Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
428 AM EST Mon Nov 30 2020

Valid 12Z Mon Nov 30 2020 - 12Z Thu Dec 03 2020

...Pacific Northwest...
Days 1...
A potent but fast moving shortwave accompanied by the left front
quadrant of an intensifying 90kt Pacific jet streak and associated
upper divergence maxima will translate onshore WA this morning,
crossing the northern Rockies tonight. This feature will be
accompanied by a mid-level relative humidity maxima, 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 have the higher QPF amounts/probabilities, 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 today to produce an
anomalously deep mid-level closed low centered over the Ohio
Valley. The 700 mb low intensifies over Lake Erie tonight and
drifts slowly northeastward to Lake Ontario Tuesday, then near the
Ontario/Quebec border. 
Cold advection wrapping around the low changes rain over to snow
across IN/MI and then OH as today progresses.
Ascent through mid-level deformation and modest frontogenesis
combine with cold advection to increase areal coverage of snow.
There are likely to be 3 areas of snow:

1) Snow belt regions adjacent to the Great Lakes are likely to
receive the heaviest snowfall from this event.
The jackpot is likely to be along much of the southern Lake Erie
shore, where synoptic ascent through height falls and upper
divergence will combine with lake effect snow supported by a
moisture channel from Lake Huron. Long duration heavy snow is
likely with rates of 1-2"/hr, and WPC probabilities for 8 inches
are high from near Cleveland, OH to Erie PA and the south towns of
southwest NY.  Local amounts of 15-18 inches are likely.
Uncertainty continues regarding duration of snow,a s initial temps
are too warm for snow, the change over occurs today into tonight.

As the low pulls further north day 3/Wed, snow will develop east
of Lake Ontario in the Tug Hill with moderate accumulations
forecast. Several inches of snow is expected before the low
departs Wed.

A band or two may develop SE of Lake Michigan near the IN/MI
border today into tonight. Rates in excess of 1"/hr are likely,
and WPC probabilities are high for 4 inches, and low for 8 inches
of snow.

2) The Southern and Central Appalachians. In these areas, rain
will change to snow on the backside of the system as NW flow and
cold advection develops. Moist northwest flow will lead to periods
of snow in the favored upslope regions along the NC/TN border, as
well as in the snow belts of WV. In WV/western MD/western PA,
northwest flow from the Great Lakes transports moisture into the
region.  WPC probabilities in these terrain regions are high for 4
inches.

3) Within the mid level deformation axis across eastern MI into
the Ohio Valley, WPC probabilities for 4 inches are high in 
western OH due to the slow eastward pivot of this axis. Event
totals of 4-6 inches are forecast on Day 1 in portions of western
Ohio.

...Southern to South Central Plains Day 3...

The models show an elongated north-south 850-700 mb trough
extending down the plains on Wed. The models show a closed low
likely forming in the base of the trough somewhere in the southern
Plains.  Lift occurring in the cold sector of the low has
potential to bring snow to portions of KS/western-central OK on
We-Wed night.
Difference sin the 850-700 mb low track ans strength lead to
differing QPF and possible snow axis/orientation, so confidence is
low given significant model spread and run to run changes.

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

Petersen