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

Valid 12Z Sun Jan 17 2021 - 12Z Wed Jan 20 2021

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

Broad cyclonic flow entrenched across the eastern CONUS will
persist through the forecast period, while waves of mid-level
energy and vorticity maxima rotate through the flow. The first of
these will eject through Maine early on D1, with a second trough
moving over the Great Lakes tonight into Monday, followed by yet a
third shortwave Tuesday night. Each of these impulses will be
accompanied by ascent through height falls and PVA, and followed
by CAA. The Lakes are mostly ice-free at this time, so strong CAA
across the warmer lake waters will setup an environment that will
become increasingly favorable for LES, and may be extremely
favorable at times. On D1, LES will be robust east of Ontario
early on D1, but the shortwave moving atop the area this aftn will
cause winds to back to the SW shutting off the heaviest LES east
of Ontario, but will begin to enhance the LES signature east of
Erie. Synoptic ascent will contribute to snowfall on D1, but the
heaviest snow will be due to LES, and WPC probabilities are high
for 4 inches in the Tug Hill and east of Erie just south of
Buffalo, NY, with local maxima of 8" possible.

The signature for LES intensifies D2, primarily east of Lake Erie.
850mb temps crash towards -12C to -15C, causing steep LRs over the
Lake, with intense forcing within the deepening DGZ and ELs
climbing above 800mb. The combination of this forcing and
instability will drive multiple rounds of intense LES on Monday,
with a single band possible producing snowfall rates reaching up
to 2"/hr. WPC probabilities on D2 are high for 4 inches south of
Buffalo, NY once again, with local maxima possibly reaching 12".
At the same time, a weak shortwave moving over Lake Superior may
spawn a wave of low pressure and enhanced instability with CAA on
N/NW winds Monday evening. This will support heavy LES across the
western U.P. of MI where WPC probabilities are moderate to high
for 4 inches.

The LES event continues and may enhance again on D3 with more
widespread LES becoming heavy downwind of most of the Lakes. This
will be due to yet another shortwave moving over the Lakes to
cause another surge of CAA and favorable low-level shear with
deepening DGZs and steepening LRs. 850mb temps crash to as low as
-20C, which raises ELs and drives the most intense forcing over
the lakes to intersect the lowering DGZ. LES on D3 is likely to be
widespread in the favored regions on W/NW flow, and WPC
probabilities for 4 inches are moderate to high in the NW L.P. of
MI, as well as east of Erie and Ontario, where some effective
fetch with a moisture tap from Lake Huron is also possible. 3-day
totals downwind of Lake Erie could exceed 2 ft, with 12-18"
possible in the Tug Hill Plateau.

South of the Lakes, a period of significant upslope snow is
possible by D3 across parts of WV. Confidence is lower here due to
soundings indicating the potential for some dry air in the upper
half of the DGZ, but moist ascent aided by upslope flow on NW
winds behind the departing shortwave has led to an uptick in WPC
probabilities which are now 10-20% for 4 inches on D3. Snow totals
over the 3 days in this region could reach 8", but should be
generally light each day until D3.


...Northern and Central Rockies...
Days 1-2...

Confluent mid and upper level flow aided by a modest eastward
moving Pacific jet streak will drive moisture inland and southeast
from the Northern Rockies towards Colorado through Monday. The
temporal duration of forcing is generally modest, but some
enhancement along the upslope/windward sides of the higher terrain
are likely to see periods of heavy snow which could add up to
significant accumulations during the next two days. Locally heavy
snow is most likely on D1 across portions of the Northern Rockies,
Absarokas, Big Horns, Bear Tooth, and Laramies, where WPC
probabilities are high for 6 inches, generally above 5000 ft, and
could reach 12" in local maxima. As the forcing and moisture
shunts southeastward on D2, modest snowfall will persist,
especially in the Absarokas where an additional 6"+ is likely, but
otherwise WPC probabilities for 6 inches shift into the CO Rockies.


...Southern Rockies/Southwest...
Day 3...

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, 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. SW 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 in the Sangre De Cristos were upslope snowfall will maximize,
but also in the San Juans and across the Kaibab Plateau where
synoptic lift will be most intense and prolonged. Snow levels will
gradually fall through D3 becoming as low as 2000-3000 ft across
the region, and this may allow for some moderate snow by late D3
to develop even along the Mogollon Rim and the higher peaks of the
San Gabriels/San Bernadinos of Southern California.


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

Weiss