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

Valid 00Z Thu Dec 15 2022 - 00Z Sun Dec 18 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...

Days 1-3...

...Northern/Central High Plains through the Upper Midwest...

The ongoing blizzard across portions of the northern Plains will
be continuing into tonight as the associated surface low drifts
only very slowly eastward due to it being vertically stacked and
blocked by high amplitude ridging to the northeast over southeast
Canada. east. The latest guidance continues to advertise a strong
upper-level jet streak rotating cyclonically to the north around
the primary trough axis, and resulting strong LFQ jet divergence
will help ventilate the facilitate secondary low development into
the Upper Midwest as the primary low begins to weaken. This second
low will then retrograde westward, with strong ascent continuing
to pivot northwestward as strong moist isentropic upglide lifts
over the Upper Midwest in conjunction with a secondary TROWAL to
enhance what is already strong synoptic scale lift.

Overall, no big changes to the previous thinking with respect to
the snowfall forecast through early Thursday as the heavier snow
areas should tend to be focused over three different regions. This
will include the the primary, but gradually waning deformation
zone pivoting over the northern High Plains of eastern MT/ND, the
secondary deformation zone strengthening over eastern SD around
the current occluded low, and within the new WCB/TROWAL lifting
northward over WI/MN in association also with a new wave of
surface low pressure developing. The heaviest additional snowfall
is expected to be with this latter feature over the Arrowhead of
northeast MN and into northwest WI, with a combination of strong
forcing overlapping at least locally with some focused low-level
moisture transport off of Lake Superior. Upslope flow into the
Iron Ranges will favor enhanced snowfall rates, and this coupled
with the larger scale environment will support snowfall rates that
could reach or locally exceed 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 on Friday and
begins to weaken.

Additional snowfall amounts going through Friday are expected to
be on the order of 6 to 12 inches across areas of far eastern MT
and adjacent areas of the Black Hills, and stretching eastward
across the Dakotas. The secondary low evolution/TROWAL mentioned
about will be favor 6 to 12 inches across large areas of
central/northern MN and central/northern WI. However, areas of the
MN Arrowhead and far northwest WI, including the local Iron Ranges
are expected to see as much as 1 to 2 feet of snow with locally
heavier amounts immediately downwind of Lake Superior where the
best moisture transport and upslope will be focused. WPC
probabilities for seeing over 12 inches of snow is maximized
across northeast MN and northwest WI. By Saturday, with the low
weakening and gradually pulling away, there will be cold air
advection advancing across the upper Great Lakes, and this will
help to locally focus several inches of lake-effect snowfall in
behind departing synoptic system.

Regarding the ice potential, along 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 some areas of freezing rain potential which will mainly be
tonight across central/northern WI, and the L.P. of MI. Several
hundreths of additinal ice accretion will be possible.


...Central Appalachians through New England...

A triple point area of low pressure developing across the southern
Mid-Atlantic by Thursday afternoon ahead of the upstream
Midwest/Plains trough will strengthen and shift northeast to
offshore of Long Island by Friday morning. The low will then cross
the Gulf of Maine on Saturday. As WAA and moisture transport
begins to intensify tonight across the central Appalachians and
Mid-Atlantic, precipitation will be developing and will be
encountering a cold low-level wedge that is in place already
across the region and especially across the Northeast. The cold
air though will be shallow in nature as the increasing warm
advection will be bringing a nose/layer of above freezing air in
the 850/750mb layer that will favor a setup conducive for sleet
and freezing rain. Marginally sub-freezing air in the boundary
layer is expected to be in place across the central Appalachians
and the interior sections of the central and northern
Mid-Atlantic, mainly west of the I-95 corridor that will
facilitate the potential for freezing rain, and the onset of
precipitation will be occurring with a fair amount of dry air that
will help to reinforce the low-level cold air through evaporative
cooling. In time on Thursday, a sufficient degree of low-level
warm air advection should ensue to help warm the boundary layer
back above freezing for many of the Piedmont areas of the
Mid-Atlantic for a changeover to rain, but the interior valleys
and adjacent central Appalachian chain should see cold air hanging
tough that will drive a prolonged period of sleet and freezing
rain potential. Also, as low pressure begins to bypass the
Mid-Atlantic, colder air aloft will begin to erode the warm nose
such that areas of sleet and freezing rain changeover to snow.
This will especially be the case across central PA and northern
PA. For some areas, including west of the Blue Ridge from far
northwest VA and the MD/WV Panhandles northward into the Laurel
Highlands, as much as a 0.25" to 0.50" of ice accretion is
expected, and a few favored higher peaks may see a bit over a
0.50" of ice. This will be in addition accumulations of sleet.
Lighter ice accumulations are expected farther east across the
Piedmont areas west of I-95 with locally as much as a 0.10" to
0.25" possible. Minimal icing is expected for the immediate
metropolitan areas from Washington D.C. to Philadelphia.

Farther to the north, as the low intensifies and shifts northeast
offshore of New England, sufficiently deep enough cold air will be
in place for heavy snow to result as strong WAA and moisture
transport couple with robust synoptic scale ascent. Areas of
south-central through eastern NY and across central New England
should see the heaviest snowfall with as much as 1 to 2 feet of
snow expected. This will especially be the case for the
orographically favored Adirondaks, and the Green and White
Mountains of VT and NH respectively. Some spotty 2+ feet snowfall
totals will be possible.


Key Messages for December 9-17 Winter Storm:
--Blizzard conditions will continue into tonight for the Northern
Plains and will be developing for the MN Arrowhead, 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 overnight and intensify into Thursday.  Blowing and
drifting snow will create difficult travel due to near zero
visibility and snow-covered roads.

--Beginning tonight, impacts from this system will spread into the
Central Appalachians and Mid-Atlantic and continue through
Thursday, before reaching the interior Northeast Friday and
Saturday.

--A wintry mix, including sleet and freezing rain is expected for
portions of the Central Appalachians and northern Mid-Atlantic
with ice accretions locally reaching a 0.25†to 0.50â€,
creating dangerous travel conditions, tree damage and scattered
power outages.

--Heavy snow accumulations will arrive across interior New York
and New England bringing significant travel disruptions and
impacts to infrastructure for Friday and Saturday.

Orrison