Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EDT Thu Apr 08 2021

Valid 00Z Fri Apr 09 2021 - 00Z Mon Apr 12 2021


...Pacific Northwest through Northern High Plains...
Days 1-3...

A potent shortwave digging through the Northern Rockies this
evening will drop southeast into the Central Plains producing
sharp height falls and PVA coincident with an upper divergence
maxima moving across the Rockies. Beneath this trough, a surface
cold front will drop southeast producing strong CAA, while a wave
of low pressure moves from Montana to Minnesota ahead of that
front. These features together will spread precipitation across
WY, SD, and NE, in the form of snow across the higher terrain.
Moderate to heavy snow is likely in the Big Horns on D1, with
heavy snow also expected in the Black Hills of SD. The
probabilities for 6" are highest in the Black Hills where N/NW
flow will drive intense upslope ascent into a saturated DGZ, and
snowfall rates of 1"/hr are possible at times. Locally more than
8" is possible in the highest terrain.

After a brief respite across the region, wet weather is forecast
to return to the Northwest by late Friday as the next system digs
along the coast of British Columbia into the region.  This will
bring a round of moderate to heavy snow into the Olympics and
northern Cascades late Friday into early Saturday, with the WPC
probabilities showing a high risk for accumulations of 6-inches or
more.  Like the previous system, this second wave is expected to
move progressively to the east, generating mainly light snows as
it moves into the northern and central Rockies on D3 where WPC
probabilities for 6 inches are confined to the highest terrain of
the Absarokas in NW WY.

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

Weiss