Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EDT Fri Mar 20 2020

Valid 12Z Fri Mar 20 2020 - 12Z Mon Mar 23 2020


...Four Corners...
Days 1-2...

A closed upper low over Nevada persists through Fri and shears out
to the east on Saturday. Despite the slow filling of this feature,
moist advection on W/SW 700mb flow will maintain sufficient
moisture such that ascent through upper diffluence and upslope
flow will produce high elevation snowfall today into Saturday. The
heaviest snow is expected on in the Wasatch and Uinta ranges of
Utah, as well as much of the western CO Rockies.  Snow amounts
taper as the upper low weakens, with little additional snow Sun.
Additional snowfall may locally be 6-12 inches by Saturday in
favored terrain.

...Southern CA Sierra Nevada...
Day 3...
The models indicate an upper level closed low off the central CA
coast on Sat shears and moves onshore Sun.  As the wave
approaches, an upper level jet maxima crosses forecast by the 00z
ECMWF to be 130 kt at 300 mb southern CA.  Ascent spurred by upper
divergence maxima near the jet core results in low-mid level
moisture fluxes and lift coming into the mountains of southern CA.
 Several inches of snow are possible in the southern CA Sierra
Nevada mountains Sunday.

...Ohio Valley to the Central Appalachians to PA...
Day 3...
The models indicate a 700-500 mb wave progressing across the Ohio
Valley early day 3 and then arriving in the Central Appalachians
and PA late day 3.  In advance of the wave, 850-700 mb warm
advection and convergence combine with moisture advection to cause
snow to develop in the northern Oh Valley and across the mountains
of northern WV, western MD, and western PA Allegheny Mountains.
The warm advection in the central Appalachians causes the snow to
change to mixed sleet and freezing rain, so light icing is
possible in the central Appalachian ridges.

The 00z guidance forecasts this system to produce more significant
snow beyond this period (Mon) in NY/New England.

The probability of greater than 0.25 inches of freezing rain is
less than 10 percent.

Petersen