Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Sun Apr 04 2021

Valid 12Z Sun Apr 04 2021 - 12Z Wed Apr 07 2021


...Northern Rockies to the North-Central High Plains...
Days 1-3...

A positively-tilted upper trough shifts southeast over western WA
today and through eastern OR tonight before the southern stream
spins off as a low over ID Monday which then tracks to the
CO/NE/KS border through Tuesday night. Decent agreement remains
with non-GFS 00Z guidance as the GFS remains more progressive with
the low starting Monday night over southwest WY. This system looks
to have a moderately cold core with snow generally contained to
higher elevations of the northern Rockies with snow levels
generally 5000 to 6000ft though they do drop to 4000ft on the
north side before the precip ends, bringing snow into the lower
elevations of ID, western WY and western MT. Widespread moderate
precipitation is expected with Day 2 snow probabilities of 4 or
more inches 40 to 70 percent from north-central ID across
southwest MT and northwest WY while Day 2 snow probabilities of 8
or more inches are generally under 30 percent.

The storm shifts east of the WY Rockies Monday night and onto the
north-central Plains through Tuesday night. Notable Day 3 snow
accumulations remain in the higher elevations though the potential
for a comma head band that brings accumulating snow to the
north-central High Plains will need to continue to be monitored,
particularly for Tuesday night. As of now Day 3 snow probabilities
for 4 or more inches is moderate for the Wind River and Bighorn
Ranges as well as the Black Hills with 10 to 20 percent probs of 2
or more inches on the Pine Ridge of northeast Neb.

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

Jackson