Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Thu Feb 04 2021

Valid 12Z Thu Feb 04 2021 - 12Z Sun Feb 07 2021

...Upper Midwest across the Great Lakes...
Days 1-3...
An amplified upper trough swings east from the northern Great
Plains today with the associated strong cold front crossing the
Upper Midwest. A brief period of rain or a wintry mix in
prefrontal precip will changeover to snow with most of the QPF
going toward accumulating snow is expected across the mid Missouri
into the mid and upper Mississippi valleys as the front pushes
east Thursday morning. Locally heavy snow is expected,
particularly with strong low-to-mid level frontogenesis along with
favorable upper jet forcing as the trough takes on a negative tilt
from northeast IA, southeast MN, across much of WI and the U.P. of
MI as well as into northern IL where Day 1 WPC PWPF are moderate
to high (higher farther north) for accumulations of 6 or more
inches. The wintry mix expected on the onset is expected to
produce minor ice accumulations (less than 1 tenth inch) across
central and eastern WI, northern IL and much of lower MI and
central OH before precipitation changes over to the snow. By late
today, phasing energy aloft will support an intensifying low that
will translate northeast across the upper Great Lakes.
Northwesterly flow will contribute to lake-enhanced snows across
the western U.P. of Michigan and the western L.P. tonight
continuing in earnest through Friday night. Day 1.5 snow
probabilities for 6 or more inches are moderately high along the
western shore of the L.P. Deep cyclonic flow will continue to
support periods of lake effect snow shows in the lee of the upper
Great Lakes through Saturday.  WPC PWPF indicates that three day
totals of a foot or more are likely across portions of the U.P.
and along the northeastern shores of Lake Michigan.  Meanwhile,
much colder air spreading east, along with low level southwesterly
to westerly flow will support the development of single-band lake
effect snows east of Lake Erie and Ontario on Saturday with low to
moderate probabilities for 6 or more inches over localized areas
east of Erie and Ontario on both Days 2 and 3.


...Pacific Northwest...Northern and Central Rockies...
Days 1-3...
A series of shortwaves embedded within strong northwesterly flow
will arrive this afternoon into interior sections of the Pacific
Northwest. These waves, along with a strong cold front settling
southwest to the crest of the northern Rockies, will support
periods of snow, with locally heavy accumulations expected across
portions of the WA Cascades northern into the central Rockies on
tonight through Saturday night. Areas impacted are expected to
include the WA Cascades, northern Idaho ranges, western to central
Montana ranges, the western and southern Wyoming ranges and the
northwestern Colorado ranges.  WPC PWPF indicates that three day
totals of 24 or more inches are likely across the WA Cascades and
northern Rockies. Cooling temperatures behind the cold front and
expanding precip in the northwesterly flow also bring about
moderate to high probabilities for 4 or more inches over the
Montana High Plains and foothills on Day 3.

Jackson