Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Sun Dec 16 2018

Valid 12Z Sun Dec 16 2018 - 12Z Wed Dec 19 2018


...Sierra and Pacific Northwest to the Northern Rockies...
An amplifying trough and associated frontal band will reach the
West Coast this evening. As precipitation moves into the West,
snow levels will be high - limiting the Day 1 threat for heavy
snow mainly to the higher elevations of CA/OR/WA, particularly the
northern Cascades. Snow levels will fall behind the trough, but
less moisture means less snow for Day 2. A long fetch of Pacific
moisture reaches the Pac NW Monday night, moving into western
Washington and Oregon. This reaches the northern Rockies along
with a strong upper jet by late Tuesday. While snow levels will
increase across OR/CA on Monday night, heavy mountain snows are
expected for the north side of the moisture axis from the northern
Cascades to the northern Rockies, especially for areas in the
northern Cascades above 5000 ft.


...Northeast U.S...
Southern stream low pressure currently over the central
Mid-Atlantic will track northeast with surface cyclogenesis along
the southern New England coast by late today. An easterly flow
ahead of this low will spread precip across the northeast through
Day 1. A surface high currently over northern New England shifts
east, leaving a dry wedge as well as the potential for a wintry
mix. A Day 1 QPF preference was given to the 00Z ECMWF/CMC/CAMs
which support a northward shift compared to prior runs as well as
the 00Z GFS/UKMET.   Dynamic cooling in the trowal of the system
should support a further increase in wintry weather coverage, with
some areas of eastern Pennsylvania and Upstate New York changing
over to all snow by late tonight.  Overall snow and ice amounts
are expected to remain light, although locally significant amounts
are possible, with WPC probabilities indicating the greatest
potential for four inches in Day 1 over the Green Mountains and
Berkshires.

A northern stream trough begins to phase with the system across
New England Monday with enhanced wrap around snow expected for
Maine. Moderately high WPC probabilities for four inches are
across northern ME for Day 2.

Jackson