Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Wed Dec 21 2022

Valid 12Z Wed Dec 21 2022 - 12Z Sat Dec 24 2022

...Central Rockies...
Day 1...

Falling heights courtesy of an amplifying 500mb trough over the
Northern Rockies will be accompanied by strong vertical motion
atop the atmosphere as the left exit region as an intense 160 knot
jet streak positions itself over the Central Rockies. Atmospheric
moisture will come in the form of residual 700mb moisture flux
originating from the Pacific. The westerly mean flow at the 700mb
level will aid in topographically-induced upslope flow, causing
heavier snowfall rates to ensue in places like northern Utah and
western Wyoming. In lee of the Rockies, vertical motion at lower
levels is enhanced by the surging Arctic front with N-NE surface
winds prompting an upsloping response along the Continental
Divide. The quick moving front will force snowfall to be confined
to one more day of snowfall, but given the moisture available and
the remarkably frigid temperatures associated with the front,
periods of heavy snow are likely in the Wasatch, Tetons, Wind
River, and northern Colorado Ranges. WPC PWPF shows high
probabilities (70-90%) for >6" of snowfall in the 70-90% range,
and moderate chances (40-60%) for >8" of snow. The WPC WSSI shows
does depict "Moderate" to even "Major" impacts in these ranges,
largely driven by the blowing snow and ground blizzard criteria.
This suggests the strong winds associated with the snowfall being
the most hazardous impacts that could lead to snow drifts and near
zero visibility along these mountain ranges.

...Southern/Central Appalachians...
Days 1-2...

A dome of Canadian high pressure Wednesday night will entrench
itself along and east of the Appalachians, leading to a classic
"cold air damming" setup across much of the Northeast and
Mid-Atlantic. Meanwhile, a shortwave trough outrunning the more
intense trough diving into the Nation's Heartland, and a
strengthening 110 knot jet streak over the Southeast, will place
favorable areas for divergent flow aloft over the Mid-Atlantic and
both the southern and central Appalachians. A frontal boundary
along the Southeast coast will lift northward Thursday morning in
response to the evolving upper level pattern and steady 290K
isentropic glide will ensue as southerly 850mb winds are sloped
vertically into the Mid-Atlantic region. The end result is periods
of precipitation over these regions, but it will be areas along
and west of the Blue Ridge with the best odds of wintry weather.
As the warm nose gets above freezing within the 900-750mb layer
early Thursday morning, surface temperatures will remain below
freezing from western North Carolina on north through the Central
Appalachians and the Shenandoah Valley. It is here where
precipitation starts off briefly as snow, before changing over to
an icy sleet/freezing rain mix. Eventually, the warm nose becomes
too large and freezing rain becomes the primary precipitation
type. Latest WPC PWPF shows an expansive area of 40-60%
probabilities for >0.1" of ice accretion with the highest end of
those probabilities in southwest Virginia around the Roanoke areas
and along that section of I-81. Farther north, areas like Garrett
County, MD and its neighboring WV counties to the south also
feature similar probabilities. Given their position farther north
with cold air likely to stay in place longer, it is here where
some localized ice accretion >0.25" are possible, along with the
potential for a couple inches of snow before the changeover to
freezing rain late Thursday morning.

...Northern Great Plains through the Interior Northeast...
Days 1-3...

...Powerful blizzard with extremely cold temperatures, dangerously
frigid wind chills, and a flash freeze likely...

A powerful and likely historic winter storm is set to unfold
across the Midwest and the Great Lakes leading up to the holiday
weekend. This storm system's story will not be told fully by just
snowfall totals, but the by the combination of intense wind gusts
and dangerously cold temperatures that are associated with the
storm. Initially, 850mb WAA over the Midwest gives rise to periods
of snow from the Northern Plains to the Upper Mississippi Valley
as temperatures are well below freezing in wake of the Arctic
frontal passage. Latest WPC PWPF shows moderate probabilities
(40-60%) of snowfall >6" over central Minnesota on Wednesday.
Speaking of the front, it will race south and east through the
Plains Wednesday evening and pass through the Middle Mississippi
Valley Thursday morning. Snow will fall in wake of the frontal
passage, otherwise known as an anafrontal precipitation shield,
thanks to the divergent left exit region of a roaring 180 knot
250mb jet streak aloft and residual 700mb moisture flux lagging
behind the front. While it is snowing, winds are gusting 30-50mph
thanks to the tight pressure gradient generated from lower
pressure along in the East, and an impressive 1060mb Arctic high
pressure engulfs the Northwest and northern High Plains (above the
99th climatological percentile and approaching monthly records in
parts of the Northwest on Thursday). The swath of snow will move
along across the Central Plains and the Middle Mississippi Valley
with most snowfall accumulations coming in below 4". That said,
the combination of intense wind gusts and increasing SLRs
(15-20:1) in wake of the front will allow for bursts of heavy,
wind driven snow that cause near whiteout and blizzard conditions
in these areas on Thursday. 

