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

Valid 12Z Fri Dec 06 2019 - 12Z Mon Dec 09 2019

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

A fast moving clipper will drop from Michigan across New York
state and through New England today. Although this system is a
fast mover, ascent will briefly be robust as weak mid-level fgen
and intensifying jet-level diffluence combine, focused across
upstate New York. This will occur in an environment with
increasing saturation as antecedent flow briefly becomes SW to add
moisture to the column. Additionally, some upslope enhancement and
lake effect is possible as the low races away tonight. WPC
probabilities for 4 inches are moderate, and highest in the
southern Green Mountains of VT, as well as downwind of Lake
Ontario and into the Southern Adirondacks of New York.


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

A vertically stacked upper low will meander off the northern
California coast before finally lifting onshore Sunday. At the
same time, a northern stream shortwave will dig across British
Columbia and into the Northwest. Moisture spreading onshore within
a robust Pacific Jet south of the California low, and within an
upper level belt of moisture confluence between the two mid-level
features, will spread precipitation across much of the West. The
most significant precipitation is likely on D1-D2 in the Sierra
and other ranges of northern CA, as well as into the terrain from
NW Wyoming through Idaho and Montana. During this period,
significant moisture advection will combine with LFQ jet position
and mid-level height falls to produce ascent and heavy snowfall.
WPC probabilities for 8 inches are high across these ranges, as
well as into the Siskiyous and Shasta/Trinities of northern
California. In the northern California ranges, D1-D2 snowfall
totals may exceed 5 feet in the highest terrain. By D3 /Sunday/
the most robust forcing will combine with better moisture
spillover further eastward into the Rockies. High WPC
probabilities for 8 inches shift east Sunday, becoming highest in
the Wasatch of Utah, the Tetons of Wyoming, and the Rockies/San
Juans of Colorado.


...Northern Plains...
Day 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. At the same time, a more substantial area of low
pressure is likely to develop across the Missouri Valley late in
the forecast period, with moisture spreading northward Sunday
night into Monday morning. There remains some uncertainty into how
far north the moisture from this secondary system will spread
before the medium range, and for Sunday/Sunday night most of the
snowfall should be associated with the modest fgen. WPC
probabilities have increased and depict a swath of 20-40% for 4
inches from eastern MT into SW MN, with more moderate snow
possible just beyond this forecast period.

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

Weiss