Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Thu Jan 20 2022

Valid 12Z Thu Jan 20 2022 - 12Z Sun Jan 23 2022

...Mid-Atlantic to Southern New England...
Day 1...

An arctic cold front will bring in colder air coincident with
precipitation to the I-95 corridor early day 1 with a burst of
snow aided by modest low-level frontogenesis and a strengthening
jet. Snowfall rates could exceed 1 inch/hour as the front sinks
southeastward through Virginia into North Carolina this afternoon
though total amounts are forecast to stay under four inches.

...Great Lakes...
Day 1...

Low to mid-level moist cyclonic flow and strong cold air advection
will support locally heavier bands of snow downwind of the Great
Lakes as surface high pressure approaches from the west overnight
tonight into early Friday. Local areas could receive several
inches of snow though WPC probabilities for at least four inches
remain low overall.


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

Shortwave just off the Washington coast this morning will push
inland as ridging builds in the northeast Pacific. This will help
dig the mid-level system south-southeastward into the Four Corners
by early Friday and close off over southern AZ or northern Mexico
by the end of day 3. Much of the moisture with the system into the
Northwest will push eastward as the upper trough digs southward,
leaving the system will progressively less moisture but enough
with which to capitalize. Moist upslope flow over the terrain and
lowering snow levels with the height falls should promote areas of
heavy snowfall for the higher terrain of the northern Rockies day
1, central Rockies day 2, and into AZ/NM day 3. 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
and to the Mogollon Rim by the weekend. 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 1-2...

An arctic cold front will dig into the Southeast and Gulf Coast
through Friday as cold surface high pressure advances east from
the Midwest to the Ohio Valley and Northeast and back toward the
southern Plains. Concurrently, 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. Farther west, a separate shortwave is expected to eject
from the Southwest U.S. and arrive out across the Rio Grande
Valley and South Texas on Friday, supporting a threat for a wintry
mix across portions of the Texas Hill Country and Deep South Texas
tonight into Friday morning. Moisture overrunning the cold surface
temperatures will support a freezing rain threat into Deep South
Texas that could exceed 0.10" of ice accumulation. Eastward, a
glaze to around a hundredth or two of ice is possible on the
northern fringe of the precipitation shield.

A larger freezing rain threat will exist over central/eastern
North and South Carolina behind the arctic front where isentropic
ascent and favorable upper-level jet dynamics ahead of an
approaching trough will favor potentially significant icing.
Though the guidance has trended east with the system overall, this
area remained less prone to that trend and the guidance continues
to show the possibility for a swath of 0.10" to 0.25+" ice
accumulations from central South Carolina northeastward into areas
of central and eastern North Carolina. As the wave moves far
enough offshore, a colder column would support a changeover or mix
with sleet and snow before the precipitation ends early Saturday.
Snowfall amounts have decreased a bit over southeastern VA into
northeastern NC as the consensus shifted eastward, but several
inches of snow is possible along and east of I-95 from Richmond
south into NC. WPC probabilities of at least four inches of snow
is moderate for this area (above 40%).


...Upper Midwest/Great Lakes...
Days 2-3...

Northern stream shortwave just atop the Northwest system day 1
will dip through the northern Plains into the Upper Midwest Friday
evening and into the Great Lakes. Cold front will have some
moisture in its warm advection side to support a couple inches of
snow across Minnesota, but waning a bit thereafter. However,
post-FROPA, northwest flow will reignite the lake effect favoring
the U.P. and western Lower Michigan by day 3, where several inches
are likely in favored areas. WPC probabilities for at least four
inches by day 3 are moderate to high there.


Fracasso/Orrison