Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 PM EST Tue Dec 31 2019

Valid 00Z Wed Jan 01 2020 - 00Z Sat Jan 04 2020

...Pacific Northwest through the Northern and Central Rockies...
Days 1-3...

An atmospheric river continues bringing robust Pacific moisture of
tropical origin into the Pac NW into tonight before diminishing
into Wednesday. The axis of the warmest air (with snow levels of
8000-9000ft) shifts south across OR tonight with much lower snow
levels Wednesday into Thursday in lesser moisture but continued
onshore flow. Moisture spreads well inland with Day 1 WPC
probabilities for one foot or more moderate to high for the
highest Cascades and across the higher peaks of eastern OR...all
of ID and western WY/MT. Snow amounts and coverage in the central
Rockies decline Wednesday night/Thursday as the upper trough
departs the Rockies and moves across the Great Plains. 

The next trough approaches the Pac NW Friday with leading warm
frontal precip reaching western WA Thursday evening with
preliminary snow levels of 4000 to 5000ft in the WA Cascades.
Heavy snow is likely with Day 3 probabilities likely for 8 or more
inches in the northern WA Cascades, though as of now the axis of
strongest moisture looks to go into BC rather than the Pac NW.


...Northeast...
Day 1...
The ongoing snow squall over upstate NY and extreme northern PA
continues to progress on the remnant front of the primary low that
tracked through the Great Lakes. However, as the low off Maine
becomes more dominant tonight (and downsloping effects occur) the
squall should quickly diminish as it crosses the Catskills and
Adirondacks, per recent runs of CAMs such as the HRRR and NAM 3km.

Energy translates east across southern Quebec tonight as the
primary system that tracked through the Great Lakes merges into
the secondary system that came up the Northeastern Seaboard. This
brings a strong west wind across the Northeast, including over
Lakes Erie and Ontario which, along with cold air advection over
the roughly 40F water, brings lake effect snow for the Tug Hill
Plateau of Upstate NY and areas east of Lake Erie well into
Wednesday. Day 1 WPC probabilities for 8 or more inches are likely
for the Tug Hill (moderate for 12 or more inches) with moderate
probabilities for 8 or more inches in far western NY downwind of
Lake Erie.


The probability of 0.25 inches of icing is less than 10 percent
Days 1-3.

Jackson