Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
452 AM EST Wed Dec 14 2022

Valid 12Z Wed Dec 14 2022 - 12Z Sat Dec 17 2022

...Significant winter storm with blizzard conditions and
significant icing to transition from portions of the High
Plains/Upper Midwest to the interior Mid-Atlantic/Northeast...

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

The ongoing blizzard will continue across the Plains today as the
surface low drifts only very slowly eastward due to it being
vertically stacked and blocked by high amplitude ridging to the
east. As an impressive upper jet streak rotates cyclonically to
the north around the primary trough axis, robust LFQ diffluence
will help ventilate the atmosphere to produce secondary low
development into the Upper Midwest as the primary low occludes.
This low will then retrograde westward, with strong ascent
continuing to pivot northwestward as strong moist isentropic
upglide lifts into a secondary TROWAL over the Upper Midwest to
enhance what is already strong synoptic lift. The most significant
change to the guidance tonight is for snowfall to be more
widespread and heavy invof the primary occluding low as colder air
collapses back to the southeast and fills in with heavy snow in
regions of deformation. Heavy snow on D1 will likely be focused in
three regions: the primary deformation axis pivoting over the
Northern High Plains of MT/ND, the secondary deformation in
eastern SD developing around the occluded low, and within the
WCB/TROWAL lifting northward over WI/MN, with a local maxima
likely in the Arrowhead where E/SE fetch taps Lake moisture,
upslopes into the Iron Ranges, and helps promote a deepening
isothermal layer beneath the DGZ to enhance SLR and snowfall
rates, which the WPC prototype snow band tool suggests will
eclipse 2"/hr tonight into Thursday. This slow moving system with
continued impressive ascent and ample moisture advection will
cause widespread snowfall through Thursday night before the
primary low finally shifts eastward D3 with drying occurring from
the NW.

WPC probabilities on D1 are high for more than 8 inches in all of
the three above regions, with locally more than 12 inches likely
where the best banding can occur. The heaviest snow today and
tonight is expected in the Arrowhead of MN from Duluth northward
where more than 2 feet of snow is expected in some areas. More
than 4 inches is likely across much of the Northern Plains outside
of these heaviest bands. During D2, snow may actually expand again
across the eastern Dakotas into MN/WI as deformation continues to
pivot southward around the occluded low, acting upon still robust
moisture transport from the east. WPC probabilities for more than
8 inches of snow are highest once again along the Arrowhead due to
Lake Superior and upslope influence, with a secondary max possible
across the Coteau des Prairies. More then 4 inches of additional
snowfall is likely as well across much of MN and the eastern
Dakotas. By D3 the low finally begins to pull way as W/NW flow
funnels drier air into the region, but this could manifest as an
uptick in LES east of Lake Michigan with a few inches of
accumulating snowfall.

Additionally, on the southern edge of the secondary moisture plume
and TROWAL blossoming over WI/MN, marginal thermals with a warm
nose aloft driven by WAA will result in modest to significant
freezing rain, which could accrete to more than 0.1" in
north-central WI and the L.P. of MI.


...Central Appalachians through New England...
Days 2-3...

Low pressure developing across the Mid-Atlantic along a triple
point will shift northeast along the coast towards New York City
through Friday morning, before redeveloping farther out to sea on
D3. The guidance differs as to how this secondary low will develop
- with the ECENS and CMCE suggesting a more inland track, with the
GEFS portraying a track well south of New England that skirts more
eastward. Confidence is lower than usual for this event, as both
the large closed low over the Upper Midwest, and a blocking low
pressure in the Canadian Maritimes will both impact the eventual
evolution. Regardless of the exact track, impressive
WAA/isentropic upglide combined with height falls, and
intensifying upper level jet diffluence will spread heavy
precipitation from the central Appalachians through New England
beginning tonight.

As WAA begins to intensify tonight downstream of the developing
wave of low pressure, precipitation will spread as far north as
the Mason Dixon line by Thursday morning. The guidance is in very
good agreement that this WAA will shift a warm nose at 850mb of
>0C northward rapidly, and it is unlikely any location south of
the Mason Dixon except the higher terrain will receive much snow
accumulation. However, cold Canadian high pressure wedged down the
coast will be enhanced both by evaporative cooling (the antecedent
airmass is quite dry) and diabatic cooling through precip loading
to keep surface temps below freezing. This setup supports at least
a brief period of IP for the Mid-Atlantic and up along the I-95
corridor towards NY, but freezing rain is likely to be the primary
p-type before changing to rain. For some areas, including west of
the Blue Ridge northward to the MD Panhandle and the Laurel
Highlands, an extended period of freezing rain is likely. Despite
precip rates intensifying which usually does not accrete
efficiently, and temps just slightly below freezing, WPC
probabilities for more than 0.25" have increased to 50-80% in this
areas, and some areas may receive damaging accretion in excess of
0.5" of ice. Lighter accretions of 0.1" or more are possible as
far south as SW VA, northward towards Buffalo, NY, and NW of the
fall line from Washington, D.C. to Philadelphia. The Thursday
morning commute could be quite trying in these areas.

Farther to the north, as the low intensifies and shifts northeast,
the strong WAA and overlapped synoptic ascent will result in
periods of heavy snow. Uncertainty continues into how far north
the warm nose will shift which has lowered confidence for eastern
PA, southeast NY, and southern New England, but areas to the NW of
there will likely receive heavy snowfall with rates of 1"/hr or
more at times. Although the duration of heavy snowfall may be
somewhat limited across PA as the warm advection shifts northward,
WPC probabilities for more than 6 inches of snow are 40-70% for
north-central PA and into the Poconos. D2 into D3 has the biggest
question mark and will depend on how the surface low tracks, but
strong moist advection and impressive synoptic ascent may result
in a pivoting area of snowfall with banded structures, especially
across parts of Upstate NY which will also benefit from E/SE
upslope flow into the terrain. Although SLRs will likely be below
the Baxter climatology December median, resulting in a heavy wet
snow, WPC probabilities are high for more than 6 inches on D2 in
the Catskills and Adirondacks, with a high probability again on D3
when the heaviest snowfall is expected. Total snowfall across
Upstate NY and into central New England will likely exceed 12
inches in much of the terrain, and may do so as well into Maine D3
but this will be more dependent on the final storm track.


Key Messages for December 9-17 Winter Storm:
--Blizzard conditions continue for the Northern Plains and develop
across the MN Arrowhead today, where 1-2"/hr snow rates and winds
gusting 45-60 mph will create near zero visibility and snow
covered roads. Expect difficult to impossible travel, scattered
power outages, and harsh livestock conditions.

--Heavy snow and gusty winds will increase across the Upper
Midwest today and intensify into Thursday.  Blowing and drifting
snow will create difficult travel due to near zero visibility and
snow-covered roads.

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

--Beginning late tonight, impacts from this system will spread
into the Central Appalachians and Mid-Atlantic, expanding into the
interior Northeast Friday and Saturday.

--Significant travel disruptions and impacts to infrastructure are
likely due to the combination of heavy snow, sleet, and freezing
rain. Continue to monitor the latest forecasts and updates.


Weiss