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

Valid 00Z Wed Dec 21 2022 - 00Z Sat Dec 24 2022

...Pacific Northwest & Northern to Central Rockies...
Days 1-2 & Day 3...

An active period of weather will continue across the Pacific
Northwest and into the Rockies as mid-level low pressure in the
Gulf of Alaska and a high pressure ridge near Hawaii combine to
squeeze the flow and drive repeated shortwaves with enhanced
moisture onshore. This is reflected in the synoptic pattern as a
series of shortwaves moving across the Pacific and into the area
within a region of confluent mid-level flow, and also via the CW3E
atmospheric river (AR) forecasts indicating multiple waves of IVT
exceeding 250 kg/m/s. With snow levels very low, at least D1 and
D2, this will result in heavy snowfall from the Cascades through
the Central Rockies, while a second piece of energy for D3 brings
snow and potentially significant freezing rain to the Northwest.

A lobe of vorticity moving across WA state this evening will
stretch southeastward into the Central Rockies by Wednesday
evening as it sheds around a stretched closed low centered over
Saskatchewan. This will be accompanied by the LFQ of a 170kt
Pacific jet streak arcing southeastward out of British Columbia,
the result of which will be large scale deep layer ascent moving
across the region. A strong arctic high north of these features
will dive southward in the wake, with a potent cold front sinking
southward rapidly. The overlap of enhanced convergence along the
front with impressive synoptic lift and periodic upslope flow
around the approaching high pressure will result in periods of
heavy snow from the Cascades through the Northern Rockies, into
the NW WY ranges, and as far south as the CO Rockies including the
Park Range by D2. The airmass behind this front is extremely cold
so SLRs will be quite high, nearing 20:1, while low-level
instability and fgen will help drive potential embedded snow
squalls with rates of 1-2"/hr at times. While snow squalls will
likely be impactful where they occur, accumulations associated
with the squalls should be minimal. However, in the terrain,
generally above 2000-3000 ft which is the forecast snow level
ahead of the front, accumulations exceeding 6 inches have a
greater than 80% chance of occurring according to WPC
probabilities in the Bitterroots, Sawtooth, Northern Rockies, Blue
Mountains, and NW WY ranges on D1, shifting into the Uintas and CO
Rockies D2. Storm total snowfall will likely exceed 1 foot in many
areas of these ranges. Snow levels crash to the surface behind the
front, but the DGZ dries out rapidly during this time. Still,
light accumulations of around 1 inch are possible for most of the
area down to the valley floors through D2.

After a brief period of shortwave ridging behind the leading
trough for D1-2, another shortwave will advect onshore OR/WA
Friday morning, with precipitation spreading inland ahead of it
Thursday evening into Friday morning, accompanied by yet another
but less impressive Pacific jet streak. This will again produce
deep layer ascent. However, WAA will be impressive with this
second batch of precipitation, so a warm nose is likely to surge
northward changing p-type, which will initially be snow, to sleet,
freezing rain, and eventually rain outside of the higher terrain.
The surface air ahead of this system is quite cold and dry, and
will likely drain out of the Columbia basin to the west,
reinforcing cold air against the Cascades and through the Columbia
Gorge, with additional cold air locked into the surrounding
lowlands. This setup will support a period of heavy snow initially
which could accumulate even in the lowlands D3, but the heaviest
accumulations are likely in the Cascades. More significantly for
the lowlands, Gorge, and valleys, an extended period of freezing
rain is likely beginning late D2 and continuing through much of
D3. Although precip rates could be heavy at times, strong easterly
winds should help maintain accretion efficiency, and as precip
changes from snow to sleet to freezing rain, heavy accretions are
becoming likely, especially in the western Columbia Gorge and
through the coastal lowlands/ranges of OR/WA. WPC probabilities
for 0.1" of accretion are above 50% in this area, and it is
possible damaging freezing rain accretions exceeding 0.5" could
occur in some places.

...Southern/Central Appalachians...
Days 2-3...

