FOUS11 KWBC 192056
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 PM EST Mon Dec 19 2022

Valid 00Z Tue Dec 20 2022 - 00Z Fri Dec 23 2022

...Arctic cold front pushes down the Plains with Blizzard
conditions for portions of the Plains and Midwest...

...Pacific Northwest, Northern Rockies, and Northern High Plains...
Days 1-2...

An upper trough currently over the Gulf of Alaska will wrap around
a deep upper low currently centered over the Canadian Rockies
through Tuesday before shifting southeast down the northern
Rockies and northern High Plains Tuesday night and Wednesday. A
plume of Pacific moisture ahead of the Gulf of Alaska wave will
shift across WA and the northern ID/MT Rockies tonight before
shifting south to northern OR and southern ID/MT Rockies.

Snow levels near sea level and moderate to locally heavy precip
rates along a nearly stationary arctic cold front look to cause
notable accumulations across Washington tonight including the
Seattle metro area along with enhanced snowfall in mountains.
Moderately high Day 1 snow probabilities for over 4 inches are
across the Puget Sound area with high probabilities for a foot or
more for the Olympics and WA Cascades with moderate probabilities
for over 8 inches in the northern ID/MT Rockies including the
Bitterroots.

The favorable upper forcing and strong low-to-mid level
frontogenesis will support heavy mountain snow spreading east from
the Pacific Northwest into the northern and north-central Rockies
Tuesday through Wednesday. Day 2 snow probabilities for more than
8 inches are high again for the Bitterroots as well as the
Absarokas, Tetons, and Wind River Range in WY with 48hr totals in
both these mountainous areas having high probabilities for over 18
inches.

In addition, as the upper trough begins its push southeast down
the northern Rockies Tuesday night the advancing arctic cold front
looks to be accompanied by lines of heavy snow and likely snow
squalls that spread over ID/WY Tuesday night/Wednesday and
possibly northern Colorado late Wednesday. Please see Key messages
below.

By Tuesday afternoon, favorable upper divergence associated with a
left-exit region of a strong NWly 250mb jet streak coming in from
the North Pacific will help spread overrunning precipitation into
the northern High Plains of Montana. Surface temperatures will be
extremely cold and well below zero during this timeframe, which
may actually temper SLRs from exceeding 20 to 1. Day 1.5 snow
probabilities for 6 inches or more are high for much of
west-central Montana mountains and valleys with the Big Belt
Mountains forming much of the eastern boundary.


...Northern and Central Plains through the Midwest/Great Lakes...
Days 2-3...

The aforementioned upper trough currently over the Gulf of Alaska
that pushes down the northern Rockies/northern High Plains Tuesday
night with an arctic cold front pushing south into the central
Plains Wednesday into early Thursday will have impressive fgen,
enhanced upper level jet support, and very cold air. This will
allow for development of widespread moderate snowfall amounts
Wednesday over the eastern Dakotas that then spreads south through
Nebraska and east through much of Minnesota and northwest Iowa.
Day 2 snow probabilities for more than four inches are centered
near the Buffalo Ridge of southwest MN.

Lee-side cyclogenesis ahead of the arctic cold front dips south to
the TX Panhandle late Wednesday before shooting east to the MO/AR
border by early Thursday then rapidly intensifying as it turns
northeast up the Midwest through Thursday. This will allow for
further expansion of the snow areas along with development of
heavy snow bands. This is seen in the Day 3 snow probabilities
which for 4 or more inches are moderate or higher from
west-central KS to central MO and northeast to the western Great
Lakes. Embedded are moderate probabilities for 8 or more inches
from southeast Neb, northwest MO through central IA to southeast
MN.

The presence of an existing wave moving along the Gulf Coast today
through Tuesday will limit the amount of Gulf moisture available
to the mid to late week system. Gulf moisture begins to stream
ahead of the system starting on Wednesday which will allow further
development of heavy snow bands for Thursday. Enhanced lift within
a deep DGZ for much of the central Plains and into the Upper
Midwest will support very efficient SLRs to make up for the lack
of initial moisture, along with surface temperatures around zero
behind the arctic cold front. High SLRs above 20 to 1 may be
short-lived for much of the central High Plains, as strong low to
mid-level winds above 50 kts fracture the falling dendrites.
However, the gusty winds (and extremely low wind chills) will add
to the potential for dangerous travel as visibility decreases and
blowing snow ensues. This system will continue to progress
eastward and become a large and powerful storm set to impact much
of the Midwest and Great Lakes with potentially heavy snow through
late this week and into the Holiday Weekend. Of particular note is
the slow motion which will allow for long duration of heavy snow
bands, particularly over the central Great Lakes.


...Southern/Central Appalachians...
Day 3...
In response to the deepening trough over the central U.S. on
Wednesday, moisture return along the East Coast will enter the
southern Mid-Atlantic and ride up along the southern and central
Appalachians Wednesday night into Thursday morning. A strong
surface high depicted by all guidance over New England during this
time frame will support cold air damming into Thursday morning and
the at least localized freezing rain over mainly the higher
terrain west from the Blue Ridge Mountains. Day 3 WPC
probabilities for at least a tenth of an inch of ice accretion are
moderate across western NC, southwest Virginia, eastern WV, into
west-central PA. The threat of freezing rain is expected to
persist into Thursday night until the arctic cold front blows
through and changes to accumulating snow.



Key Messages for Blizzard
--Significant Blizzard Developing this Week
An area of low pressure will develop Wednesday night and then
strengthen while lifting northeast into the Great Lakes by Friday
morning. This will result in blizzard conditions across portions
of the Central/Northern Plains, Upper Midwest, and Great Lakes.

--Extremely Dangerous Travel Conditions
Heavy snowfall rates of 1-2 inches per hour at times, along with
wind gusts of over 50 mph will result in near zero visibility and
considerable blowing and drifting of snow. This will lead to
dangerous, to at times impossible, land and air travel leading up
to the holiday weekend.

--Potential for Impacts to Infrastructure
The combination of heavy snow and strong wind gusts could lead to
significant infrastructure impacts as well, including scattered
tree damage and power outages.

--Dangerous Cold along with the Blizzard
Dangerous wind chills will accompany the blizzard, creating a
significant hazard for anyone that becomes stranded. Prepare now
for extreme cold and ensure outdoor animals and livestock have
sufficient shelter.

--Flood Threat for the Interior Northeast
Farther east, heavy rain falling onto a melting snowpack will
cause flooding concerns across the Northeast.


Key Messages for Cold Front and Snow Squalls
--Dangerous Cold for Much of Country
A combination of frigid temperatures and gusty winds behind an
Arctic cold front will bring dangerously cold conditions across
most of the country this week, and to the Eastern U.S. by week..s
end.

--Rapid Temperature Drops with Front
Arctic air will arrive quickly behind the cold front, with
temperatures likely to drop 25 to 35 degrees in just a few hours
as the front passes a given location.

--Hazardous Snow Squalls in the West
Snow squalls may lead to hazardous travel in the Western U.S. from
Tuesday Night into Wednesday. Snow rates over 1 inch per hour and
gusty winds may lead to sudden whiteout conditions.

--Flash Freeze Possible Farther East
From the Mid-South to the East Coast, rain before the Arctic front
could freeze in the rapidly falling temperatures and lead to icy
roads and hazardous travel.



Jackson


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