Wyoming ranges. WPC PWPF shows high
probabilities for two-day snow totals exceeding a foot for parts
of those areas.

By Tuesday night into early Wednesday, favorable upper divergence
associated with a left-exit region of a strong diving 250mb jet
streak will help spread overrunning precipitation into the
northern High Plains of Montana. Surface temperatures will be
extremely cold and well below zero during this timeframe, which
may actually temper SLRs from exceeding 20 to 1. WPC PWPF for at
least 6 inches of snow are high across the Little Belt and Snowy
Mountains, with more widespread high probabilities for at least 4
inches throughout central Montana.

...Central Plains and Middle Missouri Valley...
Day 3...
The aforementioned shortwave and arctic cold front pushing south
into the central Plains Wednesday into early Thursday will have
impressive fgen, enhanced upper level jet support, and very cold
air to allow for widespread moderate snowfall amounts. WPC PWPF
has 30 to 60 percent probabilities for at least 4 inches of snow
through Thursday morning over a large region spanning from the
central Plains to southern Minnesota. For the most part, moisture
will be limited for this section of the country on Day 3 as a very
anomalous arctic airmass surges southward. Enhanced lift within a
healthy DGZ for much of the central Plains and into the Upper
Midwest will support very efficient SLRs to make up for the lack
of QPF, along with surface temperatures around zero. High SLRs
above 20 to 1 may be short-lived for much of the central High
Plains, as strong low to mid-level winds above 50 kts fracture the
falling dendrites. However, the gusty winds will add to the
potential for dangerous travel as visibility decreases and blowing
snow ensues. This system will continue to progress eastward and is
eventually expected to become a large and powerful storm set to
impact much of the Midwest and Great Lakes with potentially heavy
snow later this week.

...Southern/Central Appalachians...
Day 3...
In response to the deepening trough over the central U.S. on
Wednesday, moisture return along the East Coast may enter into the
southern Mid-Atlantic and eventually ride up along the southern
and central Appalachians Wednesday night into Thursday morning. A
strong surface high depicted by all guidance over New England
during this time frame will support cold air damming into Thursday
morning and the potential for light freezing rain over mainly the
higher terrain and areas banked up against the Blue Ridge
Mountains. WPC probabilities for at least a tenth of an inch of
ice accretion are low (10-30 percent) across western North
Carolina and southwest Virginia. The threat of freezing rain is
expected to expand northward into the central Appalachians on
Thursday.


Snell