Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Tue Dec 20 2022

Valid 12Z Tue Dec 20 2022 - 12Z Fri Dec 23 2022

...Arctic cold front surges south down the Plains as a powerful
winter storm produces blizzard conditions for portions of the
Plains and Midwest...

...Pacific Northwest & Northern to Central Rockies...
Days 1-2...

An upper level shortwave trough positioned along the British
Columbia coast will tap into Pacific moisture and direct it
towards the Pacific Northwest and the Northern Rockies on Tuesday.
Latest guidance shows a conveyor belt of 700mb moisture flux
traversing these mountain ranges, while simultaneously, the
westerly mean flow component in the 850-300mb layer will support
strong orographically-aided vertical ascent, leading to heavier
snowfall rates in places like the Cascades, Bitterroots, and Lewis
Range. As the upper trough approaches, 500mb PVA will be maximized
just east of the trough axis, maximizing the best vertical
velocities at the upper levels of the atmosphere across these
regions on Tuesday. This setup is ripe for heavy snow, considering
temperatures are so bitterly cold across the region and more than
cold enough to support higher SLRs. Latest WPC PWPF contains high
probabilities (>70%) for snowfall totals >12" in the higher
terrain of the Cascades, Bitterroots, and Lewis Range.

As the trough and Arctic front push southeast Tuesday night into
Wednesday, the axis of best 700mb moisture flux moves into the
Sawtooth, Absaroka, Teton, Wind River, Wasatch, and northern
Colorado Rockies. This is also where an intense 160kt 250mb jet
streak's divergent left-exit region is likely to setup, further
providing additional synoptic-scale lift in an area already seeing
upslope flow from strong easterly surface winds from the
protruding Arctic high pressure to the north. 2-day totals
snowfall probabilities show 70-90% probabilities for >18" of
snowfall in the Tetons and Absaroka ranges of western Wyoming.
Many of these ranges feature "Moderate to Major" impacts according
to WPC's WSSI, indicating travel along these mountain ranges and
within their passes are likely to be very difficult, to even
impossible in some select areas.

Meanwhile, this stream of moisture is passing north of a strong
Arctic front, which is supplying bitterly cold temperatures not
only to the mountainous terrain, but valleys and coastal areas as
well. Latest WPC PWPF features 20-40% chances for snowfall totals
>4" along the Puget Sound and including the Seattle metro.

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

By 18Z Wednesday, the NW to SE oriented 250mb jet, which in terms
of wind speed is on the order of 3-4 standard deviations above
normal, will have its divergent left-exit region positioned over
the northern and central High Plains. An increase in 850mb
moisture flux over the southern High Plains will be directed north
towards the Arctic front surging south. Combined with a
strengthening wave of low pressure near the TX/OK Panhandles, this
will allow for precipitation to breakout along and north of the
Arctic front from central Kansas and Nebraska to the Upper Midwest
as it intersects the approaching frontal boundary. There will be
an initial 850mb low tracking from South Dakota to Minnesota on
Wednesday that, through 850mb WAA and frontogenesis, will foster
periods of snow across these areas on Wednesday. Latest WPC PWPF
does depict 40-50% probabilities for >6" of snowfall in
south-central Minnesota, then rising to 50-60% as the best WAA
along the front lifts into northern Wisconsin and the U.P. of
Michigan Wednesday night. As the Arctic front races south,
lingering 700mb moisture flux aloft will be oriented out of the
Southwest behind it, leading to an anafrontal setup. This means
precipitation will still fall in wake of the frontal passage
initially, and with crashing temperatures quickly falling below
freezing, it will allow for periods of snow (heavy in some cases)
to develop behind the front. WPC PWPF still shows moderate chances
(40-60%) for >6" of snowfall trough Wednesday night in central
Iowa.

