FOUS11 KWBC 271834
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
134 PM EST Wed Nov 27 2024

Valid 00Z Thu Nov 28 2024 - 00Z Sun Dec 01 2024

...Ohio Valley through New England...=20
Days 1-2...

Challenging forecast continues for the Thanksgiving holiday as a
strengthening low pressure moves from the Ohio Valley to coastal
Maine. The system begins to organize beneath a sharpening shortwave
tracking across the lower Ohio Valley very late tonight into
Thursday morning. This feature will deepen subtly as it moves east,
but remain positively tilted as it moves across southern New
England (SNE) Thursday afternoon and then into the Canadian=20
Maritimes by Friday morning. Beneath this feature, a surface low=20
will develop in response to the accompanying PVA/height falls,=20
aided by the increasingly diffluent LFQ of a poleward arcing jet=20
streak rotating around the base of the parent trough. This forcing=20
acting upon an elevated baroclinic gradient with attendant WAA=20
downstream of the developing low will help the wave to strengthen,=20
especially the latter half of Thursday as it moves across SNE and
into the Gulf of Maine. The guidance, which has been very variable
in its solutions, have trended farther north since overnight, and
while the ECMWF continues to be the northern/strong outlier, the
other models are trending towards a stronger and more northward
solution.

The track of this low is crucial for both the snowfall footprint
and snowfall amounts. The antecedent airmass downstream of the
system is marginally supportive for snowfall, with wet-bulb
temperatures at or above freezing in many areas outside of terrain.
This indicates that outside of the higher elevations, generally
above 1500 ft, it will require dynamic cooling to overcome this
column and result in accumulating snow. However, this is likely as
the guidance still suggests (and the environment conceptually
supports) an intensifying quasi-laterally-stationary band of
precipitation tracking SW to NW north of the surface low. This will
be driven by a narrow but sharply sloped axis of 700-600mb fgen
intersecting the deepening DGZ, concurrent with increasing theta-e
advection from the E/SE. With a stronger low forecast, this band
should be intense, and can support 1-2"/hr snowfall rates despite
what should be a heavy wet (low SLR) snowfall. The low SLR,=20
combined with above-freezing wet-bulb temps, snow falling during
daylight hours, and the progressive nature of the system should
limit significant snowfall in many areas.

However, where this band advects, a swath of at least moderate=20
snowfall accumulations are likely, although the narrowness of it=20
will likely limit the probabilistic guidance from capturing its=20
true potential. This results in WPC probabilities that continue to=20
be highest in the higher elevations from the Catskills, through the
Adirondacks and Greens, and into the Whites, and much of northern=20
ME, where they are 50-70% for 4+ inches, with storm total snowfall
as much as 10" possible in a few of the highest locations.=20
Otherwise, snowfall should be limited to less than 4 inches, but=20
locally higher amounts, even in valley locations, are likely=20
beneath this band, and an inch or two is possible as far southwest
as Indiana and Ohio.


..Great Lakes...
Days 2-3...

*** Long duration and widespread heavy Lake Effect Snow event=20
 begins Friday ***

A closed 500mb low centered near James Bay will spin almost in
place into Saturday as a longwave trough amplifies across the
eastern CONUS. This low will push height anomalies within the
500-700mb layer to below -1 sigma across the eastern CONUS,
reflective of the intensity of this early season arctic airmass. As
the core of this gyre spins in place, vorticity lobes will
periodically shed around it, crossing the Great Lakes, providing
surges of CAA and locally enhanced ascent. The result of this will
be several days of lake effect snow (LES) in the favored NW snow=20
belts.

850mb temps to begin D2 are progged to be generally -5 to -6C in
the east, and as cold as -11C across Lake Superior. Through Friday
and Saturday, periodic surges in CAA will cool these temps even
further, producing impressively steep lapse rates and high
inversion levels thanks to lake surface temperatures that are as
warm as +8C in Lake Superior and +15C over Lake Erie according to
GLERL. This will create lake-induced instability that at times will
exceed 1000 J/kg, supporting bands of heavy LES with rates that
will likely eclipse 2"/hr at times. While some variation in wind
speeds and direction, especially during passes of shortwaves aloft,
will cause some wavering in the position of these bands, in
general, at least through the end of D3, the heaviest snow should=20
be relatively persistent along the south shore of Lake Superior,=20
near Traverse City, MI, along the Chautauqua Ridge, and towards=20
the Tug Hill Plateau. In these areas, WPC probabilities for more=20
than 6 inches are high (>70%) both Friday and Saturday, with 2-4=20
feet of snow possible in the most long-lasting and intense bands=20
through the end of the forecast period.

The arctic-sourced cold and subsequent LES off the Great Lakes=20
through the weekend is the subject of the Key Messages which are=20
linked below.=20


...Central Plains to Mid-Mississippi Valley...
Day 3...

A fast moving clipper type low will skirt around the periphery of a
longwave trough embedded across the eastern CONUS, bringing a
stripe of snow from Nebraska through the Bootheel of Missouri. This
clipper will be driven buy a sharp shortwave digging southeast
while amplifying, working in tandem with the LFQ of a modest jet
streak following in its wake, and moving along a baroclinic
gradient left in place by a cold front. Weak WAA downstream of this
low will produce a period of 280-285K isentropic ascent Friday=20
night through Saturday, coincident with a deepening of the DGZ to
support a swath of heavy snow. The timing of the heaviest snow is
likely to be very early Saturday morning through the afternoon
which could additionally limit accumulation potential (already
somewhat muted due to progressive nature of generally weak ascent),
but WPC probabilities for 1+ inches are 30-50%, with locally 3+"
possible. Although this snowfall is minor, it will occur after a
period of cold air so it should efficiently accumulate and could
produce hazardous travel conditions.


The probability of significant ice across the CONUS is less than
10 percent.=20


Weiss



...Winter Storm Key Messages are in effect. Please see current=20
 Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=
stKeyMessage_1.png__;!!DZ3fjg!96uaeNmX1a0PZE766Pgqyk5N1qziM4MjcxWGJggfSV6nK=
kWhgIeDbS-cIzkbTkQI6kTPnN-qmki_wuv4oaIqRq4BBto$=20

$$

=3D =3D =3D
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