Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
455 PM EST Sun Jan 17 2021

Valid 00Z Mon Jan 18 2021 - 00Z Thu Jan 21 2021

...Great Lakes and Central Appalachians...
Days 1-3...

A series if wave move through low- mid level cyclonic flow to
produce bursts of lake enhanced and lake effect snow, which are
heaviest overall downstream from Lake Erie in southwest NY, where
12-18 inches is expected with locally higher totals.

On Day 1, the circulation moving from the Ohio Valley to the lower
Lakes enhances moisture aloft and provides lift.  This combines
with pre-frontal low level convergence tonight to lead to a period
of snow overnight focused in northeast Ohio across northwest PA
and southwest NY. The forward propagating boundary provides a
break to areas near the eastern shore of Lake Ontario.
A secondary boundary passes through Lake Superior with
redeveloping snow showers Monday afternoon and evening in the
Upper Peninsula of MI.

Monday night to early Tuesday, a period of lake effect snow
showers continues in the UP of Michigan and downstream in
northwest lower Michigan.
The potential for locally heavy lake effect snow east of Lake Erie
persists into Tuesday, supported by long cross lake Erie
trajectories, steep lapse rates, and near shore moisture
convergence.
WPC probabilities on D2 are high for 4 inches in southwest NY,
with local maxima possibly reaching 12". Lake effect snow
redevelops over lake Ontario into the Tug hill as moisture from
the lakes spread onshore and upslope. A period of west winds
favors a period of briefly heavy snow.

On Day 3, the parent wave of low pressure moving through NY and
then New England drives a boundary across the lakes, with winds
veering. The result of changing wind speed/direction results in
the axis of moisture convergence changing across the lakes and
thus changing location of where lake effect showers/bands will be
located. Any heavy bands persisting into Tue night should result
or dissipate on Wed in the veering flow and drying aloft.
Probabilities lower due to less confidence where bands could
persist long enough to produce heavy snow.

A few periods of upslope snow are expected across the mountains of
WV/western MD/southwest PA. The disturbance in the Ohio Valley
will produce southwest flow this evening with moisture advection
arriving already producing snow showers that will spread through
the WV to southwest PA mountain tonight. The moist southwest flow
continues Monday, along with the snow showers.
There is potential for a second wave (shown in the 18z GFS) to
cross the region early Tuesday, but there is a split in the
guidance as to whether another surge will continue.
Snow totals over the 3 days in this region could reach 8" over the
extended period, with less any particular day.  As Wed arrives, a
building low-mid level ridge results in sinking air and drying
developing, so snow shower coverage/intensity should wane.

...Northern and Central Rockies...
Day 1...

Confluent mid and upper level flow aided by a Pacific jet streak
will drive moisture southeast from the Northern Rockies towards
Colorado through Monday. Pockets of enhanced moisture and lift are
focused in the upslope/windward sides of the higher terrain to
produce periods of heavy snow. Locally heavy snow is most likely
on D1 where overlapping moisture/ascent persist the longest across
portions of the Little Belt Mountains, Absarokas, Big Horns,
Laramies, and Snow Range where WPC probabilities are high for 6
inches, generally above 5000 ft.  Local maxima of 12" are
expected. As the upper trough moves south, moisture shunts
southeast into CO and NM, with snow tapering in MT/WY as sinking
motion develops.

...Southern Rockies/Southwest...
Day 2...

The combination of a closed mid-level low forming near Southern CA
Monday night and a cold front dropping southward across the High
Plains of NM will drive large scale ascent and increasing snowfall
across the region. The cold front will produce upslope flow to
enhance precipitation in the Sangre De Cristo range of southeast
CO/NM, while divergent flow downwind of the closed mid-level
feature in tandem with increasing jet level diffluence will
produce large scale ascent lifting into the Four Corners,
continuing downstream across the San Juans and Sangre De Cristo
Mountains.

Southwest flow ahead of this trough will drive moist advection as
well, and conditions will become increasingly favorable for heavy
snowfall Monday night into Tuesday. WPC probabilities indicate a
high chance for 6 inches of snow/moderate chance of 8 inches in
the Sangre De Cristos/San Juans were upslope snowfall will
maximize. A foot of snow is expected in the favored windward
terrain.

Snow levels will gradually fall through D2, and this may allow for
some moderate snow by late D2 to develop even along the Mogollon
Rim and the higher peaks of the San Gabriels/San Bernadinos of
Southern California.

On Day 3, the low pulls away far enough that ascent weakens across
the southwest, so probabilities are heavy snow are less than 10
percent.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen