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

Valid 12Z Sun Nov 29 2020 - 12Z Wed Dec 02 2020

...Pacific Northwest...
Days 2-3...

A potent but fast moving shortwave accompanied by a jet streak and
associated upper divergence maxima will translate across the
Pacific and onshore WA Monday morning, crossing the northern
Rockies Mon afternoon. This feature will be accompanied by a brief
surge in moisture and well-defined 850-700 mb ascent, which will
spread precipitation across the WA/OR Cascades and then into the
Northern Rockies as Monday progresses.  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 2-3...

A pair of northern and southern stream 850-700 mb waves merge on
Monday, with low pressure amplifying as it moves north across the
central Appalachians and then into central NY State.
On the west side of the 850 mb low moving up through the Ohio
Valley, low level cold advection into a mid level deformation zone
changes precip from rain over to snow across MI/IN/KY/OH.

Dynamic cooling within this intense lift coincident with deepening
of the dendritic growth zone suggests snowfall within this
deformation/comma head could become moderate to heavy at times
Monday, waning as the 850 mb low pivots north into Canada Mon
night.
Several inches of snow is expected in northwest Ohio, with less
further east owing to initial warmth causing precip to start as
rain before changing over to snow.

Lake enhancement is likely on northerly flow off Lake Michigan
into areas near the IN/MI border due to long cross lake
trajectories on north/northwest flow, with lee shore convergence
helping to produce lift. This setup also supports heavy upslope
snow on prolonged NW flow which will gradually become more
entrenched into the lowering dendritic growth zone across the
Appalachians.

WPC probabilities feature several maxima for 4". These areas
include south of Lake Michigan, as well as within the primary
deformation axis progged to be in SE MI into central OH, and again
in the upslope favored regions of WV. Locally more than 6" is
possible in these areas and in the Southern Appalachians of NC/TN.

On Day 3, as the low is up in Ontario, cyclonic flow persists
across the lower lakes, with the 00z GFS showing boundary layer
vertical velocity maxima in the lee of Lake Erie across northeast
Ohio, northwest PA, and far southwest NY.
The ECMWF/GFS/NAM/Canadian regional GEM show maxima of 0.75 to 1
inch liquid equivalent QPF, supported by 1 inch maximum liquid
equivalent in the 00z GEFS Mean.  This should result in bands of
heavy snow with clusters of 8-12 inches possible in northeast Ohio
to southwest NY off Lake Erie.

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


Petersen