Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EST Fri Dec 06 2019

Valid 00Z Sat Dec 07 2019 - 00Z Tue Dec 10 2019

...Northeast...
Day 1...

A brief period of lake effect snow with some upslope enhancement
is possible tonight as a surface low pressure system quickly moves
away from the region. Activity should peak between 00Z-06Z tonight
before quickly tapering off as surface high pressure moves in and
upper-level support quickly move east. At least a few additional
inches of snow are possible downwind of Lake Ontario.


...Mountain West...
Days 1-3...

A trough/upper-level low is forecast to approach the West Coast,
finally moving onshore late Saturday into early Sunday. An
additional northern stream shortwave will dig southeastward across
British Columbia and into the Pacific Northwest during the same
time frame. A strong Pacific upper-level jet is forecast to reach
central California ahead of the strongest height falls, aiding in
ascent and large-scale forcing for precipitation. The combination
of these systems is expected to produce precipitation across much
of the West. The heaviest snows are likely D1 into D2 across the
Sierra and adjacent northern California ranges, where storm total
snowfall amounts may exceed 5 feet in the highest terrain.
Additional heavy snows (perhaps approaching or exceeding a foot)
are forecast across the higher terrain of Idaho, Montana, and
Wyoming. By late D2 into D3, the best forcing and moisture will
shift eastward into portions of the central Rockies, with moderate
to heavy snows expected for those areas.


...Northern Plains/Upper Midwest...
Days 2-3...

A northern stream shortwave diving through Montana will push a
cold front into the Northern Plains, with weak surface
cyclogenesis possible along it. This will produce a band of
frontogenesis from eastern MT, through the Dakotas, and into
Minnesota, with the potential to produce up to a few inches of
snowfall. At the same time, a more substantial area of low
pressure is likely to develop across the Midwest by Monday
morning, which could begin to enhance snowfall coverage and/or
rates across portions of the Upper Midwest and Upper Great Lakes.
WPC probabilities show a slight to moderate chance of exceeding 4
inches of snow across portions of the Upper Midwest on D3.

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

Ryan