Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Mon Oct 02 2023

Valid 00Z Tue Oct 03 2023 - 00Z Fri Oct 06 2023

...Central and Northern Rockies...
Day 1...

A slowly filling 500mb longwave trough will advect eastward
tonight and Tuesday, with 500mb height anomalies falling from -3
to -1 standard deviations according to the NAEFS ensemble by
Tuesday night. Despite the weakening, accompanying height falls,
modest PVA downstream of a pivoting vorticity impulse, and a 300mb
jet streak rotating poleward will provide sufficient ascent to
produce precipitation from the Great Basin into the Northern
Rockies. As the trough moves east, freezing levels will drop from
9500 ft to slightly below 8000 ft. These falling snow levels will
allow for moderate to heavy accumulating snow in the highest
terrain of the Absarokas, Wind Rivers, and Tetons, where WPC
probabilities reach 50-60% for 4+ inches, and local maxima greater
than 12 inches are likely in the highest terrain. Light
accumulations of snow are also possible as far east as the Big
Horn range and south into portions of the CO Rockies, but WPC
probabilities for 4 inches are generally 5-10% in these ranges.

The probability of any ice accumulation across the Continental
U.S. is less than 10 percent.

Weiss