Strong Canadian high pressure wedging down the east coast will
bank into the Appalachians and provide cold/dry surface air as
warm air begins to overrun the region downstream of an amplified
trough digging across the Central Plains and into the Ohio Valley.
This trough will become increasingly negatively tilted and close
off, driving strong downstream divergence collocated with robust
upper diffluence on the LFQ of a digging upper jet streak. A wave
of low pressure moving near the Great Lakes will advect an arctic
cold front eastward through Friday, downstream of which robust WAA
will spread moisture northward. This will precipitate initially as
snow across the higher terrain of the Appalachians from SW VA
through WV and into central PA, where a few light inches of
accumulation is possible in the highest terrain. However, the WAA
will spread a warm nose of +1C to +2C northward, causing snow to
change rapidly to IP and ZR. This will result in some modest
accretion of freezing rain which WPC probabilities indicate have a
20-40% chance of exceeding 0.1" from far NW NC through the Blue
Ridge of VA and into the Panhandle of MD and PA Laurel Highlands.
Locally, 0.25" of accretion are possible where the coldest air
holds on the longest.


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

...Significant blizzard with extremely cold temperatures and a
flash freeze likely...

An elongated closed mid-level low near Saskatchewan will dig
southward beginning Wednesday night and amplify into a short
wavelength but high amplitude negatively tilting trough, before
closing off across the Upper MS VLY Thursday night. This feature
will continue to amplify into a strong closed low with 500-700mb
heights falling to -4 standard deviations according to the NAEFS
ensemble tables. As this evolution occurs, coupled jet streaks
will develop both upstream and downstream of the primary trough
axis, with the trailing jet streak peaking above 150 kts. The
overlap of the strong mid-level low and coupled jet streaks with
accompanying robust diffluence will drive impressive omega through
a deep layer, resulting in a surface low pressure tracking from
Nebraska, to the Ohio Valley, and occluding strongly into the
eastern Great Lakes/southeast Canada on Friday. Impressive warm
and moist advection ahead of this system will spread significant
moisture northward from the Gulf of Mexico, and although the
system will be progressive into Friday, it is likely to slow as it
occludes D3, with isentropic upglide coalescing into a moderate
TROWAL to enhance snowfall across the Great Lakes.

With exceptionally cold air funneling in from Canada behind the
associated arctic front, SLRs will be quite high, around 20:1 in
many areas, so even as forcing remains transient through the
middle part of the country, a period of heavy snow is likely due
to the rapid accumulation that can occur with such fluffy SLRs.
Additionally, as the low deepens and the pressure gradient
maximizes between the low and the approaching strong surface high,
blizzard conditions will likely develop even where snowfall is
modest, producing widespread notable impacts despite snowfall
totals exceeding 4 inches only east of the Dakotas. For D1, WPC
probabilities for more than 4 inches are above 40% only across the
Coteau des Prairies of SD/MN through central MN, and increase D1.5
and into the Bayfields Peninsula of WI. An axis of heavier snow of
more than 6 inches is likely just NW of Minneapolis and into
northern WI/western U.P. of MI where moisture convergence occurs
along an inverted trough. For D2, the heaviest snowfall shifts
from central MO through both the L.P. and U.P. of MI where WPC
probabilities exceed 20-40%, with more than 6 inches likely in
parts of WI and MI. However, by D3, as the low occludes and slows
down, much heavier snowfall is likely, especially across the U.P.
and western L.P. of MI where wraparound/deformation snowfall will
be persistent, and enhanced by lake effect snowfall. WPC
probabilities for more than 4 inches are high across northern IN
and much of MI, with locally more than 12 inches likely in the
favored lake belts along the northern U.P. and far SW L.P. near
Grand Rapids. This combined with snow and strong winds may result
in blizzard like conditions for parts of Friday, although the most
significant blizzard conditions should be across the
Plains/Midwest due to the strong winds despite lesser snow
amounts. Travel later this week will be extremely difficult across
this part of the country.

Significant snowfall exceeding 4 inches is also possible on D3
downwind of Lake Erie and Ontario as westerly flow develops behind
the front.

Additionally, the potent arctic front will be racing eastward
through the period, causing extreme temperature drops of as much
as 20-30 degrees in just 6 hours, and allowing rain to change to
snow across the MS VLY, TN VLY, OH VLY, and into the Mid-Atlantic
and Northeast. While cold air chasing moisture is usually not a
favorable setup for accumulating snowfall, the rapidity at which
temperatures fall should allow the DGZ to remain saturated, at
least for a short time, with anafrontal precip persisting. This
could result in a quick burst of heavy snowfall just behind this
front, with a flash freeze also likely. Deteriorating travel
conditions are possible during this evolution, for the MS-TN-OH
Valleys on Thursday, and into the Mid-Atlantic and Northeast on
Friday.


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 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.

Weiss