Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 AM EST Tue Nov 28 2023

Valid 12Z Tue Nov 28 2023 - 12Z Fri Dec 01 2023

...Great Lakes...
Days 1-2...

Today will continue the ongoing and impressive lake effect snow
(LES) event, especially downwind of lakes Erie and Ontario. The
core of the cyclonic gyre centered south of Hudson Bay will slowly
advect eastward today in response to one final potent shortwave
shifting from the Great Lakes through the Ohio Valley, and
eventually off the New England coast by Wednesday morning. This
feature will provide additional synoptic ascent and produce at
least subtle additional moisture, but will primarily act to
enhance the CAA once again to enhance LES potential. 850mb temps
will crash to below -15C in some areas, which when moving across
the still warm lake waters will result in robust SBCAPE of
500-1000 J/kg and inversion depths that will persist as high as
15kft through D1. This should allow for focused LES bands in the
favored W/NW snow belts downwind of lakes Erie, Ontario, Superior,
and Michigan, with an upstream connection enhancing snow rates and
amounts in some areas. The snow bands will likely fluctuate in
position a little more today than they did Monday due to shifting
winds from NW early, to W, and eventually SW as shortwave ridging
develops ahead of the next vorticity maxima. This secondary
shortwave/vort on Wednesday is expected to be much weaker and
shallower than the previous, and will quickly be followed by more
zonal flow, so additional LES beyond D1 is expected to be much
lighter.

WPC probabilities for today/tonight are above 80% for the Tug Hill
Plateau , areas near Syracuse NY, and south of Buffalo, where
locally an additional 12 inches is possible. WPC probabilities for
more than 6 inches are more modest, 30-60%, along the Chautauqua
Ridge and near Traverse Bay. By D2 the greatest risk for more than
6 inches of additional LES shifts northward off Lake Ontario to
near Watertown where WPC probabilities are as high as 50%.


...Northeast and Mid-Atlantic...
Day 1...

An arctic cold front which moved across the Great Lakes and
Northeast Monday will continue to exit to the east across the
Mid-Atlantic today. Behind this front, the guidance has come into
better agreement that widespread snow showers with at least
scattered snow squalls are possible, especially late this morning
through early evening as a potent shortwave rotates through the
cyclonic mid-level flow atop the region. During this time, the
snow squall parameter reaches above 1 (favorable for snow squalls)
from western MD through northern NJ and into much of New England.
This is coincident with the high res simulated reflectivity (most
notably in the HRRR) suggesting widespread cellular activity which
will likely manifest as snow showers and snow squalls. Despite
that, there remains uncertainty into the coverage of squalls (vs
snow showers) as antecedent air is quite dry and 0-2km RH is
forecast to remain less than 60% for the eastern half of the
region. The synoptic level trough moving overhead may add some
additional moisture, which could also increase from lake
enhancement on the upstream connection to the Great Lakes. Where
moisture is sufficient, steep lapse rates will produce SBCAPE of
50-250 J/kg, highest across PA, upstate NY, and into VT,
coincident with pockets of enhanced 925mb fgen and plenty of wind
to mix through the PBL. Snow showers that develop could become
dangerous squalls despite producing minimal snow accumulation as
heavy rates and gusty winds lower visibility to near zero at
times. While the snow squall potential farther east is less
significant, heavy snow showers are possible and may reach the
coast later this evening, also producing periods of hazardous
travel.


...Southern Rockies and Four Corners...
Day 3...

A shortwave dropping along the coast of CA Wednesday night is
progged to amplify into a closed low near the Four Corners
Thursday before continuing to advect rapidly east and opening into
a wave once again over Texas on Friday. The amplification of this
feature into a closed low over the Desert Southwest will produce
increased mid-level divergence downstream, with height falls
additionally aiding in ascent towards the Four Corners on D3. At
the same time, a back door type surface front will drop SW out of
the High Plains and into the Four Corners, providing additional
ascent through convergence and post-frontal upslope flow. There is
still considerable spread as to the timing of this front, but with
at least modest increasing moisture both through elevated PW
advection from the WSW and modest theta-e ridging downstream of
the upper low, moderate snowfall is likely in the higher terrain
above 6000 ft. WPC probabilities for more than 4 inches on
Thursday reach as high as 30-50%, especially in the San Juans,
southern Sangre de Cristos, and Jemez Mountains.


...Cascades through the Sierra...
Day 3...

A shortwave ejecting from the Pacific will arc onshore Thursday
night into Friday morning while amplifying into a longwave trough
draped from British Columbia through California. This trough will
demarcate the leading edge of an incoming Pacific jet streak which
is progged to reach 110 kts and place its favorable LFQ portion
atop the Pacific Northwest the latter half of D3. Current PW and
IVT from the NAEFS ensemble tables suggest near normal moisture,
and the progressive nature of the flow will result in a relatively
transient period of high 700-500mb RH. However, snow levels will
be below many of the passes in the Cascades at 2000-3000 ft,
rising to around 4000-5000 ft in the Sierra, so any moisture wrung
out by the brief but favorable ascent will result in impactful
snow. Still some uncertainty into the amplitude and timing of this
trough, but WPC probabilities currently reflect a 40-70% chance
for more than 4 inches, highest in the OR Cascades and central
Sierra.


Weiss