FOUS11 KWBC 040902
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EST Wed Dec 4 2024

Valid 12Z Wed Dec 04 2024 - 12Z Sat Dec 07 2024

...Michigan... Days 1-2...

A strong cold front from a rapidly developing low is currently 
over northern MN and will plunge southeast across MI today. SW 
flow ahead of the front will continue to bring heavy lake enhanced 
snow (LES) off northern Lake Michigan rest of this morning. Along 
the cold front, expect snow squalls which are rather rare for MI - 
they'll produce sudden heavy snow with particularly strong wind 
gusts which are a danger to travelers. Behind the cold front, 
strong cold air advection (CAA) and modest instability set off 
multi lake effect bands over the U.P. later this afternoon and over
the western shore later this evening. The more intense lake effect
snow bands should average 1-2"/hr snowfall rates tonight 
(continuing through Thursday morning over portions of the western 
shore) per the 00Z HREF. More isolated LES banding occurs into
Thursday night as the pressure gradient rapidly tapers off.

...Eastern Great Lakes and Northeast... Days 1-3...

The types of snowfall from the eastern Great Lakes through the
Northeast Thursday are WAA/Synoptic scale, lake-effect, snow 
squalls, and upslope mountain snow. The LES and mountain snows then
persist into Saturday.

1. WAA/synoptic scale: This pattern is driven by a powerful/ 
amplifying shortwave trough currently over Manitoba that closes 
into a deep mid-level low over MI this evening before reopening 
into a negatively- tilted trough over New England Thursday 
afternoon. strong upper low tracking over the northern Great Lakes 
Wednesday evening. By 12Z Thursday, NAEFS shows 700mb and 500mb 
heights that are near the lowest observed heights at 12Z for this 
time of year in the CFSR climatology over Pennsylvania indicating
how deep this cold-core low is. 
Moisture with some Gulf of Mexico origin lifts northeast ahead of
this developing low and saturates over the eastern Great
Lakes/interior Northeast this afternoon, expanding over New 
England tonight. While temperatures have moderated across the 
Northeast in the past day, the boundary layer remains sufficiently 
cold enough to support snow as WAA increases today, especially in 
the higher elevations of the Berkshires, Green, and White 
Mountains. Expect moderate snow to begin over the southern 
Adirondacks this afternoon and then across the rest of the northern
Appalachians this evening. By Thursday morning, the surface low 
will translate to the northern New England coast. Additional 
snowfall is expected along an inverted trough on the backside of 
the coastal low as it lifts north into New Brunswick Thursday 
afternoon. Cyclonic flow will allow for upslope snow to persist 
through Thursday night in the northern Appalachians before tapering
off Friday morning. WPC PWPF continues to show high chances (>70%)
for snowfall amounts >8" in the southern Adirondacks and Greens, 
then along the White Mountains up through central and northern 
Maine. Localized amounts topping 12" are likely in the higher 
portions of these mountain ranges.

2. Lake-Effect Snow (LES): In wake of the exceptional LES event 
that occurred downwind of Lakes Erie and Ontario over the past 
week, yet another round of heavy LES is on tap starting Wednesday 
night and lasting through Thursday night, then another round Friday
night. The cold front passes Lakes Erie and Ontario late tonight with
surface-850mb winds quickly changing to the west behind the 
front and then northwesterly Thursday afternoon. Expect more 
multi-banded segments off Lake Ontario, with CAMs agreeing on 
longer single banded solutions emanating from Lake Huron and over
Erie Co PA and possibly the hard hit Chautauqua Ridge in far
western NY that could reach into central PA. Lake- effect snow 
should linger through Thursday night and begin to taper off later 
in the day on Friday as the pressure gradient weakens and winds 
over the Lakes decrease. However, flow backing to westerly Friday
night should allow some single band formation off both lakes. 
Latest Day 1.5 WPC PWPF shows high chances (>70%) for snowfall 
above 12" in northwest Pennsylvania, more like 50% for the
Chautauqua Ridge, as well as along the Tug Hill Plateau with more
like 50% for the southern Adirondacks which is after the WAA snow
through this evening. WPC PWPF continues to show around 20%
probabilities for >24" in the highest terrain of the Tug Hill 
through Thursday night. Regionally, these snowfall amounts 
combined with whipping wind gusts topping 40 mph will likely result
in significant blowing/drifting of not just the snow from this 
event, but also residual snowpack from heavy snow that accumulated 
over the past 4-5 days.

3. Snow Squalls/Mountain Snow: This strong and deep cold front will
coincide well with the left-exit region of a 100kt jet streak. By 
late this afternoon, steepening lapse rates and moistening low-mid
level soundings over both central southern Michigan will support 
snow squalls that transpire across much of the state. As the front
pushes east, the particularly deep 500-700mb heights and strong 
synoptic-scale forcing will provide sufficient enough lift along, 
along with the strong 850-700mb FGEN to foster potent snow squalls 
late tonight over Ohio, West Virgina, western Pennsylvania and the
central Appalachians. These squalls are likely to traverse the 
northern Mid-Atlantic (central PA on south to potentially northern 
MD and northern VA) in the early morning hours on Thursday. 
Boundary layer temperatures are more questionable along the I-95 
corridor, but fast moving squalls producing brief bursts of snow 
should occur in parts of the Piedmont regions of northern VA and 
northern MD. Day 1.5 PWPF for >0.1" has a curious maximum over the
northern VA Piedmont.

4. Upslope Mountain Snow: Farther south, strengthening low level 
westerlies will flow orthogonally to the SW-NE spine of the Central
Appalachians. Strong vertical ascent not only via synoptic scale 
and FGEN methods but also through orographic lift should prompt 
bursts of heavy snow to continue after the frontal squalls late 
tonight into Thursday with rapidly increasing SLRs. To make impacts
worse, 850mb winds over the Central Appalachians (including the 
Laurel and Potomac Highlands) will be in excess of 50 kts, with the
strong winds leading to not only substantial blowing/drifting of 
snow, but potential blizzard conditions that could even cause power
outages in some areas. WPC PWPF shows moderate-to-high chances 
(50-80%) for >4" of snowfall in the central Appalachians of east- 
central WV, especially above 3,000ft.

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

Jackson/Mullinax

...Winter Storm Key Messages are in effect...

https://www.wpc.ncep.noaa.gov/key_messages/La...

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