Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 PM EST Mon Dec 21 2020

Valid 00Z Tue Dec 22 2020 - 00Z Fri Dec 25 2020


Day 1...
...Central Appalachians...
A vigorous shortwave trough noted in water vapor imagery this
afternoon near Chicago will quickly advance eastward tonight
across portions of the Great Lakes, central Appalachians, and
Northeast. Strong westerly flow in its wake will lead to favorable
upslope snows on the higher terrain areas along the Appalachians.
The latest WPC PWPF show the greatest probabilities for exceeding
4 inch totals along the Allegheny Mountains of West Virginia.
Snowfall rates should taper off Tuesday morning.

...Pacific Northwest to Northern Rockies...
Shortwave troughing pushing onshore the Pacific Northwest later
today will track across Washington State into the northern Rockies
tonight. The combination of large scale favorable forcing for
ascent and moisture will support the development of moderate to
heavy precipitation across the region. The highest snowfall
probabilities remain over the northern Cascades into the far
northern Rockies.

Day 2...
A deepening area of low pressure is expected to track across the
northern Plains late Tuesday night into Wednesday morning. While
the greatest threat for higher accumulating snows will be north in
Canada, minor accumulations are forecast for portions of North
Dakota and northern Minnesota. This combination of light snow with
very strong winds could result in hazardous winter conditions. 

Day 3...
...Upper Midwest/Great Lakes...
As the low pressure lifts northeast, backside/wrap around snow
will be likely across portions of northern Minnesota, northern
Wisconsin, and the Upper Peninsula of Michigan. The tight pressure
gradient from a building Canadian high pressure will result in
enhanced lake effect snows off Lake Superior where several inches
could accumulate on Day 3 /00z Thu-00z Fri/.

Additionally, as the strong cold front marches toward the East
Coast, crashing temperatures in the low levels enhanced by a
secondary low pressure lifting northward through southern
Appalachians toward the Mid-Atlantic, could change over
precipitation from rain to snow on the backside before ending
across portions of the Great Lakes, Ohio Valley, and Tennesse
River Valley Thursday afternoon/night. Uncertainty is a bit higher
here with model spread still high with both differences seen in
the timing of the cold air and amount/depth of moisture in place
when thermal profiles do support snow versus rain.


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

Taylor