Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 PM EST Tue Nov 20 2018

Valid 00Z Wed Nov 21 2018 - 00Z Sat Nov 24 2018

...New York and New England...
Day 1...

A potent shortwave trough will dig into the Great Lakes and swing
across New England. This will reinforce the longwave trough across
the east, while tracking a weak surface low into Maine by the end
of day 1. An arctic cold front will push across the area behind
this low, and snow showers are expected to accompany this frontal
passage. Moisture is relatively limited and and the system is
progressive, so snow accumulations will likely be light. However,
synoptic ascent due to height falls and brief jet level
diffluence, combined with upslope enhancement into the terrain of
Adirondacks and points east will create some light snow
accumulations, with WPC probabilities for 4 inches less than 20
percent. Additionally, low-level instability coincident with 925mb
frontogenesis and 0-2km theta-e lapse rates less than 0 suggest
the potential for snow squalls Wednesday afternoon dropping SE
from upstate NY, through VT/NH/ME, and potentially into
Massachusetts by the evening. While additional accumulations will
be less than 1 inch, a brief period of intense snow rates is
likely during any squall.


...Great Lakes...
Day 1...

Periods of heavy lake effect snow are likely to continue through
Wednesday on the E/SE side of all of the Great Lakes. A favorable
LES setup will exist behind today's shortwave, and despite flow
becoming more from the NW on Wednesday, subtle enhancement in
synoptic ascent will combine with continued favorable conditions
for LES due to CAA to produce heavy snow in the favorable regions
SE of the lakes. The heaviest snow is likely downwind of Lake
Ontario due to upslope enhancement into the Tug Hill Plateau as
well as at least a subtle connection from Lake Huron, but WPC
probabilities are moderate for 4 inches of snow as well across the
U.P. of Michigan, the NW corner of the L.P., as well as SE of Lake
Erie. Drier air and the onset of warm advection Wednesday night
will bring an end to the lake effect snow.


...The West...
Days 1-3...

Widespread and heavy snowfall is expected across all the mountain
ranges from the Sierra Nevada in California, to the Rockies of
Colorado, and points north. 1 to 2 feet of snow is likely in the
highest terrain.

A deep trough that spans the west coast will move onshore the west
coast Thursday with an associated surface cold front. This will be
accompanied by deep Pacific moisture and jet level diffluence
producing snow in the coastal ranges as early as Wednesday night.
Significant moist advection ahead of the 700mb trough will produce
increasing column RH and as moisture spills atop the coastal
ranges, snowfall will spread from the Cascades and Sierras,
eastward into ID/WY/NV/UT/CO and AZ. Snow levels initially will be
6-8 kft, but will fall to 3-5 kft by the end of day 2. Snowfall on
day 2 will likely exceed 12 inches in the Sierra Nevadas, and WPC
probabilities show a high risk for this. The other ranges will see
lesser snow, but WPC probabilities for 8 inches are moderate
across the Cascades, as well as the Uintas and Wasatch of Utah.

By day 3, an atmospheric river (AR) event is progged to begin,
lifting into northern California and Oregon. This will be
accompanied by a strong Pacific jet maximum and anomalously high
PWAT. A reinforcing cold front beneath the AR will keep snow
levels down across OR/WA, but snow levels will climb south of the
baroclinic gradient, limiting the snow somewhat across the
Sierras. However, the Cascades of Oregon and Washington, as well
as the mountains of Idaho and western WY will experience
widespread heavy snowfall with 12 inches or more possible in the
highest terrain. Continued lower snow levels of 2-4 kft into ID/MT
will allow even the valleys to receive lighter snows Friday as
moisture spreads well inland from the coast.



The probability of significant ice (0.25-inch or more) is less
than 10 percent.

Weiss