Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 AM EST Sat Dec 07 2019

Valid 12Z Sat Dec 07 2019 - 12Z Tue Dec 10 2019


...Mountain West...
Days 1-2...

A trough/upper-level low is forecast to approach the West Coast,
finally moving onshore late tonight 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 across the Sierra
and adjacent northern California ranges, where storm total
snowfall amounts may exceed 4 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 Sunday, the best forcing within the diffluent jet
region and best 700mb moist advection will shift eastward, driving
the heaviest snow into the Wasatch, Tetons, and Colorado Rockies.
WPC probabilities are high for 8+ inches Sunday in these regions.


...Northern Plains into the Western Great Lakes...
Days 2-3...

Northern stream shortwave will dig into the Northern Plains on the
leading edge of an amplifying longwave trough which will bring
extremely cold air into the region. Ahead of this feature, a wave
of low pressure will develop along a cold front and race quickly
eastward spreading a swath of snowfall from the High Plains of MT
eastward as far as Michigan. Modest mid-level frontogenesis and
weak LFQ jet level diffluence will combine to produce moderate
snow. The trend with the overnight guidance has been to shift
everything a bit further north, and the rapid progression of this
feature should limit total accumulation, however a cold column
should produce some fluffy SLRs. WPC probabilities are generally
less than 50% for 4 inches except in the vicinity of Lake Superior
where some lake effect enhancement is possible late on day 3 as
robust CAA commences.

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

Weiss