Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
304 PM EST Mon Dec 16 2019

Valid 00Z Tue Dec 17 2019 - 00Z Fri Dec 20 2019

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

Pronounced subtropical jet streak will lift northeast into New
England while strengthening to greater than 170 kts. The right
entrance region of this jet streak will position itself over New
England, while a surface wave lifts northeast along a baroclinic
zone from near KY northeast to off the Massachusetts coast Tuesday
into Wednesday. Ahead of this feature, weak high pressure will
gradually retreat in response to strengthening WAA and the
approach of the surface low.

This initial WAA will cause precipitation to spread northward from
the Mid-Atlantic into New England. Initially, this will all be
snow as the entire column is below 0C. Additionally, some of this
could be heavy as noted by strong WAA producing some strongly
sloped fgen into the DGZ. This mesoscale forcing combined with the
larger scale synoptic ascent could produce snowfall rates of 1"/hr
across portions of New York and New England. There remains some
uncertainty into how far north the heavy precipitation will spread
as forcing gets shunted eastward, however, the trend has been for
a slightly further north and wetter solution. This suggests the
heaviest snow is likely in a swath from northern PA through
central NY and into parts of central New England, with terrain
enhancement likely. WPC probabilities for 4 inches are highest in
the Catskills and Berkshires where they exceed 50%, but are
moderately high from near Buffalo, NY northeast into the Worcester
Hills of MA and White Mountains of NH.

Further south, the WAA will gradually drive a warm nose above 0C
while low-level cold air lingers. Despite the lack of a strong
cold surface high to the north, low level northeast ageostrophic
winds persisting at the surface should lead to prolonged freezing
rain, focused in parts of southern New England. Even further to
the south, significant accretions of freezing rain are also
becoming more likely in the terrain of PA where cold air will
remain locked in the elevations as waves of precipitation spread
northeastward. This is where the highest probabilities for
significant accretions exist, and WPC probabilities for 0.25"
exceed 50% in the terrain of central PA, with probabilities of
30-50% in parts of NJ, CT, and lower upstate NY. However, there is
above normal uncertainty into amounts even on D1 as strong forcing
could produce heavy rates, which could runoff and lead to lower
accretion efficiency.

Day 2...

An arctic front will cross from the Great Lakes through Maine
Wednesday. Along and behind this front, the high-res guidance is
suggesting a potential 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 aftn/eve. 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 Minnesota Tuesday
and then race across through the Northeast Wednesday. Behind this
front, strong CAA will develop on gusty NW winds, and a brief but
robust period of Lake Effect Snow (LES) is likely. On Day 1, this
will be focused in the U.P. of MI and far NW L.P. of MI on day 1,
spreading eastward to the favored downwind locations of Lake Erie
and Lake Ontario during day 2. Pools of warm lake water
temperatures 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 exceed 20% downwind of these longer fetches near Traverse
City, MI and in far SW New York just downwind of Erie, PA.


...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 AR event is likely
starting on Thursday. While snow levels will initially be quite
low, they will rise steadily Thursday, especially west of the
Cascades, where snow levels are forecast to climb above 6000 ft.
East of the Cascades they will remain much lower, and moisture
spillover late in the period should produce snowfall as far east
as the Northern Rockies and Sawtooth Ranges. The heaviest snowfall
this period is expected in the Olympics an Cascades of Washington,
where WPC probabilities feature a high risk for 12 inches on D3,
and 2-day totals may locally exceed 3 feet. Elsewhere, WPC
probabilities are moderate for 6 inches as far south as Mt Shasta
in California, into the Blue Mountains of Oregon, The Sawtooth of
Idaho, and the Northern Rockies, with snow levels generally
2000-3000 ft in these ranges.


Weiss