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

Valid 00Z Wed Dec 14 2022 - 00Z 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...

Ongoing blizzard will continue in full force tonight as the
surface low becomes vertically stacked with the anomalously strong
low aloft. With strong blocking strengthening to the northeast,
the system will slow to a crawl tomorrow, with 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. Overall model consistency is good and there is high
confidence in an axis of very heavy snowfall (rates and
accumulations) from the High Plains of WY and MT through the
Dakotas and into MN. WPC probabilities for at least 8 inches of
snow Wed-Thu are greater than 50% for much of western ND/SD into
MT/WY as well as over much of northern MN/WI. A secondary maximum
of snowfall is likely along the Arrowhead of MN where a secondary
WCB extending towards the triple point and secondary low
development along the front over the lower MS Valley 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. Back over parts of the eastern
Dakotas, areas that have seen significant freezing rain today will
turn to snow as the dry slot fills in a bit and the column
continues to cool.

North of the occlusion, as warm air overrides a sub-freezing
boundary layer, an area of freezing rain is expected overnight
across central WI into lower MI. Surface temps will likely remain
in the upper 20s to around 30 (Tds in the mid 20s currently) 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. WPC probabilities for more
than 0.10" of freezing rain are moderate (>40%) to high (>70%)
across much of north central WI through Wednesday evening, and
generally lower than 50% into the central portions of Lower MI.


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

Deep upper low over the Upper Midwest/Great Lakes Thursday will
slowly move eastward as the blocking upper ridge over northern
Quebec closes off into an upper high. Combination of an Atlantic
Canada upper low and squeezed short wavelength upper ridging into
the eastern Great Lakes will promote a large area of upper
divergence as a southern vort max on the periphery of the Upper
Midwest upper low begins to spur cyclogenesis near the DelMarVa.
As the surface low becomes dominant from the old parent low over
the Great Lakes, warm advection preceding the occlusion will turn
to dynamic column cooling. The evolution through the Day 2-3
period will result in a variety of precipitation types through
time and increasingly heavier snow farther north. The surface low
will move along the baroclinic gradient and deepen on Friday off
the NY Bight. Pronounced synoptic lift will be enhanced by
impressive WAA and moisture as evidenced by 300K isentropic
upglide 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. The proportion of each has wavered
with recent model guidance with some recent trend toward more IP
into central 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 PA. 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. P-type probability meteograms show
this well at at point via higher QPF despite the p-type
uncertainty. WPC probabilities for more than 0.25" of ZR are high
(>70%) over the Laurel Highlands southward across far western MD
into WV with lower probabilities surrounding this area into the
Blue Ridge of VA and central MD. The probability of any freezing
rain extends eastward to the I-95 corridor but areas closer to the
coast will see a much quicker transition to rain. Much of this
freezing rain is likely Wednesday night and the first half of
Thursday. Should the column cool more quickly, more sleet is
possible especially across central PA where more than an inch is
quite possible.

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, the
North Country, 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, Adirondacks, Green Mountains into
the Berkshires, and White Mountains in NH. Additional heavy snow
is likely farther into New England on D4 as the coastal low
matures and advects to the northeast along the New England coast.


Key Messages for December 9-15 Winter Storm:
--Blizzard conditions continue 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. Expect
difficult to impossible travel, scattered power outages, and harsh
livestock conditions.

--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.

--As the storm moves to the eastern U.S., the threat for heavy
snow and ice accumulation is growing for portions of the Central
Appalachians and the interior portions of the Northeast.

--Significant travel disruptions and hazardous conditions are
possible later this week into the weekend; monitor the latest
forecasts and updates.

Fracasso/Weiss