Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Sat Apr 18 2020

Valid 12Z Sat Apr 18 2020 - 12Z Tue Apr 21 2020



...Northern Rockies...
Days 1-2...

A mid-level trough will rotate through the broad cyclonic flow
tonight through Saturday. Anticyclonic post-cold frontal flow will
promote upslope flow for eastern slopes of the MT Rockies
including Glacier NP Saturday, and then northern WY Saturday night
into early Sunday before tapering.  Ascent will be provided by
modest 700 mb convergence and frontogenesis. Day 1 WPC
probabilities are moderate to high for 4 inches in the vicinity of
Glacier National Park, and low to moderate into the higher terrain
of the Absarokas and Big Horns.  The event ends Sun as the upper
trough moves downstream and drying aloft occurs.
 
Day 2...
The probability of 4 or greater inches of snow is less than 10
percent.

Day 3...

Ca Sierra Nevada to Great Basin...
The models indicate a closed 500 mb low moves slowly onshore and
across CA into the southwest.  Significant timing differences
exist, with the 00z ECMWF having slowed down forward motion.
Moisture wrapping around the circulation moves inland into
portions of the southern CA Sierra Nevada and then into the ranges
of central NV and UT, and eventually into the mountains of western
Co.  The models are forecasting modest lift with model QPF amounts
generally under a quarter inch and resultant snow amounts in most
areas under 4 inches.  Local enhancements in terrain are possible.


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

Petersen