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

Valid 00Z Sat Dec 21 2024 - 00Z Tue Dec 24 2024


...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 this evening. By tonight, 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 tonight 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. WPC probabilities
show the greatest (30-50%) chances of 4 inches or more of snow east
of the Mississippi River associated with this current clipper
system will be across east-central WV for the Day 1 period.

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. As the low off the East Coast deepens,
easterly flow is expected to strengthen and a low level
convergence trough will set up 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. Much of the snow in this region should diminish by
Saturday morning.

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 will feature the
heaviest snowfall for the event in coastal New England with the
latest WPC probabilities showing high chances (70-90%) for
snowfall of 4 inches or more through Saturday afternoon. Areas
around Eastport, ME have a 30-50% chance of 8 inches or more of
snow through Saturday afternoon.

...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.

...Upper Midwest...
Day 3...

A low that will eject out of the northern Rockies and into the
Northern Plains on Sunday will begin to tap much colder air as it
also draws a bit of Gulf moisture north into the Upper Great Lakes
Sunday night into Monday. As the shortwave trough forcing the low
goes from being positively tilted to neutral, the increasing
divergence could cause a narrow area of snow to break out from
northern Minnesota through much of northern Michigan. The heaviest
snow totals from this low will likely fall across the eastern U.P.,
where lake-enhancement could locally increase snowfall totals. The
heaviest snows will be from late Sunday night through Monday
morning. Expect 4 to 6 inches of snow to fall across the eastern
U.P. and far northern L.P. of Michigan, with lesser amounts
elsewhere. As is typical, locally heavier snow totals are possible
where heavier and more persistent bands of snow set up, especially
if the lakes add moisture to the atmosphere.


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


Mullinax/Wegman