Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 AM EST Wed Dec 18 2019

Valid 12Z Wed Dec 18 2019 - 12Z Sat Dec 21 2019

...Northeast...
Day 1...

An arctic front ahead of a cold core low will continue to push
from the eastern Great Lakes this morning and cross the Northeast
through the rest of today. Two rounds of snow are still expected:
snow squalls immediately along the front, and lake effect snow in
the favored NW flow snow belts off Erie and Ontario.

Snow squalls are likely to become intense Wednesday afternoon from
upstate New York through much of New England. Guidance continues
to indicate that squalls could become significant despite being
fast moving, with theta-e lapse rates below 0C, collocated with
SBCape approaching 300 J/kg and robust 925mb fgen. The speed at
which these squalls rotate eastward will limit snow accumulations
to very light amounts, but briefly excessive snow rates and strong
winds could create hazardous conditions.

Behind the front, strong CAA will drive LES on NW flow,  and a
brief but robust period of lake effect snow is expected for
favored downwind locations. Open water with surface temperatures
generally around 40F (some pockets of mid 40s over Erie and
Ontario) will lead to very steep lapse rates and inversion heights
over 8kft, suggesting intense lake effect bands. The wind
trajectory suggests multi bands across most of the Lakes, but
effective fetch length into and across Lake Ontario should produce
heavy snow, and WPC probabilities for 6 inches are moderate over
the Tug Hill Plateau.


...Northwest...
Days 1-3...

A mid-level trough will moving onshore northern CA this morning
will moderate mountain snows to the far northern CA mountains
today. Following this, a zonal flow across the Pacific leading to
a prolonged atmospheric river event will occur over the Pacific NW
Thursday into Saturday night. Snow levels will initially be low,
but will rise quickly on Thursday with snow levels expected to
climb to or above 6000 ft across the region Thursday night. Higher
snow levels will develop further inland on Friday. The heaviest
snowfall this period is for the WA Cascades where snow levels
briefly raise above 4000ft (the height of Stevens Pass) Thursday
night before dropping again. Days 1 and 2 WPC probabilities for 18
inches are over the highest Cascades, and Day 3 are high over much
of the WA Cascades. 3-day totals over 5 feet are expected for the
highest peaks. Elsewhere, WPC probabilities are high for 8 inches
spreading from Day 1 down the higher OR Cascades as far south as
Mt Shasta in CA, then across the Blue Mountains of OR, The
Sawtooth of ID, and the Northern Rockies of ID/MT on Day 2 and the
far northern Rockies of ID/MT on Day 3. Interior snow levels
generally rise from 3000ft to above 6000ft as the atmospheric
river fully arrives into Friday.


The probability for significant freezing rain is less than 10
percent all 3 days. However, cold air drainage through the
Columbia River Gap on the OR/WA border should cause freezing rain
on the west side of the gap, a well known localized phenomena.

Jackson