Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 AM EST Tue Dec 01 2020

Valid 12Z Tue Dec 01 2020 - 12Z Fri Dec 04 2020



...Great Lakes/Central to Southern Appalachians...
Days 1-2...

The models continue to signal a significant lake-effect/orographic
snow event in the lee of the eastern Great Lakes this period.  A
low that is forecast to lift north across Lake Ontario today and
into adjacent Ontario and Quebec
allows a mid level deformation band to cross Lake Erie, with a
multi-lake trajectory across Georgian Bay and then Lake Erie
enhances available moisture.  Lee shore convergence aids in
producing ascent with good model agreement in producing heavy snow
in northeast Ohio to northwest PA and southwest NY. Up to an
additional foot is possible in northwest PA near Erie PA.

As the low pulls further north late Tuesday into Wednesday, snows
are expected to develop east of Lake Ontario in the Tug Hill, with
moderate accumulations forecast.  Several inches of snow are
expected before the low departs Wed.

Meanwhile, moist, cold, northwest flow will carry lake moisture
south and lead to periods of snow in the favored upslope regions
from western Pennsylvania to western Maryland and West Virginia,
with several inches possible along the favored upslope terrain
through early Wednesday.  As the low lifts north, the strength of
upslope flow weakens in the southern Appalachians, and drier air
advects into the region.  Consequently, snow should wind down in
terrain along the North Carolina/Tennessee border, with only a few
more inches today. 

...Southern to South Central Plains...
Day 2...
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 in the southern Plains. 
Lift occurring in the cold sector of the low in an 850-700 mb
deformation zone should bring snow to portions of south central to
southwest Kansas and western Oklahoma Wednesday into early
Thursday.  Differences in the precip type in southeast KS and
eastern OK continue to limit forecast confidence with this system.


The 00z NAM was an outlier in producing the heaviest QPF and
resultant snow, which appear to be a result in an anomalous surge
in the 850-700 mb jet and resultant theta-e advection and
convergence.  The NAM Conus Nest QPF was half the parent NAM,
which made it more in line with the magnitude of the
GFS/ECMWF/UKMET QPF. The NAM was given the least weighting as a
result. The ECMWF wraps heavier QPF west in OK than the GFS/UKMET
in spite of a similar 700 mb low track.

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

Petersen