Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
523 PM EST Tue Dec 04 2018

Valid 00Z Wed Dec 05 2018 - 00Z Sat Dec 08 2018

...CA Sierra Nevada Mountains to the Southern Great Basin and
Southern Rockies...

The models continue to show an upper low dropping south along the
California coast Tuesday-Wednesday.  Moisture transport and 700 mb
convergence moving inland into the CA Sierra Nevada supports
several inches of snow from tonight through Wednesday, with the
maxima shifting a bit south in tandem with the offshore
circulation and bands of 700 mb convergence during the day on
Wednesday.

WPC probabilities showing a Slight Risk for accumulations of
4-inches...and a small area where there is a risk of 8 inches...or
more for elevation mainly above 7000 ft from Wednesday evening
through Thursday.  With the area of low pressure moving southeast,
any heavy snow threat will become more confined south of a line
from the southern Sierra Nevada to the Four Corners region with
the 700 mb convergence moving east into the ranges of southern NV
and AZ into the four corners region and southwest CO by late
Thursday and Friday. WPC probabilities showing a Slight Risk for
8-inches across the higher elevations Thu-Thu night, with none of
the models showing heavy snow area.

The probability of significant icing (0.25 inches or more) is less
than 10 percent days 1-3.


...Western High Plains Adjacent To The Southern Rockies...

As the above-mentioned system makes its way eastward, crosses the
Southern Rockies and emerges over the adjacent High Plains, some
snow should be spreading out over the High Plains. At this point,
the probabilities of 4 and 8 inches of snow should still be
confined to the mountains.  However, push of colder air southward
associated with a sprawling area of high pressure over the Plains
will be in place as low level flow turns southerly and begins to
draw moisture northward...setting the stage for freezing
precipitation over parts of western Oklahoma and western Texas on
Friday.

The WPC probabilities show a slight risk of significant icing
(0.25 inches) which reflects spread in the suite of numerical
guidance.  However, some plume diagrams for total precipitation
falling as freezing rain from individual operational models and
ensemble members points to the potential for well over a quarter
of an inch.  The operational models tended to focus the greatest
threat over the Texas/Oklahoma panhandle extending eastward into
parts of western/central Oklahoma.  The plume diagram from the
ensembles extended well east and south of the operational runs. 
Regardless of whether the differences are a function of the
resolution of the ensembles vs the operational run, the synoptic
pattern is supportive of icing potential. 


...Great Lakes Days 2-3...

After a lull on day 1, light lake effect snows pick up in the lee
of Lake Superior into the Keweenaw Peninsula of the western UP of
MI as a cold front progresses across the region, with Lake Effect
snows ramping up to the lee of Lake Ontario and Lake Erie from Day
2 into Day 3.  Several inches of snow are possible centered on the
Tug Hill east of Lake Ontario and the GFS shows high layer
relative humidity and a local maxima of lift in favored upslope
areas. The GEFS Mean shows snow extending further inland into the
western Adirondacks, where several inches of snow are possible
Thu-Thu night.

The probability of significant icing (0.25 inches or more) is less
than 10 percent days 1-3.

Bann