Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 AM EST Wed Dec 25 2019

Valid 12Z Wed Dec 25 2019 - 12Z Sat Dec 28 2019


Days 1-3...

...Western U.S....

A series of weak shortwaves lifting northeast along the leading
edge of a broad longwave trough will support mountain snow showers
from central Arizona into the central Rockies on Wednesday.  While
widespread heavy accumulations are not expected, locally heavy
totals are possible.  Areas impacted by the heaviest amounts are
likely to include the northern Utah, western Colorado and
south-central Wyoming ranges.

Meanwhile, a well-defined shortwave will begin to dive south into
the base of the trough.  As the system drops quickly south, light
accumulations are expected across the mountains of northwestern
California and the northern Sierra on Wednesday.  However as an
upper low begins to close off along the central California coast,
heavy precipitation is expected to begin developing across
Southern California by late Wednesday and continue into Thursday. 
Strengthening onshore flow, along with increasing upper divergence
will support heavy precipitation into the mountains of Southern
California.  With snow levels initially between 4000-5000ft on
Wednesday and forecast to drop lower by Thursday, heavy
accumulations are likely for portions of the Transverse Ranges
into the southern Sierra and Peninsular Ranges Wednesday night
into Thursday.  Total accumulations of 1-2ft, with locally heavier
totals can be expected across the higher elevations.  Snows will
diminish as the low moves east of the region into the Southwest
Friday morning.

As the low moves east, this will bring heavy snows back into
portions of the Southwest and the Rockies on Friday into Saturday.
 Areas impacted by heavier accumulations are more likely to
include the White Mountains in eastern Arizona, as well as the San
Juan and Sangre de Cristo Mountains in southern Colorado and
northern New Mexico.

The probability of significant icing (0.25-inch or more) is less
than 10 percent.

Pereira