Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 PM EST Sun Dec 11 2022

Valid 00Z Mon Dec 12 2022 - 00Z Thu Dec 15 2022

...Northeast...
Day 1...

The shortwave trough responsible for today's snowfall across the
interior Northeast will be making its way off shore Sunday
evening. There is still adequate PVA at 500mb out in front of the
trough to promote strong vertical ascent aloft, and temperatures
over the interior portions of southern New England remain
sufficiently cold enough to support snow. With the bulk of the
event concluding, >4" snowfall probabilities via the latest WPC
PWPF are much lower <10%. That said, there is still a swath of >2"
snowfall probabilities that are as high as 50-60% along the CT/MA
state border and hourly snowfall rates could be 0.5-1.0"/hr in
some spots. This means some areas may still contend with slick
travel conditions this evening before the snow threat concludes
later tonight.

...Great Basin & Rockies...
Days 1-2...

The upper low that has blasted the West Coast with heavy rain,
copious amounts of mountain snow, and strong wind gusts is now
moving towards the Great Basin tonight. It will direct its steady
barrage of 700mb moisture flux and diffluent upper level flow
aloft more over the Intermountain West, including numerous
mountain ranges from eastern and southern Oregon on south to the
Mogollon Rim tonight into Monday. Some of the ranges that contain
50-70% probabilities of WPC PWPF snowfall >8" include the Mogollon
Rim, the San Juans, along the Wasatch and the Uintas, and on north
to the Absaroka and Blue Mountains. The latest WSSI depicts
impacts ranging from Moderate in the lower-mid level elevations in
these ranges, to "Major" along the crests of these ranges,
particularly along the Wasatch, the San Juans, and the Mogollon
Rim. By Monday evening, heavy snow moves into heart of the
Colorado Rockies and into Wyoming's Sierra Madre, Medicine Bow,
and Front Ranges. Similar WPC PWPF snowfall probabilities are
anticipated in these ranges through Monday night and into early
Tuesday morning.

...Northern & Central High Plains through the Upper Midwest...
Days 2-3...

...Blizzard conditions becoming more likely for parts of the
Plains with treacherous ice accumulations in the Midwest...

The stage is set for what will likely be a major winter storm to
impact much of the Northern and Central Plains, as well as the
Upper Midwest. This powerful upper trough traversing the
Intermountain West will place its diffluent 250mb left-exit region
over the Central Plains Monday afternoon, resulting in a deepening
wave of low pressure in lee of the Rockies over eastern Colorado.
Ahead of the developing storm, a strong 850-700mb moisture
transport will develop as the low level jet delivers plenty of
850-700mb moisture flux northward into the North-Central U.S..
Meanwhile, a cold front is set to stall somewhere over the heart
of the Dakotas and two domes of high pressure: one over the
Canadian Prairies and another over the Lower Hudson Bay, help to
supply sufficient low level cold.

In terms of where the highest confidence lies for major impacts,
it is the heavy snowfall and strong wind gusts to the northwest
and western flanks of the storm system. This includes western
ND/SD, eastern MT, and on south through eastern WY, western NE,
northeast CO, and even northwest KS. It is in these areas where
the TROWAL is likely to pivot, leading to a long-duration heavy
snow event there with hourly snowfall rates >2"/hr possible. Some
soundings just to the northwest of the strongest 850mb
frontogentical forcing contains weak MUCAPE, suggesting
thundersnow is a possibility within this axis of heavy snowfall.
Latest WPC PWPF shows 60-80% probabilities for snowfall >12" on
Day 2 for eastern SD and northwest NE, while similar percentages
are in place for >8" of snowfall in southeast Montana and
southwest ND. In addition, winds will be howling on the backside
of the storm with wind gusts >50mph. The ECMWF EFI featured values
of >0.9-0.95 in parts of far southeast WY, western NE, southwest
SD, and northeast CO between 00Z Wed - 00Z Thu. It is here where
the most intense wind gusts transpire. Blizzard conditions are
likely in these areas with dangerous travel conditions expected.
The latest WPC WSSI does contain "Major" impacts in western SD and
into northwest NE, while an expansive area of "Moderate" impacts
stretches from southeast MT all the ware to eastern ND and
northeast CO.

The lower confidence impact area resides in the Midwest, where the
roaring 850mb jet (set to be 2-3 standard deviations above normal
Monday night into Tuesday according to NAEFS) will lead to a
burgeoning warm nose at low levels. Where guidance is most
undecided is not in the development of precipitation as the column
saturates Monday night into Tuesday, but in low level thermal
profiles. CAMS are decidedly colder than global guidance, the
latter of which, does tend to erode low-level subfreezing
temperatures too quickly. WPC PWPF freezing rain probabilities
have increased from the overnight shift with moderate chances
(40-60%) for >0.10" of ice accumulating from eastern SD through
southern MN and into northern IA. There are also some spotty areas
of up to 10% probabilities for freezing rain accumulations >0.25"
in these aforementioned areas. It is worth noting the 75th NBM
percentile was as high as 0.40" in some areas, which could be a
reasonable high end for ice accumulations should the overall trend
in guidance stay on the colder side. Eventually, this front end
thump of precipitation may change over to snow, most notably in
the MN Arrowhead where WPC PWPF features 50-60% probabilities for
>8" of snowfall on Wednesday. The latest PWSSI actually depicts a
60% chance for "Moderate" WSSI criteria near and northwest of
Duluth on Wednesday. The threat for heavy snow then shifts to
northern WI Wednesday night where there is currently a 40-50%
chance for >6" of snowfall northwest of Green Bay.


Key Messages for December 9-15 winter storm from the Pacific Coast
through the Upper Midwest:
--A winter storm over the Southwest will advance through the
southern/central Rockies through Monday before intensifying and
stalling over the central High Plains Monday night into Thursday.

--Mountain snowfall rates of 1-2â€/hr will spread across the
Great Basin and Desert Southwest, resulting in difficult travel
through Monday.

--Multiple days of significant impacts to travel and
infrastructure due to snow, blowing snow, and freezing rain are
expected to begin over the Central and Northern Plains Monday
night. Blizzard conditions are expected for parts of the Central
and Northern High Plains, and travel may become impossible.

--An icy wintry mix including freezing rain and sleet is forecast
to begin Monday night on eastern portions of the Northern Plains
and expand over the northern Mississippi Valley and Upper Midwest
into Wednesday. This will lead to hazardous travel.

--While there is lingering uncertainty regarding ice accumulations
in portions of the Midwest, confidence is increasing that ice will
be problematic to travel across the region later this week


Mullinax