FOUS11 KWBC 031939
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
239 PM EST Tue Dec 3 2024

Valid 12Z Tue Dec 03 2024 - 12Z Fri Dec 06 2024

...Michigan...
Days 1-3...

Periods of snow across the state of Michigan will come in two
different ways over the next few days. The first will come via warm
air advection (WAA) thanks to a warm front traversing the northern
Great Lakes this evening. 290k isentropic ascent will act a steady
source of vertical ascent with the antecedent air-mass=20
sufficiently cold enough to support snow within the boundary layer
tonight and into Wednesday morning. WPC PWPF does depict high=20
chances (>70%) for snowfall totals >4" along the tip of the=20
Michigan Mitten, along the southern coast of Michigan's U.P. south=20
of Sault Ste Marie, and along the Keweenaw Peninsula through midday
Wednesday.

By Wednesday morning, the next Arctic intrusion plunges south
through the Upper Midwest and across Lake Superior. As strong cold
air advection (CAA) ensues, delta Ts in the surface-850mb layer
grow to as high as 40F over western Lake Superior and modest
instability for multi-band lake effect bands to tap into. Similar
highly unstable profiles are expected over Lake Michigan with heavy
lake effect streamers expected from the tip of Michigan's Mitten to
as far south as southwest Michigan starting Wednesday night. In
fact, the strong CAA working in tandem with strong upper level
divergence at the nose of a powerful 500mb jet streak (winds
exceeding values near or above those observed in the CFSR
climatology over northern Iowa) could support intense snow squalls
across southern Michigan Wednesday evening. The more intense lake=20
effect snow bands should average 1-2"/hr snowfall rates with some=20
instances of >3"/hr snowfall rates at times Wednesday night.=20

Brisk and cold NWrly flow over warm waters of Lakes Superior and=20
Ontario will continue through Thursday night along the western and=20
northern shores of Michigan's Mitten while snow gradually begins to
taper off by Friday morning. WPC PWPF shows moderate-to-high=20
chances (50-70%) for snowfall >12" along the Keweenaw Peninsula,=20
the Huron Mountains, portions of the eastern U.P. of Michigan, and=20
the northwestern tier of Michigan's Mitten through Friday morning.

...Eastern Great Lakes and Northeast...=20
Days 2-3...=20

The types of snowfall across these regions over the next 48-72
hours will transpire in four phases: WAA/Synoptic scale, lake-=20
effect, snow squalls, and mountain snow.=20

1. WAA/synoptic scale: To start, this pattern is driven by an=20
amplifying and strong upper low tracking over the northern Great=20
Lakes Wednesday evening. By 12Z Thursday, NAEFS shows 700mb and=20
500mb heights that are near the lowest observed heights at 12Z for
this time of year in the CFSR climatology over Pennsylvania. In=20
addition, while temperatures have moderated in the past 24 hours or
so, the boundary layer remains sufficiently cold enough to support
snow as WAA increases on Wednesday, especially in the higher=20
elevations of the Berkshires, Green, and White Mountains. Excellent
290K isentropic ascent via SWrly flow and upper level divergence=20
beneath the left-exit region of a 250mb jet streak will support=20
periods of snow beginning over the Adirondacks Wednesday afternoon=20
and then across the rest of the northern Appalachians Wednesday=20
night. By Thursday morning, a secondary low looks to form along the
triple point near the Maine coastline. Additional snowfall is=20
possible as an inverted trough on the backside of the coastal low=20
pivots over the region Thursday afternoon. Cyclonic flow will allow
for upslope snow to persist through Thursday night in the northern
Appalachians before tapering off by midday Friday. WPC PWPF shows=20
high chances (>70%) for snowfall amounts >8" in portions of the=20
Adirondacks, Green, and White Mountains, as well as portions of=20
northern Maine. Localized amounts topping 12" in these mountain=20
ranges are possible.=20

