Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 PM EST Sat Dec 15 2018

Valid 00Z Sun Dec 16 2018 - 00Z Wed Dec 19 2018


...Sierra and Pacific Northwest to the Northern Rockies...

An amplifying trough and associated frontal band will continue to
move across the eastern Pacific toward the West Coast on Sunday. 
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 the northern Cascades.  Snow levels will fall
appreciably as the trough moves inland Monday morning - with
probabilities for heavy snow increasing along the Sierra as a slug
of deeper moisture advects into the region as well.  However,
snows will quickly taper off as this system moves progressively
east across the Intermountain West on Monday.  Monday night,
models show the leading edge of a long fetch of Pacific moisture
moving into western Washington and Oregon Monday evening; reaching
the northern Rockies along with a strong upper jet by late
Tuesday.  While snow levels are expected to increase some on
Monday, heavy mountain snows are expected for portions of the
northern Cascades to the northern Rockies, especially for areas in
the northern Cascades above 5000 ft.


...Northeast U.S...

Overall, the models are in good agreement showing the closed low
presently over the Ohio/Tennessee valley lifting to the
east-northeast into the Mid Atlantic overnight.  Confluent flow
aloft will support cold high pressure over the northeast,
fostering a wintry mix for portions of the region as precipitation
spreads north across Pennsylvania and Upstate New York Sunday
morning.  Guidance shows the upper system deepening and assuming a
negative tilt as moves off of the northern Mid Atlantic coast on
Sunday.  Dynamic cooling on the backside 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 Sunday.  Overall snow and ice amounts are
expected to remain light, although locally significant amounts are
possible, with WPC probabilities indicating the greatest potential
for either centering across the Pocono and Catskill mountain
regions.  From Sunday evening onward, light snow and ice
accumulations can be expected to spread further northeast across
interior central New England into southern Maine.  However, the
latest model consensus indicates a surface low track far enough
offshore to hinder any widespread heavy amounts into late Monday. 
By Monday evening, models show a deep surface cyclone lifting into
Atlantic Canada, with some additional light snows spreading from
the backside of the system into eastern Maine.

Pereira