Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EST Mon Nov 14 2022

Valid 00Z Tue Nov 15 2022 - 00Z Fri Nov 18 2022

...Central Plains, Mid-Mississippi Valley, Upper Midwest...
Day 1...

Spokes of vorticity associated with modest shortwaves will rotate
cyclonically around a broad but amplified trough over the central
part of the CONUS through mid-week. The longwave trough will be
driven by a closed low spinning over southern Manitoba, and as
vort lobes rotate through Wednesday, a broad inverted trough will
spread across the Upper Midwest. Each of these shortwaves will
help drive locally enhanced ascent in an otherwise broadly forced
environment, with downstream WAA leading to 280-285K moist
isentropic upglide with mixing ratios of 4g/kg. This will result
in a broad area of light snowfall across much of the central CONUS
into Wednesday, within which there may be three local maxima.

The first will be across Missouri tonight into early Tuesday where
the isentropic upglide will drive moisture into a region of
enhanced mesoscale ascent through upper jet LFQ diffluence to
enhance 600mb fgen. This will likely result in a translating band
of heavy snowfall, within which rates will likely reach 0.5-1"/hr.
With a deepening DGZ moistened by this isentropic ascent, SLRs
will likely rise to above Baxter climo median values, but at the
same time will be somewhat tempered by warm ground, especially
after daybreak Tuesday. Still, SLRs of 10-12:1 will support at
snowfall that should overcome the hostile ground conditions and
result in a few inches of accumulation where this band sets up. At
this time, the greatest potential for more than 2 inches of
snowfall is just north of the St. Louis, MO metro area where WPC
Probabilities on D1 are as high as 60%.

The secondary area of maximum snowfall may occur across central
IA. The same moist isentropic lift will pivot NW into the inverted
trough and merge into some enhanced low-level (850mb) fgen
collocated with a slowly migrating deformation axis aligned N-S
across Iowa. The guidance is actually in very good agreement in
the placement of this feature, and as an elongated lobe of
vorticity swings across the area later Tuesday, this could allow
snowfall to persist a bit longer than other places due to the
pivoting forcing. SLRs will again be above the Baxter median,
which will likely result in a corridor of higher snowfall
accumulations. WPC probabilities are above 60% for 2 inches or
more in this area.

Lastly, across the Arrowhead of MN some lake enhanced moisture
will spread northward into the Iron ranges, supported additionally
by upslope flow. 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. WPC probabilities or more than 6 inches are
high along the NW shore of Lake Superior on D1.


...Southern Appalachians through northern New England...
Days 1-3...

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 aftn 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 tonight through the Mid-Atlantic and
Northeast D2, before exiting Maine on D3. 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 far NW NC through the Laurel Highlands of PA on
D1, with a secondary maxima occurring on D2 from the Laurel
Highlands through the Poconos, Catskills, Berkshires, and far
southern Adirondacks. WPC probabilities for 0.1" or more peak at
20-40% in these areas. Some lighter accretions of freezing rain
exceeding 0.01" are likely across much of this area, 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. However, WPC
probabilities for snowfall exceeding 2 inches is only 20-30% in
the Laurel Highlands and across much of central and northern PA
into Upstate NY and southern New England well NW of the I-95
corridor. However, 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 Maine. The heaviest snow is likely on
Wednesday.


...Great Lakes...
Days 1-3...

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,
especially on D3, and most pronounced downstream of Lake Erie and
Lake Ontario, but significant snowfall is also likely on the less
typical western side of the Lakes on Tuesday before a surface
trough swings the flow to NW. WPC probabilities on D2 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 D3, 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, and downstream
of Lake Erie and Ontario, with locally more than 12 inches likely
along the Chautauqua Ridge and into the Tug Hill Plateau. While
heavy snow will begin on Thursday downstream of the Lakes, this
could become a major, long duration, LES event for parts of these
areas into the medium range.


...Montana and Wyoming...
Days 1-3...

Waves 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. Waves of moderate snow are likely D1 south of a
pronounced shortwave, but the heaviest and most widespread
snowfall is expected from the Northern Rockies into the Absarokas
on D3 as a subsequent shortwave and accompanying upper jet streak
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 D3 in these areas, with 3-day total snowfall
potentially exceeding 12 inches in the higher terrain.

Weiss