Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
411 AM EST Thu Dec 05 2019

Valid 12Z Thu Dec 05 2019 - 12Z Sun Dec 08 2019

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

A fast moving mid-level disturbance today will eject eastward into
the Plains from the Great Basin. This feature will interact with
modest SW 700mb flow ahead of it to produce ascent in a region of
enhanced moisture. This will produce a brief period of moderate
snowfall at elevations generally above 5000 ft. WPC probabilities
are high for 6 inches in portions of the Colorado Rockies and San
Juans.

Beginning late on D2, Friday into Saturday, a deep upper low will
approach the PacNW coast. Ahead of this low, a robust Pacific Jet
reaching 130+ kts will drive into California, spreading
significant moisture into the state before spilling northeast
through much of the Northwest. Ascent will become strong in
response to height falls, PVA, jet diffluence, and SW 700mb flow
into the terrain. Snow levels south of 40N will generally be above
5000 ft, falling to 1000 ft near the Canadian border by the end of
the forecast period. Snowfall is likely to be heavy in the
mountains from the Sierra of California, and northeast through the
Oregon Cascades, the ranges of NW Wyoming, the Sawtooth in ID, and
into the Northern Rockies of MT. WPC probabilities for 8 inches
are high across these ranges, but the heaviest snowfall is likely
to be in the Sierra where D2/3 totals could reach 4 feet.


...Northeast...
Day 2...

A fast moving shortwave dropping through the persistent NW flow
across the east will drive a weak Alberta Clipper type low across
the Great Lakes and into the northeast Thursday night and Friday.
Subtle LFQ jet diffluence and modest PVA will provide deep layer
ascent, and a stripe of snowfall is likely from the northern L.P.
of Michigan through upstate NY and into interior New England. In
addition to the synoptic lift, some weak lake effect enhancement
is possible downwind of Lake Erie and Ontario, and upslope flow
will also likely enhance snowfall in the
Catskills/Adirondacks/Berkshires. The system is fast moving and
total forcing is not intense, so WPC probabilities for 4 inches
are 20% or less, and highest in the Tug Hill, Adirondacks, and
into the southern Greens of VT.


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

Weiss