Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
456 PM EDT Fri Apr 10 2020

Valid 00Z Sat Apr 11 2020 - 00Z Tue Apr 14 2020


...Northern Rockies through the Central Plains and into the Great
Lakes...

A complex and long lasting winter storm will begin tonight with a
cold front dropping southward through Montana and North Dakota.
This front will be pushed southward by an accompanying shortwave
digging through southern Canada. In addition to this cold front,
large scale synoptic ascent will begin to increase in response to
the approach of a second northern stream shortwave and the LFQ of
a jet streak entering from the NW. These features in tandem may
spawn a weak wave of low pressure along the boundary, aiding in
upslope flow behind the cold front. This will spread precipitation
in the form of heavy snow from near Glacier N.P. southeast into
the Big Horns of WY and towards the Black Hills of SD. WPC
probabilities are moderate to high for 4 inches across the entire
zone, including the foothills of the mountains, with more than 12
inches likely in the higher terrain.

By Sunday, more aggressive synoptic forcing begins to take shape
as both a northern stream and southern stream impulse begin to
interact. As this occurs, the southern jet streak will intensify,
while a northern streak begins to lift off to the northeast. These
features together above a sharp baroclinic zone /cold front/ will
drive lee cyclogenesis in eastern CO, and this surface low will
deepen rapidly as it lifts northeast into the Great Lakes by
Monday. While there remains considerable spread into the exact
track of this surface low, guidance is beginning to converge on a
track that brings it up into central lower Michigan, with a stripe
of heavy snow to the NW of the track. There is likely to be a band
of heavy snow from Nebraska through eastern SD, northern IA,
southern MN, WI, and into the U.P. of MI where strong mid-level
deformation overlaps intense 850-600mb sloped fgen. This band will
likely pivot northeastward Sunday and Monday, bringing with it
heavy snow rates which may exceed 1"/hr as the best forcing
intersects a deep DGZ. The heaviest snow of all three days may
occur across the U.P. of MI, however. It is here, where the
deformation may begin to pivot instead of translate northeast, and
some enhancement occurs due to moist northerly flow along Lake
Superior. WPC probabilities for 4 inches are high from the Black
Hills of SD eastward into far NW IA, and then northeast into the
U.P. of MI. Across this zone, there is likely to be banded
precipitation and some amounts may be much higher. This may be
most significant in the U.P. where WPC probabilities for 12 inches
are already moderate to high.


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

Weiss