Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 PM EST Sun Nov 29 2020

Valid 00Z Mon Nov 30 2020 - 00Z Thu Dec 03 2020

...Pacific Northwest...
Days 1-2...

A potent but fast moving shortwave accompanied by the LFQ of an
intensifying 90kt Pacific jet streak and associated upper
divergence maxima will translate across the Pacific and onshore WA
Monday morning, crossing the northern Rockies Monday night. This
feature will be accompanied by a brief uptick in mid-level
moisture noted by 850-600mb RH exceeding 90%, coincident with
robust omega. Snowfall during this event will be primarily in the
higher terrain of the Olympics, WA and OR Cascades, and the
Northern Rockies.  WPC probabilities are highest in the WA
Cascades, which will be orthogonal to the confluent mid-level
flow, and are high for 8 inches, and moderate for 12 inches.
Elsewhere, WPC probabilities indicate a moderate chance for 4
inches in the Olympics, northern OR Cascades, and into the
Northern Rockies, such as the Bitterroots.


...Ohio Valley/Great Lakes/Central to Southern Appalachians...
Days 1-3...

Two distinct shortwaves, one in the southern stream and one in the
northern stream, will phase together during Monday to produce an
anomalously deep mid-level closed low centered over the Ohio
Valley. This feature will then pivot slowly northeastward into
upstate New York on Tuesday, before crawling further northward
into Quebec Province Wednesday as it gets displaced by yet another
strong mid-level low digging into the Central Plains. The
combination of this upper low and PVA with a coupled jet structure
will produce height falls and surface cyclogenesis, with the
surface low strengthening quickly as it lifts from the Southern
Appalachians into Quebec.

As the low lifts northward, precipitation will spread ahead of it
to encompass much of the Southern Appalachians, Ohio Valley, and
Great Lakes. Initially much of this precip will be rain as the
column is too warm for snow. However, a rapid changeover from rain
to snow will begin Monday from NW to SE as intense ascent through
mid-level deformation and modest frontogenesis combined to
dynamically cool the column. Additionally, CAA will commence just
above the surface and through the mid-levels as the low deepens
and pulls of to the northeast. A cooling column will lead to a
lowering and deepening of the DGZ, into which robust forcing will
drive precipitation which will change to heavy snow from NW to SE.
There are likely to be 3 areas of heavy snow:

1) Within this deformation axis across eastern MI into the Ohio
Valley. WPC probabilities for 4 inches are high in far SE MI and
western OH due to the slow eastward pivot of this axis. While snow
rates are not progged to be intense, maybe briefly reaching 1"/hr
at times, prolonged moderate to at times heavy snow should
accumulate to modest accumulations late Monday and Tuesday.

2) The Southern and Central Appalachians. In these areas, rain
will change to snow on the backside of the system as NW flow and
CAA develops. NW flow with a moist DGZ will lead to periods of
heavy snow in the favored upslope regions along the NC/TN border,
as well as in the snow belts of WV. In WV, there may be additional
support as NW flow from the Great Lakes adds moisture to the
column. WPC probabilities in these terrain regions are high for 4
inches, with locally 10" possible before the event winds down
Tuesday night.

3) Favored N/NW/W snow belt regions of the Great Lakes. It is this
area that is likely to receive the heaviest snowfall from this
event. S/SE of Lake Michigan and Lake Superior, northerly flow
with steep lapse rates atop still warm lake water temperatures
will produce a favorable environment for heavy LES. Multi bands
are likely off Superior, but a single band may develop SE of Lake
Michigan especially Monday night. Rates in excess of 1"/hr are
likely, and WPC probabilities are high for 4 inches. The jackpot
is likely to be along much of the southern Lake Erie shore, where
synoptic ascent through the deformation axis will combine with LES
and an effective moisture channel from Lake Huron. Long duration
heavy snow is likely with rates eclipsing 1"/hr, and WPC
probabilities for 8 inches are high from near Cleveland, OH to
just south of Buffalo, NY. Local amounts in excess of 15 inches
are likely. As the low pulls further north the latter half of D3,
additional heavy snow will develop east of Lake Ontario with
moderate accumulations forecast.


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


Weiss