Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 PM EST Tue Dec 17 2019

Valid 00Z Wed Dec 18 2019 - 00Z Sat Dec 21 2019

...Great Lakes/Northeast...
Day 1...

An arctic front will drop quickly from the Great Lakes through
Maine tonight into Wednesday. This front will produce two rounds
of snowfall: one in scattered snow squalls immediately along and
behind the front, and a second area with Lake Effect snow in the
favored NW snow belts.

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 (upper 30s on Superior and some pockets of
mid 40s over the rest of the lakes) 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 increase off
Superior-Michigan and Huron-Erie, and even off the Georgian Bay
into Lake Ontario should produce heavy snow, and WPC probabilities
for 6 inches are moderate.


...Northwest...
Days 2-3...

Mid-level trough will move onshore California Wednesday, followed
by a pronounced Pacific jet and zonal flow across the Pacific
leading to a prolonged atmospheric river event. While snow levels
will initially be low, they will rise steadily Thursday and
Thursday night with snow levels expected to climb above 6000 ft
across the region Thursday, and these higher snow levels will
develop further inland on Friday. The heaviest snowfall this
period is for the WA Cascades where Day 2 and 3 WPC probabilities
feature a moderate risk for 18 inches, with 2-day totals over 3 to
4 feet for the highest peaks. Elsewhere, WPC probabilities are
high for 8 inches down the higher OR Cascades as far south as Mt
Shasta in CA, across the Blue Mountains of OR, The Sawtooth of ID,
and the Northern Rockies of ID/MT, with snow levels generally
rising from 3000ft to above 6000ft as the atmospheric river fully
arrives.


The probability for significant freezing rain is less than 10
percent all 3 days.

Weiss