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QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Fri Dec 20 2024

Valid 12Z Fri Dec 20 2024 - 12Z Mon Dec 23 2024

...Great Lakes...
Day 1...

The gradually weakening clipper system will track across the Lower
Great Lakes this morning with lingering periods of snow across
much of the region, although most totals will be on the light side.
As the storm tracks east towards the Mid-Atlantic, northerly winds
will accelerate over Lakes Superior, Michigan, and Huron and lead
to a handful of lake-effect streamers containing occasional bursts
of heavy snow on Friday. Latest WPC probabilities show low chances
(10-20%) for snowfall totals >4" in parts of northwest Indiana,
near Traverse City, MI, and along the coastal areas of Michigan's
thumb. Otherwise, additional snowfall totals will generally hover
around 1-3" in parts of Michigan, Indiana, and western Ohio.

...Central Appalachians to Northeast....
Days 1-2...

As the remnant circulation of the weakening clipper system
approaches the central Appalachians, its residual moisture will
flow directly into the mountain range Friday evening. By Friday
night, as low pressure off the East Coast deepens and high pressure
builds in from the north, low-level NW winds will be favorably
oriented into the Central Appalachians with some periods of snow as
far south as the Smokeys. Upslope ascent will continue into the
central Appalachians through Friday night and gradually taper off
by Saturday afternoon. Snowfall totals in east-central WV will
likely range between 1-4" with localized totals up to 6" possible
in the tallest peaks of the WV Appalachians and the Smokeys.

Farther east, an area of low pressure will organize along a
strengthening frontal boundary off the East Coast this morning.
This low will strengthen as low-level WAA to its north and a jet
streak couplet takes shape over the Gulf Stream. Meanwhile, broad
upper level divergence out ahead of the 250-500mb mean trough axis
will supply sufficient ascent aloft in tandem with the residual
moisture from the clipper system to support periods of snow over
northern PA and western NY. Colder temperatures aloft supporting
higher SLRs will also support As the low off the East Coast
deepens, easterly flow is expected to strengthen and a low level
convergence trough will setup over the northern Mid-Atlantic coast.
Latest CAMs guidance is keying on this convergence boundary
setting up over the Delaware Valley and stretching north into the
Poconos. WPC probabilities do show moderate chances (40-60%) for
snowfall >4" in parts of the Poconos, while there are low-to-
moderate chances (30-50%) for snowfall >2" in parts of the Lehigh
Valley, Delaware Valley, and into northern New Jersey. Localized
amounts in the highest elevations of the Poconos have low chances
(10-20%) for snowfall amounts topping 6". There is a chance for
minor accumulations (coating-2" along I-95 from the Philadelphia
metro on north through the NYC and Tri-State metro regions this afternoon

Coastal New England sports the higher "boom" scenarios given the
combination of ocean-enhanced snowfall over eastern MA Friday
afternoon and the deformation axis of the coastal storm extending
far enough west to produce heavy snow over Downeast Maine Friday
night into early morning Saturday. Latest guidance is starting to
come into better agreement with a swath of 1-3" of snowfall over
eastern MA with some CAMs suggesting higher end totals (>4" within
the range of possibilities. Downeast Maine could feature the
heaviest snowfall for the event in coastal New England with WPC
probabilities showing moderate-to-high chances (50-70%) for
snowfall >4" through Saturday morning.

...Pacific Northwest...
Days 2-3...

A pair of Pacific storm systems will bring rounds of high
elevation mountain snow to the Olympics, Cascades, Blue, and
Sawtooth Mountains this weekend. Snow levels will generally be as
low as 4,000ft in the Cascades, but the heavier snowfall totals
(>6" will likely be confined to elevations >5,000ft in the
Olympics and Cascades. In the Blue and Sawtooth Mountains, >6"
snowfall totals will likely be confined to elevations >6,000ft.
These heavier totals will generally be above pass level in these
mountain ranges, so expect the more hazardous travel impacts to be
in the more remote and complex terrain of these mountain ranges
this weekend.


The probabilities for significant ice accumulations are less than
10 percent.

Mullinax
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