Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
443 AM EDT Mon Apr 05 2021

Valid 12Z Mon Apr 05 2021 - 12Z Thu Apr 08 2021

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

A positively tilted upper trough shifting southeast from the
Pacific Northwest this morning will split into its own southern
stream closed low later today. This low will then track to
northern UT through tonight and north-central CO through Tuesday
before ejecting east across the central Plains/KS Tuesday night
through Wednesday. Moderate precip, snow levels 4000 to 5000ft
over southwest MT will be enhanced tonight as the closed low slows
and is able to draw in moisture of Gulf origin west across WY as
lee-side low pressure develops near the CO/KS border. It's on the
eastern slopes of the Big Horn, Wind River, and Absarokas that the
moisture, topographical lift and cold combine to produce the
heaviest snow of the system late tonight into Tuesday with these
areas with 70 to 80 percent likelihood for 6 or more inches in Day
1.5 snow probabilities. Snow levels drop to ground level tonight
in the Big Horn Basin which is beneficial for the area of
north-central WY which is currently under extreme drought
conditions.

As the low spills onto the plains and the lee-side surface low
becomes dominant, TROWAL formation northwest of the low looks to
line up well with the Black Hills and south through the Pine Ridge
of Northwest Neb. Day 2 snow probabilities for the Black Hills are
40 to 50 percent for 6 or more inches with 10 to 20 percent values
along the Pine Ridge. That much of this comma head banded snow
will occur during the daylight hours of Tuesday does not bode well
for accumulations and elevation should factor heavily for
accumulating snow. A lack of cold air farther east should being a
fairly quick end to snow on the northwest side of the low early
Wednesday as the low lifts toward the Upper Midwest.


...Pacific Northwest...
Day 3...

The next trough shifts down the Alaskan Panhandle Tuesday night
and the BC Coast Wednesday before shifting southeast across WA
Wednesday night. A moderate surge of Pacific moisture ahead of the
trough/cold front brings moderate precip rates and snow levels
around 2500ft. Day 3 snow probabilities are moderate to high for 8
or more inches in the Olympics and WA Cascades.



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

Jackson