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

Valid 12Z Tue Dec 31 2019 - 12Z Fri Jan 03 2020

...Pacific Northwest through the Northern and Central Rockies...

Days 1-3...

Confluent flow setting up over the northern Pacific will drive a
plume of enhanced moisture into the Northwest with convergence and
ascent on the nose of the 700 mb jet
today across WA State, continuing inland into the ranges of
northern ID and northwest MT.
Heavy snow is likely in the northern WA Cascades at higher
elevations, with a few feet expected. 8 to 12 inches is expected
in the ranges of northern ID, with several inches in the mountains
of northwest MT.

On Wed as an upper shortwave and associated cold front move
inland, the moisture and lift should move southeast across the OR
Cascades inland through southern ID and western WY, followed by
northern NV, northern UT, and then northwest CO. Several inches
are expected across the impacted ranges, with higher amounts
expected due to longer duration snow in the WA Cascades and
Bitterroots of ID.

Probabilities are high for 12 inches in the highest terrain of the
Cascades, Bitterroots  and then the WY Tetons through Day 2, with
lower probabilities forecast across the other ranges.

Snow amounts and coverage in the central Rockies decline on Thu as
the upper trough departs the Rockies and moves across the Great
Plains.  The models show a new plume of moisture poised to move
onshore in Vancouver Island and then into WA State Thu night. 
Upper divergence in the right entrance of a jet streak over
British Columbia favors ascent in the Wa Cascades, with several
inches of snow possible.


...Great Lakes...

Days 1-2...

As the 700 mb low moves east across southeast Ontario, expect lake
effect snows to ramp up and continue into Tue night downwind of
Lake Erie and Lake Ontario, where amounts exceeding 6 inches are
possible (snow totals may exceed a foot across portions of the Tug
Hill Plateau).
Heavier snows off Lake Eris focus in southwest New York downstream
from the west-southwest boundary layer flow and lakeshore
convergence in southwest NY.  Snow across the Great Lakes should
quickly wind down on Wed as the upper low moves northeastward away
from the region, and low-level winds back and drier air moves
across the lakes.


...Maine...

Day 1...

A developing coastal low moves along the New England coast,
nearing Boston this morning and moving up the coast of Maine, and
then into New Brunswick this evening.
Mid level deformation and frontogenetic cross central and northern
Maine.
Snow is already moving into the area and is expected to continue
with the region lying in the left exit region of an approaching
upper-level jet streak and strongly diffluent flow.
The event winds down tonight as the upper trough and sfc low
departs. An additional
8-10 inches is expected in northern Maine, with a few locations
possibly reaching a foot for the event total (before and during
this forecast period).


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

Petersen