Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 AM EST Wed Nov 16 2022

Valid 12Z Wed Nov 16 2022 - 12Z Sat Nov 19 2022

...Northern New England...
Day 1...

A fast moving and strengthening nor'easter near Long Island this
morning will 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 northern New
England and parts of northern Upstate New York. 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 continue to spread northward within strong WAA
and potent/moist SE 850mb flow. The mid-level warm nose should
transition areas over to sleet and light amounts of freezing rain
from central VT/NH to parts of central Maine. This system will
produce heavy snowfall across the northern 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. 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 8 inches across northern Maine.


...Northern to Central Rockies and nearby Front Range...
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. This
jet streak has trended slightly north and better oriented for
upper diffluence over north-central CO on Thursday night. Moist
advection ahead of this front will also 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.
Conditional symmetric instability is noted within forecast
soundings, which may help expand moderate-to-heavy snow bands
farther east into the High Plains of CO. WPC probabilities for
more than 4 inches are above 80%, highest in the Absarokas, where
locally more than 12 inches is possible. Into D2, 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 and just east of the I-25 corridor into Denver and
the Palmer Divide.


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

***Major lake effect snow event likely to begin tonight and last
through the upcoming weekend***

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 and
mean layer flow becomes northwesterly across the Upper Great Lakes
and west-southwesterly over the Lower Great Lakes. 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 eventually to the east-southeast of Lake Erie and the
Chautauqua Ridge into D2 due to multi-bands developing across
Erie. A few snow squalls may also extend eastward across parts of
northern PA on Thursday away from Lake Erie underneath a cutting
shortwave and within steep low-level lapse rates. WPC
probabilities are above 90% for 4 inches on D1 downwind of
southern Lake Michigan and east of Lake Erie, with locally 12
inches possible as convective snowfall rates eclipsing 2"/hr
become likely. Additional moderate-to-heavy LES is likely D2 and
D3 in the western U.P., as well as L.P., of MI.

The most significant 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 higher than 80%, with
locally much higher totals likely. Downstream of Lake Erie and
Ontario, potent, convective, single bands are becoming more likely
Friday as fetch becomes ideal and aimed right at the Buffalo metro
area. Extreme snowfall rates of 3"/hr into parts of Upstate NY are
possible. WPC probabilities are high for 12 inches of snow within
24 hours ending 00z 11/19, with locally much higher totals
measured in multiple feet possible near Buffalo, NY over the
course of the entire forecast period.

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.

Key Messages for Lake Effect Snow in the Great Lakes:

--Confidence is increasing that several days of heavy lake effect
snow will occur east of the Great Lakes through the upcoming
weekend.

--Lake effect snow is expected to begin late Wednesday, with
periods of heavier snow continuing through Sunday. The most
intense snowfall will occur Thursday into Friday.

--Snowfall will at times likely be accompanied by lightning,
thunder, and rates exceeding 2â€/hr. This will produce near zero
visibility and snow covered roadways making travel hazardous to
nearly impossible.

--Snow amounts in excess of 12 inches are likely in many locations
E/SE of the Great Lakes. The heaviest snowfall is expected in the
Buffalo, NY metro area and far northwest PA where several feet of
snow are possible.

--Very cold temperatures will accompany this event with high
temperatures possibly more than 20 degrees below normal by the
weekend.


Snell/Weiss