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

Valid 00Z Wed Nov 16 2022 - 00Z Sat Nov 19 2022

...Mid-Atlantic through northern New England...
Days 1-2...

A fast moving nor'easter will develop tonight near the NJ coast
and then lift northeast across Cape Cod, into the Gulf of Maine,
and then into the Canadian Maritimes by Thursday morning. This low
will deepen gradually as it makes this trek, leaving a swath of
snow, sleet, and freezing rain across the interior Northeast and
Mid-Atlantic. This nor'easter will be driven by deep layer ascent
created through height falls and mid-level divergence downstream
of a trough rotating into Quebec, with a shortwave pivoting across
New England. Additionally, a strong 130kt upper jet will streak
across the TN VLY and arc northeast, placing favorable LFQ
diffluence atop the northeast. Together these will work to produce
cyclogenesis, resulting in the nor'easter.

Precipitation will expand northward on pronounced WAA driving
enhanced theta-e ridging into the Mid-Atlantic and northeast. This
WAA will produce some initial snow/sleet almost everywhere NW of
I-95, but will quickly transition to sleet, freezing rain, and
then rain outside of the higher terrain as far north as southern
New England. This could result in modest accretions of freezing
rain tonight into early Wednesday in the Poconos, Catskills,
Berkshires, and southern Adirondacks where WPC probabilities for
0.1" of accretion are 20-50%. Lighter freezing rain accretions of
0.01" or more are possible from the Laurel Highlands through much
of Upstate NY and southern New England, but outside the major
metropolitan corridor.

Farther to the north, this system will produce heavy snowfall
across the Adirondacks, Greens, and much of northern NH/ME. The
WAA across ME will be coupled with impressive 285K isentropic
ascent, and a band of heavy snowfall is likely to pivot in
northern ME. The guidance has trended a but farther NW/warmer with
today's solutions, but heavy snowfall rates of 1"/hr are expected
along this front end WAA band into northern Upstate NY and
northern New England, with brief rates of 1-2"/hr possible in
northern Maine in the most robust banded structure. This will
result in snowfall accumulations that have a greater than 70%
chance of exceeding 4 inches from the Adirondacks eastward,
especially in the higher terrain, with local totals exceeding 10"
possible in far northern Maine.


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

...Major lake effect snow event likely to begin on Thursday...

A long duration and heavy lake effect snow event is likely to
begin late Wednesday and then ramp up impressively through Friday
as a large scale trough envelops the eastern 2/3 of the CONUS.
Embedded within this trough, spokes of shortwave energy will
rotate cyclonically from NW to SE, producing rounds of enhanced
ascent and re-energizing the subsequent CAA. Lake water
temperatures according to GLERL range from as warm as +6C in Lake
Superior to as high as +14C in Lake Erie and Lake Ontario.
Pronounced CAA through the period will drop 850mb temps from -6C
to -10C Wednesday, to as low as -10C to -15C Friday, creating
intense lake-induced instability reaching potentially 2000 J/kg as
inversion heights climb towards 10,000 ft. While wind trajectories
will vary at times with the passage of each shortwave, shear
should become ideal at times for single intense bands of LES,
especially downwind of Lake Erie and Ontario, with some additional
moisture /effective fetch/ off Lake Huron aiding. Additionally,
this intense instability and lake wind convergence could result in
meso-lows over Lake Superior to additionally enhance LES at times.

For D1, the most intense LES is forecast south of Lake Michigan
and west of Lake Huron due to flow around a wave of low pressure
moving into Ontario. Lake enhancement here could result in more
than 4 inches of snow in some areas, with the highest
probabilities focused in northeast lower Michigan due to longer
fetch and some potential for 1-2"/hr rates. During D2, the flow
becomes more typical and focusing on the downwind/east side of the
Lakes, with intense banding developing off Ontario into the Tug
Hill Plateau and E/SE of Erie south of Buffalo and along the
Chautauqua Ridge due to multi-bands developing across Erie. WPC
probabilities are above 90% for 4 inches, with locally 12 inches
possible as convective snowfall rates eclipsing 2"/hr become
likely. Additional moderate LES is likely D2 in the western U.P.,
as well as L.P., of MI.

The most significant day of LES during this forecast period
appears to occur Friday as shear becomes westerly over Lake
Michigan and Superior, becoming W/SW over Lake Erie and Ontario.
This occurs in conjunction with additional tumbling of 850mb temps
to enhance instability, and convective snow rates moving onshore
of many of the typical lake effect belts are progged for Friday.
Heavy snowfall from the Bayfields Peninsula through the Keweenaw
is expected, along with multiple bands shifting onshore from near
Grand Rapids to Traverse City, MI. In these areas, WPC
probabilities for more than 6 inches are as high as 50%, with
locally much higher totals likely. Downstream of Lake Erie and
Ontario, potent, convective, single bands are becoming more likely
Friday, which could focus extreme snowfall rates of 3"/hr into
parts of Upstate NY, including the Buffalo metro area and possible
Watertown as well. WPC probabilities are moderate to high for 12
inches already on D3, with locally much higher totals measured in
multiple feet possible near Buffalo, NY. ]

This lake effect event will continue through the upcoming weekend
with additional significant accumulations likely. The medium range
discussion from WPC outlines some of this additional threat.


...Northern to Central Rockies
Days 1-2...

Broad cyclonic flow enveloping much of the CONUS will maintain a
trough axis near the Great Lakes this week. Upstream of this axis,
lobes of vorticity will rotate out of Canada and southward across
the Rockies, producing waves of enhanced lift. A potent vorticity
streamer will move out of Alberta and down across Montana and into
WY/CO Thursday into Friday, combining with the LFQ of a fast
moving upper jet streak to produce deep layer ascent across the
region. This will help push a surface cold front southward, along
which the enhanced baroclinicity will be aided by the upper jet
streak to produce a period of robust fgen to enhance ascent. Moist
advection ahead of this front will help expand precipitation, and
with a column that is anomalously cold for mid-November, this will
result in snowfall with high SLRs to efficient accumulations,
especially in the terrain from the Northern Rockies through the
Absarokas, and into the Laramies and northern CO Rockies. WPC
probabilities for more than 4 inches are above 80%, highest in the
Absarokas, where locally more than 12 inches is possible. Late D2
into D3, this snow will pivot southward towards the Front Range of
CO creating additional accumulations exceeding 4 inches, with a
few inches of snow possible along the I-25 corridor into Denver
and the Palmer Divide.


Weiss