Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 PM EST Mon Dec 12 2022

Valid 00Z Tue Dec 13 2022 - 00Z Fri Dec 16 2022

...Great Basin and Rockies...
Day 1...

Combination of deep upper low over Utah and peripheral vorticity
swinging through the base of the larger-scale troughing will
advance the system out of the Rockies in basically two parts. Each
will be a focus for snowfall in the Great Basin/central Rockies as
well as across southeastern AZ into NM, driven by a 130kt jet.
Snow levels will continue to lower in concert with the cold core
upper low as precipitation gradually winds down by Tuesday.
Several additional inches are likely for the White Mountains in AZ
and into the San Juans.

...Central Appalachians and Mid-Atlantic...
Day 3...

Maturing central CONUS system will run into expanding blocking
over Hudson Bay (GFS 500mb heights around +3 sigma), forcing an
elongation of the front and eventual development of a new area of
low pressure near the DelMarVa by late Thursday. In Atlantic
Canada, coalescing mid-level lows will act to help strengthen the
jet off New England (>150kts) in advance of the 130+ kt jet across
the MS Valley, promoting broad scale lift up and over the surface
front. With marginally cold surface temperatures and advancing
warm air advection aloft, a mixed ptype event will overspread much
of the region late Wednesday through Thursday. Though the surface
high will anchor down the coast preceding the arrival of
precipitation, it will not continue to reside in a place to supply
cold air through the event. This would allow some warming to
change any frozen precip to rain closer to the coast, but interior
locations will likely see a prolonged event. By the end of day 3,
surface low development near the DelMarVa may act to enhance a
colder northeasterly flow into the central Appalachians, perhaps
turning any mixed precip to snow. There remains a fair amount of
uncertainty in the lower thermal profiles but the greatest
probabilities of snow (>4") lies over north central PA (still
low-end near 10% through 00Z/16). Guidance has been trending
toward a more impactful freezing rain event for the Laurel
Highlands southward through eastern WV and western VA/MD where
current WPC probabilities for at least 0.25" icing are high
(>70%).


...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 large bowling ball closed low ejecting from the Central
Rockies overnight will consolidate and deepen over the Central
Plains Tuesday, with 700 mb heights falling to -4 sigma compared
to climo (and at the min over the 30-yr CFSR climo period for this
time of year) according to the NAEFS ensemble tables. This
deepening low will combine with phased jet streaks south of the
primary trough curving cyclonically and arcing poleward to place
impressive LFQ diffluence overlapping the most robust height falls
and PVA at 500mb. This will result in surface cyclogenesis in the
lee of the Rockies, and this system will move slowly northeastward
in response to the highly amplified/blocky pattern. As the low
becomes vertically stacked by Wednesday, the upper/mid-level
features will largely stall, allowing broad warm air advection to
carry a formidable moisture plume (PWs ~99th percentile) northward
from the Gulf across the Corn Belt and into the TROWAL over the
Northern Plains. Cold, dry airmass will take a bit to moisten
through the column, but precipitation will become heavy rather
quickly across the region. This will include snow to the north and
northwest of the low and an area of sleet and significant freezing
rain near where the mid-level low stalls over eastern SD.

Classic evolution of a quickly maturing then slowly decaying
cyclone will unfold as the system wraps up and occludes Tuesday
into Wednesday. Deformation band will likely for to the NW of the
low in an area of supportive FGEN and slantwise convection which
will result in snowfall rates of 1-2"/hr per the WPC Snowband
Probability Tracker page. Heavy snow well over a foot is likely
where this band may stall and pivot, and WPC 48-hr probabilities
for >12" are high (>70%) over much of western SD and northwestern
NE and moderate (>40%) from central SD westward into southeastern
MT and parts of eastern WY. Strong winds will create blizzard
conditions and blowing/drifting snow. Broader WAA-driven snow to
the east will be locally enhanced over the Arrowhead of MN as the
occlusion halts its northward progression due to blocking to its
northeast. Varying degrees of vertical thermal advection will
modulate the ptype amounts between sleet/freezing rain over
eastern SD into IA/MN until the dry slot shuts off most QPF by
early Wednesday.

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 bring in >0C air to the 750-850mb layer atop near
and sub-freezing 2m temps, promoting accretion on already cold
surfaces. Without a strong re-supply of cold air, many places may
change to a cold rain from ~I-80 southward whereas places farther
north will have a colder starting point and may hang to
sub-freezing temperatures for some time. This may be counteracted
by higher QPF and thus likely lower accretion, but even so the 12Z
CAM guidance does show potential for >0.25" icing. As such, WPC
probabilities for at least 0.10" are moderate (>40%) from northern
IA northwestward and high (>70%) over northeastern SD. Over north
central WI and into Lower MI, north of the occlusion, marginally
cold 2m temps may support a prolonged period of light to moderate
icing as less intense WAA off the deck maintains a ZR profile. WPC
probabilities for at least 0.10" of icing are generally low
(10-40%) to moderate (40-70%) over WI and Lower MI.


Key Messages for December 9-15 Winter Storm:
--A winter storm will advance from the Central Rockies into the
High Plains tonight. Blizzard conditions will likely develop over
the Central to Northern High Plains. Heavy snowfall rates of
1-2"/hr and strong winds gusting to 50-60 mph will likely make
travel very difficult to impossible.

--This system will likely track slowly northeast across the
Central Plains from Tuesday into Thursday, producing a broad area
of heavy snow, strong winds, and icy conditions from the Central
into the Northern Plains and Upper Midwest. Widespread impacts on
travel and infrastructure are expected.

--An icy mix, including freezing rain and sleet, is forecast to
begin tonight across the Plains and shift into the Upper Midwest
on Tuesday. Freezing rain accretions may exceed 0.25" in some
areas, creating dangerous travel conditions and isolated power
outages.

--Strong winds and cold temperatures will produce dangerous wind
chills and harsh livestock conditions.


Fracasso/Weiss