Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
444 AM EDT Mon Mar 15 2021

Valid 12Z Mon Mar 15 2021 - 12Z Thu Mar 18 2021


...Upper Midwest/Great Lakes/Central Appalachians...
Day 1...
An expansive northwest-southeast oriented area of precipitation
associated with a deep low moving into the central Plains will
continue to shift northeast, with rain changing to snow across
southern Minnesota, Iowa, and northern Illinois this morning. 
Some areas, especially those across southeastern Minnesota and
northeastern Iowa could see several inches of snow supported by a
period of strong upper divergence and low-to-mid level
frontogenesis.  This is supported by the several HREF members,
which show snowfall accumulations of over an inch/hr as the
developing bands move east across the region later this morning
and early afternoon.  The overnight WPC PWPF shows a widespread
area of 50 percent or greater probabilities for snow accumulations
of 4-inches or more centered over southeastern Minnesota,
northeastern Iowa, and far southwestern Wisconsin.  Within this
area, WPC PWPF shows a narrow stripe of high probabilities for
snowfall accumulations exceeding 6-inches.  Farther southeast, the
forecast remains more uncertain, with a combination of
precipitation types lifting north across central to northern
Illinois, Indiana and Lower Michigan later today.  Still expecting
mostly sleet and/or snow, especially across northern Illinois and
northern Indiana into Lower Michigan, where low-to-mid level
frontogenesis will help to support heavier precipitation rates --
keeping the column cool.  Freezing rain may be more of a threat
farther to the south across central Illinois and Indiana, although
significant ice accumulations are not expected, with probabilities
continuing to decrease.  Farther to the east, a wintry mix with
accumulating ice is possible as precipitation spreads across the
central Appalachians Monday evening and overnight.  WPC PWPF
continues to show some high probabilities for ice accumulations of
0.10-inch or more along the southern Allegheny Mountains by 12Z
Tuesday.

...California to the Central and Southern Rockies...
Days 1-2...
A compact upper low dropping south into Northern California Monday
morning is forecast to continue southeast across California into
the Southwest by early Tuesday.  Mountain snows will spread south
along the Sierra and into the Transverse ranges later today, with
locally heavy amounts possible across the higher terrain.  Expect
generally light accumulations farther east across the Nevada and
Utah mountains, as well as across the higher terrain of northern
and central Arizona.  This system is expected to move east across
the Southwest and into the southern Rockies on Tuesday.  As the
system moves across New Mexico and into the Texas Panhandle late
Tuesday into early Wednesday, a period of strong northeasterly
flow along the northwest side of the low, along with favorable
forcing aloft, may support a period of heavy snow along and east
of the Sangre de Cristo Mountains in southeastern Colorado.

Pereira