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

Valid 00Z Thu Dec 22 2022 - 00Z Sun Dec 25 2022

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

Strong mid-level height falls beneath a digging 160kt jet out of
western Canada will promote broad lift over the Colorado Rockies
tonight into Thursday. At the surface, a strong arctic front will
race down the Front Range with sharply dropping temperatures and
strong northerly flow. The front will be slower to move through
the higher terrain west of Denver where the northern edge of a
Pacific moisture plume will enhance topographically-induced
upslope flow over parts of the Wasatch/Uintas and especially into
the Flat Tops, Gore Range, and Sawatch. WPC probabilities of at
least 6 inches of snow are above 50% in the central Colorado
ranges with much lighter amounts east of I-25.

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

A mid-level shortwave moving through the Mid-South tonight will
tap a surface boundary off the Georgia coast and lift
northeastward on Thursday, bringing a surge of moisture with it.
In advance of the strong/digging jet over the Plains, the eastern
half of the longwave trough will amplify in tandem, sharpening the
divergence aloft and promoting lift across the Mid-Atlantic into
the Northeast. SSE flow will bring in milder temperatures and
above normal moisture to the region on 40-55kt 850mb flow which
will lift up and over the colder surface boundary layer. This will
yield an area of freezing rain and some sleet over the central
Appalachians from Virginia through western MD and into south
central PA. High pressure over Atlantic Canada will attempt to
hold in colder air at the surface but this will eventually be
eroded as low pressure from the south moves northward along I-95
in VA and I-81 in PA in advance of the arctic front to the west.
WPC probabilities for at least 0.10" ice are moderate (>40%) to
high (>70%) along I-81 in VA and through eastern WV into far
western MD. Some areas may receive more than 0.25" ice. Where
temperatures are colder through the column, several inches of snow
are possible in portions of the MD/VA panhandles into southern PA,
though probabilities of at least 4" are low.

Fracasso

...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. 850mb WAA on D1 shifts over the Upper Midwest and into the
Great Lakes producing a swath of snow as temperatures are well
below freezing in wake of the Arctic frontal passage. Latest WPC
PWPF shows moderate probabilities (30-60%) of snowfall >6" from
northeast Iowa through the U.P. of MI on Thursday. Speaking of the
front, it will race south and east through the Plains tonight and
pass through the Middle Mississippi Valley and into the Ohio
Valley during Thursday. Snow will fall in wake of the frontal
passage, thanks to the divergent LFQ of a 170 knot 250mb jet
streak aloft and residual 700mb moisture flux lagging behind the
front. While this anafrontal snow will likely accumulate only
modestly, brief snowfall rates of 1+"/hr and strong winds could
result in lowered visibility and challenging travel.

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 fluffy SLRs
which could approach 20:1, limited only by the likelihood of
dendritic fracturing due to strong winds beneath the DGZ, and
additional lake enhancement will make these areas (U.P. of
Michigan, northwest Michigan) a focus for the heaviest snow, aided
additionally by the pivoting deformation that may occur W/NW of
the surface low. These are the areas with the best odds of seeing
the most prolific snowfall rates and heaviest totals for the
event, reflected by WPC probabilities that are 70+% for 8 inches,
and it is likely that some areas will receive more than 2 feet of
snow by the time the storm winds down. 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.

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. While cold air
chasing moisture is rarely a setup for snow accumulation, the
rapidity at which the column cools enables the DGZ to remain
saturated even as temps fall to below freezing near the surface.
This combined with dynamic cooling through impressive ascent could
result in light accumulations anywhere along the front, but with a
period of difficult driving likely as heavy snow rates fall and
help produce a flash freeze potential. 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 and
NH White Mountains where there is a moderate chance 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.

Mullinax/Weiss


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

A shortwave moving onshore the Pacific Northwest coast Friday will
be preceded by a warm front and accompanying low/mid level WAA to
spread moisture onshore. Moisture will funnel inland within a
modest atmospheric river progged by CW3E to exceed 250 kg/m/s.
While the column initially will be cold enough to support snow
across most of the Pacific Northwest noted by NBM snow levels at
the surface, warm air nosing in from the Pacific at 850mb will
slowly melt snowflakes aloft to cause a change in p-type. The
airmass ahead of this system is quite cold and dry noted by dew
points in the -10s, which will support rapid wet bulb effects as
precip falls beginning Thursday night. While a burst of heavy snow
is likely in most areas to start, the warm nose aloft will cause a
rapid transition to IP and then ZR. The models today have trended
a bit more robust with the cold layer depth which could support a
longer duration of IP before switching to ZR. Nevertheless, light
to moderate precip rates which generally feature efficient
accretion will be enhanced by robust easterly winds, especially
along and through the Columbia Gorge and into the Willamette
Valley and surrounding lowlands. This could result in extremely
efficient ice accretion as reflected by conceptual models, and the
WPC probabilities for freezing rain have increased this aftn. The
chance for more than 0.25" is 50-70% on D2 for the Coastal Ranges
of OR and points northeast through the Columbia Gorge and the
lowlands of SW Washington, with these probabilities extending
eastward through the Gorge and into the western Columbia Basin on
D3. Both Portland, OR and Seattle, WA could receive significant
ice accretion, and locally more than 0.5" is possible near the
Gorge. This will result in extremely dangerous travel for the
Friday before the holiday weekend. Farther to the north, heavy
snow will likely remain the primary p-type across the WA Cascades
where WPC probabilities for more than 8 inches are above 50%,
including for Stevens Pass.

Weiss


Key Messages for Blizzard
--Widespread Very Gusty, and Potentially Damaging, Winds
A powerful winter storm system will bring widespread wind gusts
over 40 MPH to a large portion of the Central and Eastern U.S.
over the next few days. In some spots, the wind gusts could
approach or exceed 60 MPH. This could lead to damage and power
outages.

--Blizzard Conditions Where Wind Combines with Snow
The winter storm will produce new snow primarily in the Midwest,
Great Lakes, and interior Northeast. Even small amounts of snow
could combine with the gusty winds to produce blizzard conditions.
Additionally, existing snow cover in the northern Plains could
combine with the winds to produce a ground blizzard, even in the
absence of new snowfall.

--Extremely Dangerous Travel with Whiteout Conditions
Where blizzard, or near-blizzard, conditions occur, expect
periodic whiteouts with near zero visibility and considerable
blowing and drifting of snow. Traveling in these conditions will
be extremely dangerous, to at times impossible. You risk becoming
stranded and more vulnerable to the bitterly cold weather.

--Life-threatening Cold Temperatures
Dangerous wind chills will create a significant hazard for
travelers that become stranded. If you must travel, prepare for
extreme cold. In some areas, being outdoors could lead to
frostbite in minutes. Ensure outdoor animals and livestock have
sufficient shelter.

--Coastal and Inland Flooding Threat for the Northeast
Heavy rain falling onto a melting snowpack may cause flooding
impacts in urban areas or along rivers and streams, and minor to
isolated major coastal flooding will be possible due to onshore
winds.


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.