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

Valid 00Z Mon Apr 05 2021 - 00Z Thu Apr 08 2021


...Northern Rockies and High Plains...
Days 1-3...
An upper trough dropping south across western Canada will continue
to flatten an upper ridge extending into the northwestern U.S. 
Models continue to show a split in the trough developing, with a
closed low forming within the southern stream as it settles
southeast across the Pacific Northwest on Monday.  The system is
expected to turn east -- moving across southern Idaho Monday night
and then along the Wyoming-Colorado border on Tuesday, and into
the central Plain by early Wednesday.  This will usher in much
colder temperatures from the Northwest to the northern Rockies on
Monday into Tuesday -- supporting some lower elevation snows
across portions of Idaho, western Montana, and Wyoming.  Still
expecting mostly light to moderate snow accumulations for most
areas.  While the WPC PWPF shows high probabilities for storm
total accumulations of at least 2-4 inches covering much of
central Idaho, southwestern Montana, and western Wyoming,
probabilities drop off quickly for higher thresholds.  High
probabilities for accumulations of 8-inches or more are largely
confined to the higher elevations of the western Wyoming and
southwestern Montana ranges, including the Absaroka and Wind River
ranges.  As the system moves east, rain changing to snow expected
farther east across portions of eastern Wyoming into western South
Dakota and western Nebraska on Tuesday, with significant
accumulations likely across the Black Hills.  The latest PWPF
shows snow accumulations of 4 inches or more are likely, with
locally heavier amounts possible for the Black Hills.  At least an
inch or two is likely farther to the south across southeastern
South Dakota and the northern Nebraska Panhandle, with some
guidance members showing some potential for locally heavier totals.

...Northern Maine...
Day 1...
A deepening closed low centered east of Nova Scotia is expected to
drift back to the west Sunday night into Monday, before weakening
and moving back east on Tuesday.  Snow is expected to push from
east to west across much of interior Maine Sunday evening through
the overnight, with an inch or two likely across northern Maine,
with some locally heavier amounts possible across the mountains.

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

Pereira