Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Fri Oct 16 2020

Valid 00Z Sat Oct 17 2020 - 00Z Tue Oct 20 2020


...Northern Rockies...
As a sheared mid-to-upper level shortwave drops southeast into the
region, models shows a surface wave developing east of the
Canadian Rockies and tracking southeast across central and eastern
Montana Friday evening.  This, along with a trailing frontal zone
settling southwest across the region, is expected to support
accumulating snow across portions of western and central Montana
and northern Wyoming, with locally heavy totals possible,
especially along the southwestern Montana and northern Wyoming
ranges late Friday into Saturday.  As the lingering boundary
continues to support periods of snow across this region,
additional energy dropping farther to the west into the Pacific
Northwest may support some heavy amounts west of the Divide over
northwestern Montana on Sunday.

...Northern Plains and Upper Midwest...
Models continue to show the previously noted clipper system
turning quickly to the east across South Dakota Friday night
before lifting northeast across southern Minnesota and Wisconsin
on Saturday.  Guidance continues to show low-to-mid level
frontogenesis supporting a stripe of accumulating snow across
northern North Dakota to northern Minnesota.  While the
progressive nature of the system should limit the potential for
widespread heavy accumulations, mesoscale banding is expected to
produce periods of moderate to heavy snow and enhance the
potential for locally heavy accumulations.  This is shown by WPC
PWPF which shows pockets of probabilities greater than 50 percent
for accumulations exceeding 4-inches.  As the low lifts across
northern Wisconsin into the western U.P. of Michigan,
lake-enhanced totals appear likely along the higher terrain of
Minnesota's North Shore.

...Northeast...
In response to a negatively-tilted upper level shortwave moving
from the Ohio valley into the northeastern U.S., low pressure is
expected to deepen and move north along the New England coast
early Saturday, before crossing eastern Maine into Atlantic
Canada.  While all rain is expected for most areas, a change to
wet snow is expected for the mountains of northern England.  While
mostly minor amounts are expected, some areas including the
Presidential Range in northern New Hampshire could see more than a
few inches of accumulation.

For Days 1-3, the probability of significant ice accumulations is
less than 10 percent.

Pereira