Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
458 AM EST Tue Dec 13 2022

Valid 12Z Tue Dec 13 2022 - 12Z Fri Dec 16 2022

...Central Appalachians and Mid-Atlantic...
Days 2-3...

An anomalously strong closed 500mb low moving across the central
CONUS will slowly shift eastward as it runs into a short
wavelength but high amplitude ridge across the Atlantic coast.
Impressive downstream divergence and spokes of vorticity rotating
around this larger gyre will work together with increasingly
coupled jet streaks over the Mid-Atlantic to drive surface
pressure falls and secondary cyclogenesis over the Southeast on
Thursday. This low will move along the baroclinic gradient along
its front, and intensify as it shifts offshore. Pronounced
synoptic lift will be enhanced by impressive WAA lifting northeast
on impressive moisture 300K isentropic upglide atop a Canadian
wedge of high pressure with mixing ratios above 6g/kg. While the
antecedent airmass is not extremely cold, it is quite dry, such
that temperatures will wet bulb down to below freezing as precip
saturates the column. This will enhance the wedge, and although
some models are depicting snow as far south as the Appalachians in
southern VA, 850mb temps are generally progged to be above 0C as
far north as the Mason-Dixon line so the primary p-type is likely
to be ZR/IP from VA into PA.

As precip continues to expand northeastward Wednesday night and
Thursday, the in-situ wedge will strengthen through precip falling
into it, and a prolonged period of freezing rain is likely,
especially in the terrain from SW VA through western Upstate NY.
There is some uncertainty into how much ZR can accrete as there
appears to be a lack of impressive dry advection to offset heating
due to latent heat release of freezing, and precip rates may also
become impressive as ECMWF EFI tables indicate an increasing plume
of instability surging northward. Still, WPC probabilities for
more than 0.25" of ZR are above 50% from the Blue Ridge of VA
northward along the Laurel Highlands, with lesser but still
notable freezing rain possible as far east as I-95 from
Washington, D.C. to Philadelphia, and extending into western
Upstate NY. Much of this freezing rain is likely Wednesday night
and the first half of Thursday.

Farther to the north, the strong WAA will likely result in a burst
of heavy snowfall where the column remains entirely below
freezing. Snow ratios near the transition zone will be low
suggesting a heavy and wet snow for parts of PA, which is also
reflected by high pWSSI probabilities for moderate impacts due to
snow load. As precip continues to expand into Upstate NY and
western New England on D3, periods of moderate to heavy snow are
likely as the WAA overlaps with the robust deep layer synoptic
ascent. The heaviest snow is progged in the terrain due to the
overall marginal low-level thermal structure of the column within
the Canadian high pressure, and WPC probabilities for more than 4
inches reflect this, peaking above 70% in the Catskills, northern
PA, Poconos, and southern Adirondacks. Moderate probabilities
extend into the Berkshires as well. Additional heavy snow is
likely into New England on D4 as the coastal low matures and
advects to the northeast along the New England coast.


...Northern/Central High Plains through the Upper Midwest...
Days 1-3...

...Significant winter storm to bring blizzard conditions and
significant icing to portions of the High Plains into the Upper
Midwest...

The blizzard will rage in full force today as the surface low
becomes vertically stacked with the anomalously strong low aloft
to slow the system to a crawl, and spread heavy snowfall across a
large portion of the region as impressive WAA continues to feed
moisture meridionally into the system. Forcing for ascent will
maximize as the most intense LFQ diffluence curves northwestward
over the low, with an impressive TROWAL overlapping a deformation
axis resulting in pivoting bands of 1-2"/hr snowfall. While the
guidance has trended just a hair southeast today, overall model
consistency is good and there is high confidence in axis of very
heavy snowfall (rates and accumulations) from the High Plains of
WY and MT through the Dakotas and into MN. Southeast of there, an
axis of moderate to heavy freezing rain within the mixed p-type
transition zone will occur from the Coteau eastward through
central WI and lower MI, but the dry slot is progged to fill in a
bit more rapidly today as the low occludes, suggesting even these
areas of SD/MN may get some heavy snow as precip turns back to
snow.

The heaviest snowfall is almost certainly going to occur where the
deformation pivots NW of the surface low and collocates with
theta-e lapse rates <0C/km suggesting CSI. These 1-2"/hr snowfall
rates moving across the same areas with generally fluffy SLR of
15:1 will accumulate to more than 12 inches of snow as shown by
WPC probabilities for 12+ inches above 70% for the Pine Ridge of
NE, the Black Hills of SD, and much of western SD into far SW ND
and eastern MT. This is where blizzard conditions could be severe.
East of this pivoting band, more broad WAA snow or a shorter
duration of deformation snow will accumulate to more than 6 inches
from the NE High Plains of CO through central NE, into the eastern
Dakotas and across MN/WI. A secondary maximum of snowfall is
likely along the Arrowhead of ME where a secondary WCB extending
towards the triple point and secondary low development will surge
across Lake Superior, adding column moisture, and efficiently
upsloping into the Iron Ranges. More than 2 feet of snow is
possible in isolated locations across the Arrowhead in addition to
the broader deformation axis across the northern High Plains.
These heavy snow rates combined with strong winds gusting over 50
mph and nearly 2 feet of snow accumulation will likely cause
impossible travel during the event.

Farther to the south and east, significant freezing rain is
possible over parts of eastern SD into southeastern ND and
southwestern MN with additional enhancement over central WI during
the event. Strong warm advection will surge a warm nose above
850mb to above 0C, while surface temps remain marginally just
below freezing. Across the higher terrain of the Coteau and other
areas farther north into WI, surface temps will likely remain in
the upper 20s to around 30 which will be enough to preclude the
self-limiting processes inherent to freezing rain without strong
advection of cold dry air to maintain sub-freezing wet bulb
temperatures. Even in these areas, WPC probabilities for more than
0.25" peak around 20% due to heavier rain rates which will not
efficiently accrete. WPC probabilities above 20% for 0.1" of
freezing rain extend from the Coteau across much of southern and
central MN, WI, and into the lower peninsula of MI.


Key Messages for December 9-15 Winter Storm:
--Multiple days of heavy snow, strong winds, and periods of
freezing rain will create major to extreme impacts across the
north-central U.S this week.

--Blizzard conditions are expected for the northern and central
High Plains where 1-2"/hr snow rates and winds gusting 50-60 mph
will create near zero visibility and snow covered roads. Difficult
to impossible travel, scattered power outages, and harsh livestock
conditions are expected.

--Heavy snow and gusty winds will likely spread across the
Northern Plains into the Upper Midwest Wednesday and Thursday. 
Blowing and drifting snow will create difficult travel due to low
visibility and snow covered roads.

--A wintry mix, including freezing rain and sleet, will continue
across the Plains and shift into the Upper Midwest through
Wednesday. Freezing rain accretions may exceed 0.25" in some
areas, creating dangerous travel conditions and isolated power
outages.

--Strong winds and cold temperatures will continue even after this
storm ends, creating bitterly cold wind chills into the weekend.

Weiss