Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Tue Dec 24 2019

Valid 12Z Tue Dec 24 2019 - 12Z Fri Dec 27 2019


Days 1-3...

...Western U.S....

A well-defined mid-to-upper level shortwave centered east of
northern Baja will continue to move through the base of a long
wave trough -- lifting northeast across northwestern Mexico and
the southwestern U.S. on Tuesday.  On Tuesday into early
Wednesday, this will produce widespread precipitation across the
Southwest into the central Rockies, including potentially locally
heavy snows across central into eastern Arizona along the Mogollon
Rim into the White Mountains, as well as the mountains of southern
Utah and western Colorado.  Expect upstream energy moving across
the region to generate additional light snows across portions of
the region through Wednesday into early Thursday.

Meanwhile, another well-defined shortwave is forecast to drop
south along the California coast on Wednesday.  Generally light
snow accumulations are expected across the northwestern California
ranges and the Sierra early Wednesday, with heavier accumulations
beginning to develop farther south later in the day as an upper
center begins to close off along the central California coast. 
Strengthening onshore flow, along with increasing divergence aloft
will support moderate to heavy precipitation developing across
Southern California by Thursday morning.  With snow levels between
4,000-5,000ft, heavy snowfall accumulations are likely along the
Transverse Ranges Wednesday night into early Thursday.  Then as
the low continues to drop south, heavy snows will likely shift
farther south along the Peninsular Ranges on Thursday.

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

Pereira