As the trough amplifies, the 500mb low will dive south through the
Midwest, reaching minimum heights that are among the lowest
observed on record for late December. The trough will tap into
both Gulf of Mexico and Atlantic moisture and direct them north
across the eastern U.S.. As the 850mb low forms over southern
Michigan Thursday night, northerly winds will begin to strengthen
over Lakes Superior and Michigan. The combination of 15-20:1 SLRs
and additional lake enhancement will make these areas (U.P. of
Michigan, northwest Michigan). These are the areas with the best
odds of seeing the most prolific snowfall rates and heaviest
totals for the event, on top of the expected 40-50+ mph wind
gusts. Latest WPC 72-hr PWPF shows >18" snowfall probabilities up
to an impressive 70-80% there. It is also worth noting these high
of probabilities exist for >12" of snow on south into southwest
Michigan. As the low lifts into Lake Huron on Friday morning,
850mb winds out of the WSW will be an astonishing 70 knots, not
only leading to a potentially significant and damaging wind event
off Lakes Erie and Ontario, but also intense lake effect snow
bands that would produce 2-3"/hr snowfall rates. The same can be
said for those in the U.P. of Michigan and northwest Michigan. The
wind impacts can not be understated, as not only will these lead
to prolonged blizzard conditions (resulting in dangerous and
impossible travel conditions) but could also mean downed trees and
power lines at a time when bitterly cold temperatures will stick
around well into the holiday weekend. Those in the most heavily
impacted areas of the Great Lakes should ensure they have plan
accordingly for a long duration heavy snow event from Friday into
Christmas Eve.

Farther south, the fast moving Arctic front will prompt periods of
rain to occur within the warm sector, but once the front passes
through, temperatures will plummet to subfreezing levels late
Thursday into Friday. This allows for precipitation to change over
to a combination of graupel/rain, then eventually all snow as the
atmospheric column quickly cools below freezing. Upslope flow into
the central Appalachians could lead to some receiving localized
totals >4" (WPC probabilities 10-20% in northern WV and western
PA). Farther north where the cold air is still wedged in from the
cold air damming setup over the interior Northeast, snow will fall
heavily in the parts of the Adirondacks, Finger Lakes area of
central New York, and into the Berkshires and Green Mountains.
Probabilities for >6" of snow are highest in the Adirondacks where
there is a moderate chance (40-60%) for those totals Thursday
night into early Friday. Meanwhile, the front will make its way
east through the Mid-Atlantic Friday morning where a brief burst
of heavy snow could result in significantly reduced visibility
along the I-95 corridor from central Virginia to southern New
England. Accumulations would likely be very light, but there is
also the potential for a flash freeze in these areas east of the
Appalachians. Blustery winds will help to evaporate much of the
rainfall that fell on Thursday, but any standing areas of water
could freeze Friday evening and lead to slick spots of sidewalks
and roads Friday night into Christmas Eve morning.

With such a widespread extent of strong wind gusts, accumulating
snowfall, and bitterly cold temperatures, this will undoubtedly
lead to numerous and widespread travel delays and cancellations
Thursday, Friday, and likely lingering into Christmas Eve. With
such a busy time for travel across the country, those traveling in
the eastern half of the continental U.S. should prepare
accordingly for cancellations and take caution when driving
leading up to the Christmas weekend.

...Pacific Northwest...
Days 2-3...

A warm front associated with a storm system in the northeast
Pacific will advance east towards the Pacific Northwest ushering
in a tongue of 850mb moisture flux to the region Thursday evening.
The warm nose aloft will quickly get above freezing within the
900-750mb layer, but temperatures below 900mb in the Columbia
River Gorge, Portland metro, the Willamette Valley, and along the
lee side of the Coastal Range  will contain subfreezing
temperatures and very dry dew points, making wet bulb temperatures
exceptionally cold. This is a classic set up for sleet and
freezing rain, as anomalous high pressure in the Columbia River
Basin enacts its own form of cold air damming in the valleys of
western Oregon and Washington Thursday night into Friday. The
850mb moisture transport is not strong, but persistent, which will
allow for precipitation rates to be lighter. This however can be a
more efficient means of ice accretion, and thus the expectation is
for significant ice impacts in the region. Latest WPC PWPF depicts
moderate (40-60%) probabilities for >0.25" of ice in the
aforementioned areas above, as well as some lower elevations of
the Cascade Range. The Cascade Range, in fact, features 48 hour
probabilities of 10-20% for >0.50" of ice accretion. Ice accretion
this high would support possible tree damage and downed power
lines. There is also lower chance probabilities (10-30%) for
>0.25" ice accretion as far north as Olympia, Tacoma, and the
southern Seattle metro. Latest PWSSI does depict a heightened risk
(60% odds) of "Moderate" impacts in the metro areas along I-5 from
Seattle to Portland.


Key Messages for Blizzard
--Significant Blizzard Developing this Week
A powerful 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
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.

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

--Life-threatening Cold
Dangerous wind chills will create a significant hazard for
travelers that become stranded. Prepare now for extreme cold and
ensure outdoor animals and livestock have sufficient shelter.

--Flood and Wind Threat for the Northeast
Farther east, heavy rain falling onto a melting snowpack will
cause flooding concerns Friday, while wind gusts of 50+ mph could
lead to power outages and tree damage.


Key Messages for Cold Front and associated Snow Squalls/Flash
Freeze
--Dangerous Cold for Much of Country
Frigid temperatures and gusty winds behind a powerful 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.

--Increased Impact if Power Outages Occur
The impact of the cold will be increased if any power outages
occur due to the winter storm.

--Rapid Temperature Drops with Front
Temperatures are likely to rapidly drop 20 to 35 degrees in just a
few hours as the front passes a given location.

--Considerable Travel Impacts Possible
Snow squalls, or a brief burst of moderate to heavy snow (1 to 2
hours), are likely to occur immediately behind the Arctic cold
front from the Intermountain West, to the Central Plains, Ohio
Valley, Mid Atlantic, and Northeast.

 -Snow will be accompanied by gusts to 40 MPH, potentially
creating sudden whiteouts.
 -Snow will coincide with rapid temperature drops, which will
likely lead to flash freezing and icy spots developing on
roadways. Any flooding in the Northeast may also re-freeze.

Mullinax