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

Valid 12Z Tue Dec 17 2019 - 12Z Fri Dec 20 2019

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

The mid-level wave over the Midwest with the surface low over the
central Appalachians this morning will shift northeast today with
the surface low pushing off Nantucket this evening as a cold core
low approaches Lake Superior. A swath of snow north and west of
the low level low will shift across the Northeast from NY State to
northern New England as overrunning flow with low to mid level
frontogenesis promotes moderate rates. WPC Day 1 has lower
probabilities for six inches of snow from the Hudson Valley to
north-central Maine and north of the northern MA border.

Ahead of the low...from northeast PA across southern New England a
warm nose overrunning low level cold air promotes freezing rain
and sleet today. WPC Day 1 probabilities for a tenth of an inch of
ice is moderately high, particularly from southern NY and CT
across the border into MA.

Days 1-2...
An arctic front will push east from the Great Lakes through Maine
Wednesday. Along and behind this front is the threat for fast
moving snow squalls racing across upstate NY, into
Northern/Central New England. Modest instability (SBCape ~ 300
J/kg) combined with theta-e lapse rates less than 0, and high
values of the snow squall parameter suggest snow squalls are
likely Wednesday afternoon/evening. While snow amounts are likely
to be minimal, briefly very heavy snow and strong winds could
create hazardous conditions.


...Great Lakes...
Days 1-2...

An arctic cold front will move southeast from Lake Superior
Tuesday and cross Lake Ontario early Wednesday. Behind this front,
strong CAA will develop on gusty NW winds, 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 should produce heavy snow, and WPC probabilities for 6
inches are moderate off Lakes Erie and Ontario.


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

Mid-level trough will move onshore California Wednesday, followed
by a pronounced Pacific jet and zonal flow across the Pacific.
This will advect increasing moisture into the Pacific Northwest
beginning Wednesday, and a long duration atmospheric river event
is expected to start Thursday. 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. The
heaviest snowfall this period is for the WA Cascades where Day 3
WPC probabilities feature a moderate risk for 18 inches on D3 with
2-day totals over 3 to 4 feet for the highest peaks. Elsewhere,
WPC probabilities are moderate for 6 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.


Jackson