Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 AM EST Wed Jan 19 2022

Valid 12Z Wed Jan 19 2022 - 12Z Sat Jan 22 2022

...Great Lakes...
Days 1-2...

Cyclonic flow behind a modest clipper system which will eject to
the east will leave CAA in its wake with 850mb temps crashing to
-20C. N/NW flow driving this CAA will produce favorable conditions
for LES S/SE of the lakes in the typical snow band regions.
Despite the DGZ sinking to near the surface, strong forcing into
this zone combined with inversions rising, especially late D1,
will likely lead to snowfall rates of 1"/hr within wavering bands
of heavy snow. WPC probabilities D1 are moderate to high for 4+
inches across the northern U.P., near Traverse City within the
L.P., and due south of Lake Michigan near Gary, IN. During D2
drier air begins to advect into the area shrinking the inversion
depth and limiting snow growth, causing a reduction in both
duration and coverage of LES bands. WPC probabilities for 4 inches
remain right along the southern Lake Michigan shore but are
generally less than 10% across NW IN.


...Ohio Valley to the Northeast...
Days 1-2...

A cold front trailing from a clipper ejecting through New England
will move into the Ohio Valley and stretch across the Mid-Atlantic
Wednesday night into Thursday morning. Along this front, a weak
southern stream shortwave is progged to lift northeast, producing
modest cyclogenesis along the boundary which will then shift
rapidly northeast and weaken off the coast Thursday aftn. While
the surface wave is likely to be weak, the combination of
low-level baroclinicity and upper LFQ diffluence should induce
some robust mid-level fgen driving robust ascent into a deepening
DGZ. Forecast soundings indicate a situation with rain changing to
snow, with a burst of heavy snow possible during transition from
central KY through eastern MD and into far southeast New England.
HREF snow fall rate probabilities for 1"/hr are modest, and snow
falling after rain will struggle to accumulate. However, heavy
enough rates with a rapidly cooling column should produce 1-2" of
snowfall in a narrow corridor, with local maxima above 3" possible
as shown by the WSE 90th percentile. This will likely impact the
Thursday morning commute for the I-95 corridor from Washington,
D.C. to New York City.


...Pacific Northwest through the Central Rockies...
Days 2-3...

Longwave ridging D1 will begin to shift east as a shortwave digs
out of British Columbia and races southeast into the Pacific
Northwest late D2 /Thursday night/ and then amplifies into a
closed low near the Four Corners as it digs south around a potent
ridge blossoming near the OR/CA coasts. This feature will be
accompanied by a 130+kt jet streak digging southward from Canada,
and the overlap of LFQ diffluence with the mid-level height falls
and PVA will spread precipitation from Washington state D2 to as
far south as New Mexico D3. This system is likely to be fast
moving, but PW anomalies are as high as +2 standard deviations on
D2 across the NW, waning to +0.5 D3 towards the Four Corners.
Within this moisture plume, the ascent will produce snowfall in
much of the terrain, with snow levels east of the Cascades
hovering around 3000-4000 ft, but may rise above 6000 ft in OR/WA
during the most robust WAA. WPC probabilities for 6+ inches of
snow are moderate in the WA Cascades, the Northern Rockies, and
the Tetons on D2, shifting to the CO Rockies, San Juans, and
Sangre De Cristos on D3.


...Gulf Coast to the Southern Mid Atlantic...
Days 2-3...

An arctic cold front will dig into the Southeast and Gulf Coast
Thursday followed by cold surface high pressure surging across the
southern tier. As this happens, a positively tilted mid-level
trough will begin to amplify across the Central Plains into Texas,
with mid-level SW flow and associated WAA spreading northeastward.
The overlap of these features will produce increasingly moist
isentropic ascent from 285-295K, and on D2 some light freezing
rain is likely to develop across the Coastal Plain and Piedmont of
the Carolinas where WPC probabilities for 0.1" are as high as 20%.

A much more significant winter weather threat develops D3 from
south Texas all the way to southern MD. As the aforementioned
trough begins to shift eastward, warm and moist upglide will
intensify. At the same time, a subtropical jet streak will begin
to arc poleward leaving favorable RRQ ascent draped across the
region. This will not only help to drive low pressure development
across the Southeast, eventually shifting to a coastal low, but
also increase ascent to spread a large swath of precipitation from
near the Rio Grande Valley, along the entirety of the Gulf Coast,
and into the southern Mid-Atlantic. The guidance has trended a bit
colder with the surface cold layer, and where the most robust WAA
occurs, this will support light to moderate freezing rain and
sleet from the Hill Country of TX through the Upper Coast, and as
far east as the Panhandle of FL. WPC probabilities for 0.01" of
ice are as high as 30% along this corridor. Further to the east, a
major ice storm is likely as the high pressure becomes continually
reinforced through both precipitation and northern stream
confluence near New England. Heavy rainfall rates may limit
accretion across the Carolinas and into the Tidewater region o VA,
but WPC probabilities are above 40% for 0.25" of freezing rain on
D3, with locally up to 0.5" of ice possible.

Further to the north, as the low pressure over the Southeast
begins to consolidate and shift offshore, a swath of moderate to
heavy snowfall is likely to develop well NW of the center from the
Upstate of SC northeast through southern MD and DE, including the
I-95 corridor from Washington, D.C. to Baltimore, MD. There
remains considerable uncertainty into the track of this low and
how far west/north the precipitation and snow will spread, but
current WPC probabilities are high for 4+ inches across
north-central NC into far southeast VA, with a sharp cutoff in
probabilities along the Mason-Dixon line.


Weiss