Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EST Fri Jan 22 2021

Valid 00Z Sat Jan 23 2021 - 00Z Tue Jan 26 2021

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

Continued cyclonic flow across the Lakes will lead to more LES in
the favored eastward bands. Tonight into Saturday, this is most
likely downwind of Erie and Ontario, as well as across the western
U.P. of MI as CAA enhances behind yet another impulse rotating
within the flow. The heaviest snowfall is likely downwind of Lake
Ontario where despite less than ideal fetch on a more NW flow, an
effective moisture tap from the Georgian Bay and Lake Huron will
help enhance snowfall. Heavy snow is also likely at times downwind
of Lake Erie due in part as well to a Lake Huron feed. Westerly
flow developing across Lake Superior will channel some heavy LES
towards the Keweenaw Peninsula as well. WPC probabilities for 4
inches are high in these locations, with local maxima exceeding 6"
possible south of Syracuse, NY SE of Lake Ontario.


...The West...
Days 1-3...

An amplified longwave trough across the far West will be
reinforced by several impulses rotating down from the Gulf of
Alaska, and then pivoting towards the Great Basin as closed
features.

On D1, a mid-level low will pivot down the coast of CA and become
a closed feature Saturday evening before opening and ejecting
northeast through the Four Corners on Sunday. A persistent and
slowly intensifying jet streak downstream of this trough will keep
a the modest diffluent LFQ stretching from the Southern CA coast
into the Central Rockies, providing moisture and aiding ascent
across the region. As this trough expands eastward, snow levels
will crash, starting as high as 7000 ft D1 across the Four
Corners, and falling to 3000 ft in Southern CA and the central
Rockies. Where mid-level divergence and upper diffluence overlap
atop the moisture stream, heavy snow is likely above these levels.
WPC probabilities for 6" are high on D1 across the San Juans, CO
Rockies, Uintas, Wasatch, southern Sierra, and other ranges of the
Great Basin. Isolated maxima above 12" are possible in the highest
terrain of the San Juans. By D2, the best moisture shifts east
while ascent maximizes to the south. This will lower snowfall
across the region, but additional heavy snow is likely. WPC
probabilities are highest for 6 inches in the San Juans once
again, as well as spreading across the Mogollon Rim and White
Mountains of AZ.

As the first system exits to the east, a more powerful shortwave
will dive along the coast from WA to CA, with multiple closed lows
possibly developing across the coast. This will have a two pronged
effect of significantly lowering snow levels, while also
transporting Pacific Moisture along the coast and then inland
towards the Four Corners once again. Height falls will be the
primary driver of ascent this period as the jet energy stays
mostly west of the trough and over the Pacific Ocean. However,
pronounced increase in mid-level RH suggests moderate to heavy
snow will spread down the coast, and on Monday snow levels may
fall as low as 500 ft in OR/WA, and to near 2000 ft in southern
CA. While the heaviest snow will once again occur in the terrain,
there is potential for lowland snow accumulating above 1" in
WA/OR, including the Columbia Gorge. In the Cascades and coastal
ranges, WPC probabilities are moderate for 4 inches. Further
south, heavier snow is likely to being to impact the Sierra,
Mogollon Rim, and other ranges of Southern CA and the Great Basin
as moisture gets channeled eastward concurrent with intensifying
ascent. It is becoming increasingly likely that heavy snow will
accumulate in these ranges, and WPC probabilities are high for 8
inches in the Sierra, San Bernadinos, and Mogollon Rim D3.
Isolated totals above 12" are likely, and more heavy snow is
expected to impact these same ranges into D4, just beyond this
forecast period.


...Northern/Central Plains to Upper Midwest...
Days 1-3...

Two distinct storm systems are set to impact this region over the
next 3 days.

The first is a modest mid-level wave which will shed from a closed
low over the Great Basin and eject northeast across the Plains
Saturday through Sunday. While the amplitude of this feature is
modest, it will be embedded in moist mid-level confluent flow,
with weak height falls and accompanying LFQ jet diffluence
providing ascent, aided by intensifying isentropic lift providing
WAA. Although forcing and moisture is transient for this event,
snowfall could over-perform. A cold column noted on regional
soundings will allow for high SLRs, aided by a deepening DGZ as an
isothermal layer develops on the WAA. During the period of
heaviest snowfall, the DGZ appears to be quite deep, and as the
best lift intersects this saturated DGZ, heavy snow is likely. WPC
probabilities for 4 inches are above 60% from far eastern SD
through extreme western WI, with local maxima above 6" possible.
The greatest snow accumulation is likely 00Z-12Z Sunday, with
rapid waning occurring thereafter as the system pulls away to the
northeast.

As the first system pulls away, a more significant and larger
scale event will begin as the Great Basin low ejects into the
Plains as a more robust shortwave lifting from OK to IL Sunday
into Monday. This shortwave will be accompanied by an intense
jet-level divergence maxima, and surface low pressure is likely to
develop and head northeast into Illinois while deepening by the
end of the forecast period. An increasing Gulf moisture tap with
WAA/isentropic ascent will spread moisture northward from KS to IN
D2 into D3. This increasing WAA is likely to drive a warm nose
above 0C, creating a mixed p-type scenario with sleet and freezing
rain likely across a large area. The guidance still features
considerable spread in the intensity and placement of this low,
but at this time feel the northern solutions (non-NAM) was the
most likely. With this track, a wave of sleet/freezing rain is
likely to impact a large corridor from eastern KS through northern
IN, and WPC probabilities on D3 are as high as 30% for 0.1" of
freezing rain accretion. WSE plumes indicate the potential for
much higher amounts, especially across Missouri, but confidence is
not high at this time. Some light snow is likely north of the
freezing rain with the WAA, but heavier snowfall should begin to
develop on the N/W side of the low as it deepens and lifts late
D3. An intense mid-level fgen band is progged to length NW of the
surface low as rapid CAA commences, and this will quickly cause
precipitation to change to snow within the developing comma. WPC
probabilities currently indicate a 30-40% chance for 4" of snow
from central KS into southeast IA.


...Mid-Atlantic/Central Appalachians...
Day 3...

A low pressure system organizing near the Arklatex late Sunday
will drape a warm front northeastward towards the Mid-Atlantic,
and then race northeast along it. Moisture sourced from the Gulf
of Mexico will spread northward on warm/moist advection associated
with this front, expanding precipitation northeastward into the
Central Appalachians and Mid-Atlantic Sunday night. As this
precipitation expands, a wedge of high pressure will be in place
from the northeast, and is likely to become reinforced and
entrenched by precipitation falling into it. At the same time the
WAA will drive a warm nose quickly northward, and guidance
suggests that despite a brief burst of snow at precip onset, most
of the precipitation by the end of D3 will be in the form of sleet
or freezing rain from western VA northward into the Laurel
Highlands of PA and much of MD. Guidance still features
considerable spread in the low-level thermal structure so
confidence in the exact evolution is moderate. However, there is
an increasing signal for a period of freezing rain which could
become impactful. WPC probabilities for 0.1 inches are as high as
60% in the Panhandle of WV and western MD, into the Laurel
Highlands, with low probabilities for 0.25". Light freezing rain
accumulating to less than 0.1" is likely across much of the rest
of this area. With differences in low-level thermal structure
persisting, it is possible that this may end up more snow/sleet if
secondary low-development occurs more rapidly, or more rain if the
WAA overwhelms the low-level cold air. Future updates will
hopefully narrow the spread and increase confidence in the exact
evolution, but it does appear a wintry mix of moderate to heavy
precipitation is likely beginning Sunday night.

Weiss