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

Valid 00Z Thu Jan 20 2022 - 00Z Sun Jan 23 2022

...Mid-South/Ohio Valley to the Northeast...
Days 1...

An arctic cold front move cross the Ohio Valley and move into the
Mid-Atlantic region tonight through 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 afternoon. While the surface wave is likely to be weak, a
combination of at least modest low-level frontogenesis, mid-level
vort energy, and favorable upper-jet support should interact with
the arrival of cold air behind the frontal passage to yield a
period of light to locally moderate snowfall accumulations from
central Kentucky east-northeast across the central Appalachians
for the overnight period, and then crossing the north-central
Mid-Atlantic region Thursday morning. The latest HREF and NBM
guidance does favor the potential for snowfall rates to approach
or locally exceed 1 inch/hour, including for portions of the I-95
corridor from Washington D.C. to New York City. Aside from the
snow, there will also be a narrow concern for some light freezing
rain too for areas especially areas of the Mid-South and Ohio
Valley tonight prior to the arrival of deeper cold air and the
precipitation pulling away to the east.

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

Low to mid-level moist cyclonic flow and strong cold air advection
in behind a shortwave trough and arctic cold front passage will
support heavy lake-effect snow showers and squalls overnight and
on Thursday downwind of the Great Lakes. The activity should begin
to wane Thursday night as high pressure builds overhead and the
low-level fetch over the lakes slackens. Some of the heaviest
snowfall is expected to be downwind of Lake Michigan where areas
of the northwest L.P. of Michigan and also areas of far southwest
lower Michigan and northwest Indiana will see locally more
organized/intense bands with snowfall rates that could approach 2
inches/hour. Some additional storm totals going through Thursday
night are expected to 8 to 12+ inches. Locally heavy totals with
somewhat lesser amounts are expected locally across the U.P. of
Michigan for this period.


...Pacific Northwest through the Central Rockies...
Days 1-3...

Longwave ridging early in the period will shift east as a
shortwave digs out of British Columbia and races southeast through
the Pacific Northwest on Thursday and into the Four Corners region
Thursday night and Friday. Rather strong forcing will accompany
the amplifying energy as it drops down through the Intermountain
West and this coupled with moist upslope flow over the terrain and
lowering snow levels with the height falls should promote areas of
heavy snowfall for the higher terrain. The heaviest accumulations
will tend to be over the northern Washington Cascades of the
Pacific Northwest; the Sawtooth, Bitterroots and Absaroka Range of
the northern Rockies; and the Colorado high country with an
emphasis gradually into the Sangre De Cristo range involving the
central Rockies toward the end of the period. Locally over 1 foot
of new snow is expected and especially for the Sawtooth and
Bitterroots where the more favorable mid-level upslope flow will
be noted.


...South Texas/Gulf Coast to the Southern Mid Atlantic...
Days 1-3...

An arctic cold front will dig into the Southeast and Gulf Coast
Thursday and Friday as cold surface high pressure builds advances
east from the Midwest to gradually the Ohio Valley and Northeast.
As this happens, a positively tilted mid-level trough will begin
to slowly amplify from the Midwest to the Mid-Atlantic states with
the energy then advancing offshore on Saturday. Meanwhile, a
separate shortwave is expected eject from the Southwest U.S. and
arrive out across the Rio Grande Valley and South Texas on Friday.
The latter of the two pieces of energy is expected to drive a
threat for sleet and freezing rain across portions of the Texas
Hill Country and Deep South Texas beginning by Thursday night and
especially Friday morning as the energy taps subtropical moisture
to south and pulls it northward over the shallow sub-freezing
temperatures that should be locally in place behind the strong
cold frontal passage into the South. Areas of Deep South Texas may
locally see in excess of a 0.10" of ice accumulation. As the
energy pulls east across the Gulf of Mexico on Friday, it may
skirt areas of the central Gulf Coast region too for some very
light icing concerns.

A much more significant winter weather threat though will evolve
across the Southeast coastal plain and the southern Mid-Atlantic
region as the aforementioned Midwest to Mid-Atlantic trough
evolution eventually begins to interact with the aforementioned
cold front passage. Isentropic ascent and favorable upper-level
jet dynamics ahead of the approaching trough will favor a swath of
precipitation developing and spreading in coverage across the
Southeast beginning Thursday night through early Friday. Gradually
a couple waves of low pressure will develop along the
aforementioned front which will consolidate Friday night into
Saturday offshore of the southern Mid-Atlantic region. This
coupled with the amplifying energy aloft will bring precipitation
northward into the southern Mid-Atlantic region with an emphasis
on areas of central/eastern North Carolina and southeast Virginia.
Currently the thinking is that a swath of 0.10" to 0.25+" ice
accumulations will occur from central South Carolina northeast
into areas of central and eastern North Carolina prior to
sufficient cold air arriving to yield a changeover or mix with
sleet and eventually snow. By later Friday night and early
Saturday, as low pressure organizes offshore of the Mid-Atlantic,
an area of moderate to heavy snow is forecast for areas of
especially northeast North Carolina and southeast Virginia.
Locally as much as 4 to 8 inches of snow will be possible across
this region prior to the storm exiting on Saturday.

Orrison

~~~ Key Messages for Winter Storm Impacting Portions of the South
and Mid Atlantic Jan 20-22 ~~~

-- A strong cold front will move across the Gulf Coast States and
into the Southeast on Thursday.

-- A wintry mix, including freezing rain, will likely develop
north of the front, impacting areas as far south as South Texas
and the central Gulf Coast late Thursday into Friday.  This could
result in dangerous travel conditions.

-- An area of low pressure is expected to develop over the
Southeast on Thursday and then move northeast along the Carolina
coast on Friday. This low will spread snow, sleet, and freezing
rain from the Southeast into the Mid-Atlantic states.

-- Significant ice accumulations are likely in the Carolinas. This
could result in dangerous travel conditions and scattered power
outages.


-- Moderate to heavy snow is possible from central North Carolina
through the DelMarVa Peninsula.

-- Changes in the track and strength of this system can have
significant impacts on precipitation types and amounts.