Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 PM EST Fri Jan 15 2021

Valid 00Z Sat Jan 16 2021 - 00Z Tue Jan 19 2021

Days 1 to 3...

...Midwest/OH Valley to the Great Lakes/Central Appalachians and
Northeast...

A slow-moving and vertically stacked low pressure system currently
over the Midwest will shift gradually east across the OH Valley
tonight and cross the central Appalachians, northern Mid-Atlantic
and lower Great Lakes region on Saturday. As the strong height
falls approach the East Coast later tonight, a robust shortwave
will lift out of the base of the upper trough and up along the
Mid-Atlantic coast which will foster secondary cyclogenesis along
the attendant cold front edging offshore. This low center will
then lift inland across southern/central New England by Saturday
evening and then through central/eastern Maine by early Sunday. As
all of this occurs, deep layer south to southeast flow will ride
northeast up across New England bringing strong warm-air advection
and a robust fetch of Atlantic moisture for widespread moderate to
heavy precipitation. While there will be little in the way of cold
air initially in place to support widespread snow, the thermal
profiles do appear to be conducive for locally heavy, wet, and
elevation-dependent snowfall for portions of NY and New England.
The main focus should on elevations especially above 1500 feet in
the Adirondacks, the Berkshires, and the Green and White
mountains. Adjacent areas of western Maine will also be a focus
for heavy snowfall. The strong and mild Atlantic inflow should
keep snow levels somewhat elevated at least through midday
Saturday across the Northeast, but as the stronger height falls
arrive and low-level cold air begins arriving around the back side
of departing low pressure, the snow levels will begin to fall and
reach some of the interior valley locations across NY and New
England later in the day. The current forecast is for as much as 6
to 12+ inches of snow for the aforementioned higher elevations of
the Northeast going through early Sunday, with the heaviest
accumulations generally over the Green and especially White
mountains where storm totals of 12 to 18+ inches will be possible.

Meanwhile, the deep layer cyclonic flow coupled with multiple
embedded shortwave impulses will continue to favor broken areas of
light to occasionally moderate snow across portions of the Midwest
this evening and then spreading into the OH Valley by Saturday
morning. This energy coupled with cold and relatively moist low to
mid-level upslope flow into the central Appalachians should favor
an axis of heavier accumulating snowfall from central WV into far
western MD and southwest PA. Locally some areas of central WV may
see in excess of 6 inches of snowfall going through Sunday as the
focus will transition increasingly from synoptically forced
snowfall to more of a lake-enhanced snowfall threat as the
boundary layer winds gradually involve more of the flow over the
Great Lakes going through Saturday night and early Sunday.
Portions of western NY involving the Tug Hill plateau will also
see an increasing focus of heavy lake-enhanced snowfall Saturday
night and Sunday around the departing low center over the
Northeast with a highly favorable cold, moist and convergent
low-level fetch coming off the relatively warm waters of Lake
Ontario. As much as 6 to 12 inches of snow will be possible here.
Elsewhere, the broad deep-layer cyclonic envelope of this system
will also maintain the threat of accumulating lake-effect snowfall
for even areas of the U.P. of MI where north to northeast
low-level flow over Lake Superior will support locally 6+ inches
of additional snow for areas east of the Keweenaw Peninsula.

As the main closed low/trough pushing into the Northeast begins to
pull away by late Sunday, there will be a new vigorous upper
trough/closed low center dropping southeast across the Midwest and
the OH Valley going through early Monday. This may bring locally a
few additional inches of snow to the central Appalachians and
portions of western NY.


...Northern/Central Rockies...

Multiple shortwave troughs will be advancing southeast down across
the northern/central Rockies going through this weekend and early
next week, with one impulse sliding progressively southeast
tonight and early Saturday, and then a stronger shortwave digging
southeast and into Intermountain West by Monday. Stronger
jet-aided forcing associated with the shortwave impulses coupled
with relatively moist mid-level flow should favor heavy snowfall
for portions of the Bitterroots and Sawtooth where as much as 6 to
12 inches will be possible going through Monday. However, somewhat
stronger upslope flow should tend to favor even heavier snowfall
totals that may exceed 1 foot across the Belts, Beartooths and
Bighorns. Some of this heavier snowfall should make it farther
south down into south-central WY and far northern CO with the
Medicine Bow mountains likely to see as much as 1 foot of snow as
well.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Orrison