Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 AM EST Tue Dec 10 2019

Valid 12Z Tue Dec 10 2019 - 12Z Fri Dec 13 2019

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

Low pressure lifts away through eastern Canada today, leaving
strong CAA in its wake. This CAA moving across the Great Lakes
will produce periods of moderate to heavy Lake Effect Snow (LES)
on W to NW flow. The moisture is shallow, but forcing is robust
and prolonged with steep lapse rates and an 8kft inversion height.
The heaviest snow on D1 /Tuesday/ is likely in the Keweenaw
Peninsula where Lake Superior fetch and upslope combine, as well
as NW parts of the L.P. of MI where the effective fetch length
increases due to pre-saturation off Lake Superior. WPC
probabilities on D1 are low to moderate for 6 inches. By day 2,
the flow becomes more westerly, and the favored snow belt regions
downwind of Erie and Ontario feature the highest WPC
probabilities, which are high for 4 inches.

On D3 /Thursday/ a clipper and associated jet streak will race
southeast from central Canada and towards the Great Lakes. The
combination of a very cold column, jet level diffluence, and
subtle height falls will produce a swath of moderate snow despite
rapid progression of this system to the east. WPC probabilities
are moderate for 4 inches in northern WI and the U.P. of Michigan.


...Tennessee Valley through New England...
Days 1-3...

A strong cold front crossing the northeast and down through the
southeast will bring much colder air to these regions Tuesday and
Tuesday night. As this occurs, precipitation within a divergent
region ahead of a southern stream shortwave will continue to
spread northward in an anafrontal precipitation setup. Initially,
the precipitation will all be rain, but as strong CAA continues
the column will cool and p-type will transition to snow late
Tuesday in the Southeast, and early Wednesday into the Northeast.

While antecedent temperatures are very warm, and rainfall will be
widespread Tuesday, some snow accumulation is likely. This is due
to the likelihood for intense snowfall rates as a robust stripe of
7000-600mb fgen collocated with the RRQ of a 170+kt upper jet
streak to produce intense and deep layer ascent into the cold air.
The dynamic cooling of the column associated with heavy
precipitation will further enhance snowfall possibilities, and
HREF probabilities for 1"/hr have increased. The duration of
forcing and cold enough temperatures overlap is short, so total
snowfall should be limited. However, a few inches of snow are
possible as evidenced by WPC probabilities for 4 inches eclipsing
20% in the terrain of WV/VA, with 10-20% across southern New
England. In the I-95 corridor, a burst of heavy snow is likely,
potentially coinciding with Wednesday morning rush hour. While the
ground will be warm, heavy snow should overcome this briefly, and
WPC probabilities for 1" are as high as 50% for the urban corridor.

A secondary band of precip is progged by CAMs to cross the TN Vly
later this evening. This precip is mainly snow over TN, but mainly
ice on the higher portions of Smokey Mountains. This raises the
risk for a quarter inch of ice on those peaks to 10% tonight.

On day 3, a more pronounced southern stream low pressure may
blossom and spread precipitation from the Gulf of Mexico atop a
wedge of surface high pressure Thursday night. With cold surface
temps, especially in the western Carolinas and SW VA, the
precipitation may begin as freezing rain. There remains quite a
bit of spread in guidance in the thermals, but some freezing rain
is likely, and WPC probabilities for 1/4" are as high as 20%.


...Pacific Northwest...
Days 2-3...

Prolonged and robust Pacific Jet streak will angle into the
Pacific Northwest beginning Wednesday. This jet streak will drive
significant moist advection into the West, with small
perturbations embedded within the zonal flow accentuating synoptic
lift. Periods of heavy snow are likely on day 2 in the Washington
Cascades, with widespread heavy snow spreading eastward as far as
the Colorado Rockies and all ranges in between as the nose of the
jet shifts eastward. WPC probabilities feature a high risk for 12
inches from the Washington Cascades and Oregon Cascades, into the
Sawtooth, Northern Rockies, and Grand Tetons, with snow levels as
low as 1000 ft near the Canada border, to as high as 5000 ft in
Colorado. Total 2-day snowfall could eclipse 4 feet in the
Washington Cascades, with more than 2 ft possible in the other
highest terrain.


Weiss