Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Tue Apr 06 2021

Valid 00Z Wed Apr 07 2021 - 00Z Sat Apr 10 2021

 ...Northern Rockies onto the North-Central High Plains...
Day 1...

A closed upper low ejecting east out of the Intermountain West
tonight tracks through the central Plains during the day on
Wednesday. As the upper low becomes positioned over eastern CO
overnight, low level warm air advection aloft aids in the
formation of a narrow stripe of 925-700mb frontogenesis over
southern ND and northern NE. This is supported on both global and
CAM guidance, but thermal profiles within the boundary layer are
causing a wider range of snow accumulation outcomes. In addition,
where this corridor of low level forcing determines the
orientation of a pivoting deformation zone that could lead to
locally heavy snowfall rates tonight and into Wednesday morning.
Day 1 probabilities suggest as much as a 30 percent chance for 4
or more inches of snow in southwest SD and northwest NE.
Meanwhile, persistent upsloping flow over the Black Hills keeps
periods of moderate-to-heavy snow in the forecast overnight and
Wednesday morning. Probabilities of snow totals exceeding 4 inches
are up to 50 percent in the higher elevations of the Black Hills.

Snowfall looks to diminish from late Wednesday morning into the
afternoon hours as low level forcing weakens and drier northerly
flow is entrained on the northwest flank of the upper low. Daytime
heating will also limit accumulations as rates lessen and boundary
layer temperatures warm.


...Pacific Northwest and Northern Rockies...
Days 1-2...

A potent upper level trough diving south off the British Columbia
coast introduces an influx of Pacific moisture to the Pacific
Northwest Wednesday afternoon and into Wednesday night. Snow
levels will fall in the Olympics and Cascades as the trough tracks
overhead Wednesday evening. In addition, a steady barrage of
upsloping westerly flow leads to typical topographic enhancement
of snowfall rates in the more orographically favored locations.
The result is accumulating snowfall in the Olympics and Cascade
Range where moderate probabilities of greater than 8 inches of
snow are present. As the upper level disturbance heads east, warm
air advection out ahead of it will induce periods of mountain snow
in the Northern Rockies. The Bitterroots and western MT ranges can
expect periods of snow to transpire Thursday morning with the
higher elevations ranging between low and moderate probabilities
for greater than 4 inches of snowfall. Snow is also forecast to
reach parts of the Absarokas, Tetons, and Big Horns late Thursday
and into Friday, but the best forcing aloft resides north of these
mountain ranges, and thus lesser totals are anticipated at this
time.


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

Mullinax