2. Lake-Effect: In wake of the exceptional LES event that has
unfolded downwind of Lakes Erie and Ontario, yet another round of
heavy snow is on tap starting Wednesday night and lasting through
the end of the work-week. The cold front passes through Wednesday
night with surface-850mb winds over Lake Erie changing from a
250-260 wind direction between 03-06Z to 280-290 after 09Z. A=20
similar wind shift occurs over Lake Ontario Thursday morning. Delta
Ts between the surface and 850mb will range between 32-38F over=20
the open waters of Lakes Erie and Ontario, and modest instability=20
will be at any lake effect band's disposal. By Thursday afternoon,
winds shift more to pure NWrly flow. This should allow for more=20
multi-banded segments off Lake Ontario, while some CAMs are=20
hinting at longer single banded solutions emanating from Lake Huron
that could reach into northwest Pennsylvania. Lake-effect snow=20
should linger through Thursday night and begin to taper off later=20
in the day on Friday as the pressure gradient weakens and winds=20
over the Lakes decrease. Latest WPC PWPF shows high chances (>70%)
for snowfall above 12" in northwest Pennsylvania, far western NY=20
south of Buffalo, and along the Tug Hill Plateau. The Tug Hill in
particular could be hit quite hard from the combination of WAA at
the onset, then the lake-effect on the backside of the storm on
Thursday. WPC PWPF shows low chances (10-30%) for snowfall >24" in
the highest terrain of the Tug Hill through Friday. Regionally, these
snowfall amounts combined with whipping wind gusts topping 40 mph=20
will likely result in significant blowing/drifting of not just the=20
snow from this event, but also residual snowpack from heavy snow=20
that accumulated over the past 4-5 days.

3. Snow Squalls/Mountain Snow: This strong cold front, which based
on some guidance shows cross-section FGEN plots extending as far=20
up as 500mb, will coincide well with the left-exit region of a=20
robust 500mb jet streak. By late Wednesday afternoon, steepening=20
lapse rates and moistening low-mid level soundings over both=20
central southern Michigan will support snow squalls that transpire=20
across the southern tiers of the state. As the front pushes east,=20
the thermal gradient will not be as tight as it could've been had=20
this cold frontal passage occurred during the daytime hours. That=20
said, the aforementioned deep 500-700mb heights and strong=20
synoptic-scale forcing will provide sufficient enough lift along,=20
along with the strong 850-700mb FGEN to foster potent snow squalls=20
Wednesday evening over Ohio and western Pennsylvania. These=20
squalls are likely to traverse the northern Mid-Atlantic (central=20
PA on south to potentially northern MD and northern VA) in the=20
early morning hours on Thursday. Boundary layer temperatures are=20
more questionable along the I-95 corridor, but fast moving squalls=20
producing brief bursts of snow could occur in parts of the Piedmont
regions of northern VA and northern MD.=20

4. Mountain Snow: Farther south, strengthening low level=20
westerlies will flow orthogonally to the SW-NE spine of the Central
Appalachians. Strong vertical ascent not only via synoptic scale=20
and FGEN methods but also through orographic lift should prompt=20
bursts of heavy snow Wednesday night with rapidly increasing SLRs.=20
To make impacts worse, 850mb winds over the Central Appalachians=20
(including the Laurel and Potomac Highlands) will be in excess of=20
50 kts, suggesting the strong winds will likely lead to not only=20
substantial blowing/drifting of snow, but potential blizzard=20
conditions that could even cause power outages in some areas. WPC=20
PWPF shows moderate-to-high chances (50-70%) for >4" of snowfall in
the central Appalachians of east- central WV, especially above=20
3,000ft.


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

Mullinax


...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!9hzyLjVZY9PhSi8VOVHuU4_jhOGuf0gZBksUiZHV5-Q7a=
mMXf1ptRBLx1BGIaxeMaZLzO1cQ2wjLRfnlH5ZTbpowwk4$=20


$$

=3D =3D =3D
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