By Thursday, the 200-300mb layer is beginning to take on a
negative tilt over the Middle Mississippi Valley towards the
evening hours. As the wave of low pressure steadily strengthens
beneath the divergent left-exit region of the 250mb jet streak
over the central High Plains, a second jet streak is set to form
over New England, placing its divergent right-entrance region over
the Ohio Valley. The upper low in the Northern Plains will
continue to deepen as it tracks into the Middle Mississippi Valley
with a surge in 850mb moisture flux originating out of the Gulf of
Mexico and off the East Coast. As the 850-700mb low forms, the
plume of moisture resulting in periods of snow through out the
Upper Mississippi Valley will become directed more to the
northwest flank of the low, where the most intense vertical
motions will set up. The final result is periods of heavy snow,
rates sometimes 2-3"/hr, where the TROWAL sets up on the storm's
western flank. Meanwhile, strong N-E winds at 850mb racing over
Lake Superior and Lake Michigan will become enhanced further from
lake effect processes. Latest WPC PWPF shows an expansive area of
>4" snowfall probabilities >70% across much of the Upper
Mississippi Valley and into the Upper Great Lakes through Friday
morning. When it comes to >12" snowfall totals, the areas just
inland off the coast in Michigan's U.P. feature 30-40%
probabilities.

Winds in wake of the frontal passage will be strong and
potentially destructive, gusting as high as 45-60 mph. This
combination of snow and whipping wind gusts means whiteout and
blizzard conditions within these areas of snowfall are possible,
making for a dangerous combination for travel on both the ground
and by air. The latest WPC WSSI currently features "Moderate"
impacts from eastern Kansas on north and east to southern
Minnesota and southwest Wisconsin. There still remains some
uncertainty in storm track, which could result in adjusting
snowfall probabilities. However, whether it be 4" or 8", the snow
does fall during a period of time where winds are exceptionally
strong, making near zero visibility and tall snow drifts hazardous
to travelers leading up to the holiday weekend.

...Southern/Central Appalachians...
Day 3...
In response to the strengthening trough over the central U.S.
Wednesday night, a fetch of low level moisture from the Atlantic
ocean will enter the southern Mid-Atlantic and ride north along
the southern and central Appalachians late Wednesday night into
Thursday morning. A strong surface high depicted by all guidance
over New England will lead to a cold air damming setup Thursday
morning from the Smokey and Blue Ridge Mountains of western North
Carolina on north through the Blue Ridge of Virginia, the central
Appalachians, and Laurel Highlands. Day 3 WPC probabilities for at
least a tenth of an inch of ice accretion range between 20-40%
across these regions, with southwest Virginia featuring the
highest probabilities (50-70%). The threat of freezing rain is
expected to persist into Thursday night until the arctic cold
front blows through and changes to accumulating snow. Should
totals begin to approach or surpass a quarter inch, there could be
an increased threat for possible tree damage and power outages.

...Pacific Northwest...
Day 3...

A Pacific storm system approaching the region will direct its
moisture into the Pacific Northwest at a time where bitterly cold
subfreezing temperatures are entrenched along the I-95 corridor
from Seattle on south into the Willamette Valley. NAEFS 1000mb
temperatures are shown to be as cold as 3-4 standard deviation
below normal at the same time as an influx of 850-700mb moisture
overruns the wedged subfreezing air at the surface. The warm nose
aloft will lead to a wintry mix of sleet and freezing rain,
although at this range, it is still unclear how much the
precipitation falls in either form. Currently, WPC PWPF shows
moderate chances (40-60%) of freezing rain accretion >0.1" from
southwest Washington into northwest Oregon. This slug of rich
Pacific moisture will continue to funnel into the Pacific
Northwest through the day on Friday, resulting in a potentially
prolonged period of freezing rain. Should confidence rise in
freezing rain being the primary precipitation type, more
significant ice accretion totals (>0.25") could occur in these
areas and result in more detrimental impacts in the impacted
region.


Key Messages for Blizzard
--Significant Blizzard Developing this Week
A storm system 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 travelers that become 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 Friday.


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 weekâ€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.


Mullinax