Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
338 AM EST Tue Nov 15 2022

Valid 12Z Tue Nov 15 2022 - 12Z Fri Nov 18 2022

...Southern Appalachians through northern New England...
Days 1-2...

The first nor'easter of the season will develop mid-week spreading
a mixture of snow, sleet, and freezing rain across the region. A
wave of low pressure will develop off the VA coast Tuesday night,
and then lift northeast to around Cape Cod Wednesday afternoon and
then into the Canadian Maritimes on Thursday while slowly
deepening. This surface low will develop in response to a broad
trough digging across the Upper Midwest and Great Lakes, which
will then pivot across New England by Thursday morning. Downstream
of this trough axis, a SW to NE oriented jet streak will intensify
to 130 kts, placing intensifying LFQ diffluence from the
Appalachians through New England. The overlap of this diffluence
with the mid-level divergence/height falls will help to drive the
aforementioned cyclogenesis.

As the low develops, WAA downstream of the wave will increase and
combine with the deep layer ascent to expand precipitation from
the Southern Appalachians this morning through the Mid-Atlantic
and Northeast into tonight, before exiting Maine early Thursday.
While the WAA is likely to become impressive, which will cause a
transition in p-type from snow to sleet to freezing rain and
eventually rain, significant accumulations of wintry precipitation
are expected.

For freezing rain, the greatest potential for more than 0.1" of
accretion will be focused in the highest terrain of the
Appalachians from the Appalachian spine of west-central VA through
the Laurel Highlands of SW PA through tonight, with a secondary
maxima occurring on Wednesday throughout the Poconos, Catskills,
Berkshires, and far southern Adirondacks. WPC probabilities for
0.1" or more peak at 30-50% in these areas. Some lighter
accretions of freezing rain exceeding 0.01" are likely across much
of this area and as far south as western NC, but should remain NW
of the major metropolitan areas along the I-95 corridor.

A brief period of moderate to heavy snow may occur at onset across
much of the Central Appalachians and into the higher terrain of
the Mid-Atlantic and southern New England as WAA drives intense
but brief ascent to help dynamically cool the column. 00z HREF
depicts this potential across the northern Potomac Highlands
through central PA with mean 1-hr snowfall totals of 0.5-1"
between 20z Tues and 01z Weds, implying the potential for at least
1"/hr rates. As a result, WPC probabilities for 4 inches of snow
ending 12z Weds have increased into the 10-20% range. As the low
moves near Cape Cod and into the Gulf of Maine, impressive 285K
isentropic ascent will spread moisture northward on mixing ratios
of 4-5 g/kg. This will support a 6-12 hour period of heavy snow,
before a pronounced dry slot races northeastward drying the DGZ
which will shut off the snow and result in a period of additional
freezing drizzle. The heaviest snow is likely in northern ME where
a significant pivoting deformation band may develop with snowfall
rates of 1+"/hr on the periphery of some -theta-e mid-level lapse
rates. The duration of this band will heavily impact how much snow
occurs, but there is increasing confidence that more than 6 inches
of snowfall will fall, especially in the higher terrain, from the
Adirondacks through the Greens, NE Kingdom of VT, northern Whites
and into much of northern ME. The heaviest snow is likely on
Wednesday. WPC probabilities of at least 10% for greater than 8
inches of snow extend from the northern White Mts of western ME
through a majority of northern ME.


...Minnesota Arrowhead and Great Lakes...
Days 1-3...

For the Arrowhead of MN on Tuesday continued lake enhanced
moisture will spread eastward into the North Shore, supported
additionally by upslope flow. Additionally, significant moisture
and high SLRs in the cooling airmass aided by instability aloft
due to steep lapse rates beneath the upper low will help produce
waves of heavy snow rates of more than 1"/hr. As the flow shifts
to more of a northerly direction in reaction to the developing low
pressure system near New England, lake effect snow picks up across
the U.P. of Michigan downwind of Lake Superior. WPC probabilities
or more than 6 inches are high along the NW shore of Lake Superior
today.

A long duration of heavy Lake Effect Snow (LES) is likely to begin
Wednesday and persist beyond this forecast period. The amplified
trough swinging eastward through the Great Lakes and into New
England which will drive the nor'easter in New England will leave
broad cyclonic flow in its wake with pronounced CAA. Additionally,
weak spokes of vorticity will continually rotate eastward within
this cyclonic flow to enhance deep layer ascent, with each one
driving a supplemental surface trough eastward. These will enhance
CAA, and 850mb temps are progged to fall to -8C to -12C, while
lake surface temps remain +8 to +14C according to GLERL, highest
in Lake Erie and Lake Ontario. This will produce an increasingly
favorable environment with steep low level lapse rates to produce
sufficient instability, deepening inversion depths, and plentiful
moisture to support LES. Minor wind trajectory differences will
significantly impact the placement of these bands and subsequent
snowfall, but it does appear a significant LES event will begin by
Wednesday evening, most pronounced downstream of Lake Erie and
Lake Ontario, but significant snowfall is also likely on the less
typical western side of the Lakes today before a surface trough
swings the flow to NW. An even smaller scale heavy LES event is
also possible across northern IN on Wednesday with favorable NW
flow. WPC probabilities for today (D1) feature a moderate risk for
more than 4 inches west of Lake Superior, Michigan, and Huron,
with locally more than 8 inches possible near Alpena, MI. By
Wednesday, The flow becomes more typical and pronounced for LES,
with WPC probabilities for more than 4 inches exceeding 40% in the
western U.P., southwest lower Michigan/northern IN, and downstream
of Lake Erie and Ontario, with locally more than 12 inches likely
along the Chautauqua Ridge and into the Tug Hill Plateau. In
response to a diving shortwave into the Midwest, flow is expected
to become more southwesterly by Thursday night. This will provide
better fetch off Lake Erie and Ontario and the potential for a
very strong and long duration snow bands beyond this forecast
period. Snowfall totals could be measured in feet near and just
south of Buffalo when all is said and done.


...Montana and Wyoming...
Day 2...

A strong lobe of vorticity rotating NW to SE on the upwind side of
cyclonic flow which envelops most of the CONUS will spread waves
of precipitation in the form of snow from the Northern Rockies
into the Central Rockies, although moisture will be more limited
to the south. Widespread snowfall is expected from the Northern
Rockies into the Absarokas on D3 as a subsequent shortwave,
accompanying upper jet streak and arctic cold front pivot
southeast across the region. The result of this evolution will be
a band of enhanced fgen which should drive some heavier snow
rates, especially in the presence of a cold airmass leading to
high and fluffy SLRs nearing 20:1. WPC probabilities are high for
4 inches on Wednesday in these areas, with 3-day total snowfall
potentially exceeding 12 inches in the higher terrain.

Snell/Weiss