Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
411 AM EST Mon Dec 09 2019

Valid 12Z Mon Dec 09 2019 - 12Z Thu Dec 12 2019

...Great Lakes...
Days 1-3...

Wave of low pressure beneath a northern stream trough moving along
a cold front will continue to push eastward today and northeast
through the Great Lakes tonight. A band of robust mid-level
frontogenesis will produce moderate to heavy snowfall from eastern
MN into the U.P. of MI, and WPC probabilities in this zone are
high for 4 inches. However, much heavier snowfall is likely on the
south side of Lake Superior in N WI and the western U.P of MI due
to significant Lake Effect enhancement. In a narrow zone along the
southern lake shore, WPC probabilities are high for 8 inches.

As the low pulls away into day 2, strong CAA will commence across
the lakes. This will produce steep low-level lapse rates and a
favorable environment for periods of heavy Lake Effect Snow (LES).
The heaviest snowfall on D2-D3 is likely in the favorable westerly
snow belts of the Keweenaw Peninsula, NW L.P. of MI, and downwind
of Lake Erie and Ontario where WPC probabilities are moderate to
high for 4 inches both days.


...Tennessee Valley through New England...
Days 2-3...

A strong cold front will cross southeast from the Great Lakes
Tuesday into Wednesday. As this occurs, a southern stream impulse
lagging back into TX will slowly migrate northeast, producing
favorable divergence and ascent in an anafrontal precip type
setup. At the same time, an intense jet streak of 170+ kts will
pivot northward, arcing into Canada, and leaving the favorable RRQ
for diffluence and ascent from the TN Vly through New England.

Initially, the column will be too warm for p-type issues and rain
will overspread the region. However, as the column begins to cool,
p-type transition is likely, and snow will eventually become
dominant from west to east. The guidance has continued to show
this transition more quickly, and intense frontogenesis will
likely lead to heavy precipitation and faster cooling due to
dynamic effects. This suggests a burst of snowfall is possible
before the precipitation exits to the east, with the heaviest snow
likely in the terrain from WV and into the Poconos, Catskills, and
Berkshires, where WPC probabilities for 6 inches are as high as 30
percent. Further south and east, the warmness of the antecedent
airmass will take longer to overcome and cause a transition to
snow, but it is still likely into the I-95 corridor and as far
south as TN/AR. In these regions, WPC probabilities are low for 2
inches, but the trend has been upward, especially from D.C.
northward where forcing becomes intense into the DGZ. Warm temps
and rain Tuesday afternoon will initially inhibit accumulations,
but rates should become great enough to overcome this, and the
Wednesday morning commute of the I-95 corridor cities could become
hazardous.


...Pacific Northwest...
Day 3...

Mid-level trough moving onshore the Pacific Northwest Wednesday
will be accompanied by a 120kt jet streaking across the northern
Pacific Ocean, advecting moisture onshore. 850-500mb flow becomes
unidirectional from the west, and moisture spills into British
Columbia and then southeast into Washington/Idaho/Oregon/Montana.
Prolonged moisture fetch will produce heavy snow in British
Columbia, but lesser south of the international border. Still, WPC
probabilities are high for 6 inches in the Olympics and Cascades
of Washington, with lesser amounts likely into the Northern
Rockies.


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


Weiss