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

Valid 00Z Sun Apr 19 2020 - 00Z Wed Apr 22 2020



...Montana, Wyoming, South Dakota...
Day 1...

A modest mid-level trough dropping NW through broad cyclonic flow
will interact with a cold front draping across MT/WY to induce a
wave of low pressure tonight and Sunday. Northeast flow behind the
cold front will become upslope into the terrain of the Absarokas,
Big Horns, and Black Hills of SD, which will enhance the synoptic
ascent driven via height falls and 700mb fgen. A band of moderate
snow is likely in the higher terrain of these ranges, generally
above 6000 ft. WPC probabilities for 4 inches are high in the
Absarokas, and moderate elsewhere.
 

...Sierra Nevada through the Four Corners...
Day 3...

A closed low dropping into central CA will open, but remain a
potent vorticity impulse as it digs through the Great Basin and
into the Four Corners Tuesday. Sharp height falls coincident with
weak upper diffluence and robust 700mb convergence will drive
ascent in a swath west to east along and north of this impulse.
Moist advection associated with this feature will produce
precipitation across much the area, but the thermal structure is
such that only the highest elevations, generally above 8000 ft,
will receive heavy snowfall. WPC probabilities for 4 inches are
moderate to high in the Sierra, Wasatch, Uintas, and San Juans on
D3, with some lighter accumulations also possible in the NV
mountains of the Great Basin.


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

Weiss