Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EDT Tue Oct 20 2020

Valid 00Z Wed Oct 21 2020 - 00Z Sat Oct 24 2020


...Northern Rockies to northern Great Plains...
Days 1-3...
A significant early season snow event is likely stretching from
the Northern Rockies and into the northern Great Plains Wednesday
night through Friday morning.

Shortwave trough rotating around the large cyclonic gyre centered
near the Hudson Bay will dig into the Northern Rockies late
Wednesday and then pivot across the northern Plains, exiting the
western Great Lakes on Friday. This trough will be accompanied by
a strong jet max which will place its RRQ atop the region to
enhance deep layer ascent, with the interaction of the two driving
surface low development in the High Plains, and elongating into
the upper MS VLY. Northwest of this low, a steeply sloped
mid-level wave will move from WY northeast into the Arrowhead of
MN, and a swath of heavy wintry precip is likely.

Guidance has trended a bit south this aftn, and the preferred
blend included a heavy weight of the GFS/GEFS/ECENS/ECMWF due to
their consistency and agreement. This suggests there will be heavy
snowfall in the terrain of ID/MT southward into WY. With a more
broad band of snow from far northern SD into ND, eastward across
MN. Some of this snowfall could be very heavy as isentropic ascent
at 290-295K lays beneath an fgen band which is likely to intensify
through the RRQ of the upper jet streak, and some -EPV suggesting
potential CSI. The combination of these forcings should produce
snowfall rates which could well exceed 1"/hr at times. However,
the strongest ascent may overlap the p-type transition zone, and
forecast soundings indicate the potential for sleet, with some
freezing rain also possible. It is possible that some dynamic
cooling will overcome this subtle warm nose, especially in ND/SD,
with more robust WAA driving a better chance for mixed precip into
MN.

The heaviest snow from this event is likely in two areas. The
first is in the high terrain of the Northern Rockies, Absarokas,
and Big Horns, initially at 4000-5000 ft but falling to 1000-2000
as the event winds down. WPC probabilities are high for 8" across
these ranges, with amounts in excess of 12" possible. The second
maxima is more associated with the better mesoscale lift NW of the
surface wave into SD/ND where WPC probabilities for 6" exceed 40%
from south-central ND, along the ND/SD border, eastward towards
the Arrowhead of MN. Locally 12" is possible in a few locations.


...Washington Cascades...
Day 3...

A shortwave embedded within the broad cyclonic flow, and an
associated Pacific jet streak will dig down from British Columbia
Friday to spread moisture into Washington State. Ascent through
height falls and an associated diffluent LFQ will produce ascent
as mid-level RH briefly saturates invof a 700mb wave tracking into
Idaho. Snow levels are forecast to be generally 2000-3000 ft, so
precip should be rain in the lowlands. However, the synoptic
ascent combined with low-level upslope flow could produce heavy
snow, primarily in the Washington Cascades. WPC probabilities for
6" are greater than 50%.


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

Weiss