Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 AM EST Fri Feb 21 2020

Valid 12Z Fri Feb 21 2020 - 12Z Mon Feb 24 2020


...Southern Great Basin/Central to Southern Rockies/Central High
Plains...
Days 2-3...

An upper low will lift onshore Southern California on Sat. Ahead
of this system, deep and robust moist advection will spread
northward on low-level southerly flow out of the Pacific and into
the Four Corners region. PWAT anomalies become quite impressive
during this time, with moisture riding into the cold sector in the
ranges of central to southern NV and UT, then into CO.  Snow
occurs in the mid level deformation zone along the 700 mb low
track as it progresses across southern NV and UT on Day 2 and then
across CO on day 3.  The heavier amounts are expected in the UT
central to southern Wasatch. Heavy snow is possible in the
mountains of the Mogollon Rim, Wasatch, San Juans, Sangre De
Cristos, and the front range of the Colorado Rockies. 

The forecast gets trickier on to the Plains as the models forecast
a lee cyclone development on the high Plains.  Differences in the
strength of the cyclone and how much cold air is available lead to
difference in snowfall potential, with the deeper cyclones in the
NAM and Canadian leading to heavier snowfall amounts in the CO
Plains. 
The UKMET and ECMWF were considerably lighter with snowfall
potential. 

...Pacific Northwest...
End of Day 2 and Day 3...

Snow begins to develop in the WA Cascades as a weakening low level
boundary comes ashore with 700 mb convergence bands crossing the
Olympics and Cascades as a brief period of a coupled 300 mb jet
crosses the region.  The snow picks up in intensity/coverage on
Day 3 as stronger 700 mb theta-e advection and convergence maxima
stream onshore and across western WA and northwest OR. Once the
primary upper trough progresses inland, a second jet max streams
onshore late Sun afternoon/evening into WA. Heavy snow will be
confined to the Olympics and Cascades, with WPC probabilities
indicating potential for a foot of new snow in the WA Cascades on
Day 3.

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

Petersen