Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 PM EST Thu Jan 28 2021

Valid 00Z Fri Jan 29 2021 - 00Z Mon Feb 01 2021

...Western U.S....
Days 1-3...

The slow moving plume of Pacific moisture into central CA begins
to shift a bit quicker to the southeast tonight as a reinforcing
wave trough digs down the northern/central CA coast before
ejecting east over the Desert SW Friday. The focus of the
precipitation will shift south down the Sierra Nevada this evening
to the transverse and peninsular ranges of southern CA later
tonight into Friday before tapering off Friday afternoon. Day 1
snow probabilities for 18 or more additional inches are moderate
to high for the southern half of the Sierra Nevada and the San
Gabriel/San Bernardino Mtns while the probabilities for 8 or more
inches include the San Rafael/Sierra Madre of the southern central
Coast Ranges as well as the ranges of central NV, the Sawtooth to
the ranges near Yellowstone NP, the Spring Mtns west of Las Vegas
and the highest points of southern UT/northern AZ (expanding down
the Mogollon Rim and to the San Juans of CO for Day 1.5).

Precip ahead of the next shortwaves which are associated with a
Gulf of Alaska low will reach the Pacific Northwest coast Friday
night with southwesterly flow and subsequent height falls
producing a new round of heavy mountain snow for the Olympics and
Cascades. Day 2 snow probabilities are moderate for 8 or more
inches in the Olympics with snow levels of 2500ft. Day 3
probabilities for 8 or more inches expand to the WA Cascades (with
snow levels around 3000ft) and northern CA Klamath Cascades where
snow levels will be around 4000ft.


...Central Plains...Midwest...South-Central
Appalachians...Mid-Atlantic...
Days 2/3...

The main trough ejecting east from CA on Friday becomes negatively
tilted as it crosses the southern Rockies Friday night, turning
northeast and closing off over the south-central Plains Saturday
morning and slowing on approach to the Midwest Saturday afternoon.
Gulf of Mexico moisture is added to elevated Pacific moisture as
the storm spills across the Plains Friday night into Saturday with
increasing rain coverage over the southern Plains. As this rain
lifts up the Plains antecedent cold conditions allow a light
freezing rain threat to develop over eastern Neb, central IA, into
north/central IL Saturday. Sleet and snow then form farther north
Saturday afternoon. Day 2.5 ice probabilities for a tenth inch or
more are limited to east-central IA. As the low develops and the
warm conveyor belt wraps around the low and cold air to the north
is drawn in, heavy snow is likely to develop first ahead of the
system on warm air advection, then wrapping around the north side.
Confidence has increased in the track being a little farther north
(with preference given to the 12Z ECMWF) than before from eastern
KS Saturday afternoon across central MO Saturday night and
south-central IL/IN on Sunday. Day 3 snow probabilities for 6
inches or more are low to moderate from eastern IA across northern
IL and far southern WI and across northern IN (just into southern
MI) to northwest OH. Enhancement from Lake Michigan on
easterly/northeasterly flow looks to favor the Chicago and
Milwaukee metro areas on Sunday.

Cold air damming develops with a surface ridge from a 1034mb high
over northern Quebec extending down the eastern side of the
Appalachians Saturday night/Sunday with overrunning precip leading
to a freezing rain/sleet threat for a swath along the
central/western NC/VA border and up through central WV where Day 3
freezing rain probabilities for a tenth inch or more are low to
moderate (highest in SW VA where there are 20 percent
probabilities for a quarter inch or more). Warm air advection
overrunning the cold air damming wedge allows front end snow to
cross the Mid-Atlantic on Sunday with locally (though likely
transient) heavy snow possible. Day 3 snow probabilities for 4 or
more inches are moderately high for the south-central Appalachians
of WV/VA and low for northern VA to south-central PA.


...Eastern New England...
Day 1...

A reinforcing mid-level northern stream low will push southeast
from the Adirondacks to central New England tonight. This will
influence the developing low well offshore and allow an inverted
surface trough to develop toward the central Maine coast tonight
and swing south to Cape Cod Friday. The combination of low level
forcing from the trough and elevated instability from the cold
core low as well as increasing oceanic fetch as strong winds veer
from NWly to Nly tonight will allow bands of snow to develop and
affect the immediate Maine and southeast Mass coasts Friday,
potentially lingering into Friday evening for Cape Cod. Locally
heavy snow in squalls will mainly be offshore with low Day 1 snow
probabilities for six or more inches near the central Maine coast
and outer Cape Cod.


Jackson