FOUS11 KWBC 281918
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
218 PM EST Thu Nov 28 2024

Valid 00Z Fri Nov 29 2024 - 00Z Mon Dec 02 2024

...Northern New England...=20
Day 1...

Strengthening coastal low will continue to advect northeast along
the coast of Maine before ejecting into the Canadian Maritimes
Friday morning. As this low tracks northeast, it will continue
impressive warm and moist advection onshore into Maine, with the
accompanying theta-e ridge pivoting back into VT/NH, at least early
on D1, before gradually drying out and shifting exclusively into
Maine. By Friday morning, enough dry air should work into the
column behind the departing system to bring an end to the snowfall.

The column will remain marginally conducive for snow with forecast
wet-bulb temperatures right around 0C. However, impressive ascent
both through synoptic and mesoscale forcing, will force banded
structures to overcome the modest temperatures, resulting in heavy
snowfall rates which will dynamically cool the column. The WPC
prototype snowband tool continues to indicate that translating
bands of 2"/hr snows are likely across Maine, which will rapidly
accumulate despite low SLRs (a heavy and wet snow). The most
uncertainty is near the coast where temps will be above freezing,
but at least some mixing is possible during the heaviest
precipitation rates. Still, the greatest accumulations are likely
across portions of central and northern Maine, especially in the
higher terrain, where WPC probabilities for more than 6 inches of
snow are above 70%, and locally as much as 10" is possible. Since
this snow could be heavy and wet, hazardous travel and impacts to
infrastructure, including power outages, are likely.


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

*** Heavy Lake Effect Snow event tonight into Tuesday ***

The impressive mid-level closed low stalled near James Bay will
meander nearly in place through Saturday before finally drifting
southeast towards northern New England by the end of the forecast
period. Even during this transition, broad but amplified troughing
will persist across the eastern CONUS, with spokes of vorticity
repeatedly cycling through the flow and crossing the Great Lakes.
The primary adjustment to the flow regime through the weekend will
be a shift to more northerly winds vs W/NW as the core of the low
moves eastward, and while this will cause a shift in the placement
of the primary snow bands, intense lake effect snow (LES) will
continue for many days.

850mb temps crashing during periods of CAA behind vorticity maxes
will create 850-sfc temperature gradients that are as much as +20C
or more at times due to lake temps that are well above normal of
+8C to +15C. This will result in impressive lake-induced
instability of more than 1000 J/kg and inversion levels peaking
above 10,000 ft. With impressive ascent off the lakes, this will
result in widespread LES bands, with multi-bands likely off Lake
Superior and Lake Michigan, and potential intense single bands=20
shifting off of Lakes Erie and Ontario, especially D1-D2 when an
upstream connection is also likely. Some of these bands may=20
additionally contain lightning and thunder, especially east of=20
Lakes Erie and Ontario, producing snowfall rates of 2-3"/hr at=20
times. The heaviest snow accumulations are likely east of these two
lakes, where WPC probabilities for more than 12 inches are high
(50-90%) both D1 and D2. Downstream of the other lakes, WPC
probabilities for 12+ inches D1 are moderate (30-50%) along the
southern Lake Superior shore and near Traverse City, with lighter
accumulations likely on D2.=20

During D3 the mid-level pattern finally begins to shift leading to
a more pronounced northerly flow across the Lakes causing the bands
to pivot more S/SE than Friday or Saturday. While this will bring a
break to some areas, it will likely lead to enhanced LES across new
areas, including potentially along the eastern shore of Lake
Michigan and pivoting more towards the southern shore of Lake Erie
and south of the Tug Hill (eventually approaching the Finger Lakes
region D4) off of Ontario. Rates will still be impressive however,
and these bands will only slowly pivot, so WPC probabilities D3
remain elevated for 12+ inches (30-50%) near the Chautauqua Ridge
and Tug Hill Plateau, with lower probabilities of 10-30% for the
central U.P. Locally, 2-4 feet is likely in the more persistent
bands, with locally as much as 5 feet possible in the most intense
snowfall.

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


...Central Plains to Mid-Mississippi Valley...
Day 2...

Fast moving clipper-type low will dig southeast around the=20
periphery of a large-scale longwave trough over the east to produce
a swath of moderate to heavy snow. This clipper low will be driven
by a sharp shortwave and accompanying vorticity maxima which will=20
swing out of the Central Plains Saturday and move progressively to=20
the east through Sunday morning. Although the amplitude of this=20
feature will be modest, it will be favorably placed in conjunction=20
with the LFQ of a trailing jet streak, with downstream 285K=20
isentropic ascent surging moisture and precipitation out ahead of=20
it. Additionally, a residual baroclinic zone will lay across the=20
region behind a front, providing focus for the surface wave to=20
strengthen, albiet modestly. The overlap of the accompanying fgen=20
with the isentropic ascent and synoptic lift should result in an=20
expanding swath of precipitation, falling as moderate snow in most=20
areas. The system is progressive, but antecedent cold conditions=20
and moderate snowfall rates should accumulate efficiently, and=20
while there remains some latitudinal spread in the axis of heaviest
snowfall, and the guidance has backed off on the QPF just a bit
today, WPC probabilities remain moderate (around 30%) for 2+
inches, highest in the vicinity of St. Louis, MO.


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!-w52YNNrCToIJOh6eEC6BnRRkiVkxTJUhW3V2sBN0ZqNG=
gt1g3FvgESoJmxV8AuhP73M-1A98eEblm16n3OXA5R6jK0$=20

$$

=3D =3D =3D
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