Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 PM EDT Tue Mar 23 2021

Valid 00Z Wed Mar 24 2021 - 00Z Sat Mar 27 2021

...Minnesota...
Day 1...
An upper low will continue to track north from the central Plains
and lower Missouri Valley -- spreading precipitation across much
of the Upper Midwest and western Great Lakes through late Tuesday
into Wednesday.  Still expecting mostly rain across much of the
region with the exception of northern Minnesota.  As the system
begins to lift northeast through the western Great Lakes and ahead
northern stream trough, cold air on the backside of the system
will support a changeover to a wet snow beginning overnight and
continuing into Wednesday across portions of northern Minnesota. 
With the 12Z runs, the WPC PWPF has shown increasing probabilities
for snow accumulations of 4-inches or more across parts of the
northern Arrowhead.

...Southwest to the Southern Rockies...
Days 1-3...
As a deep upper trough/low moves east across the Southwest, expect
precipitation including high elevation snow to continue across
portions of Arizona, while spreading east and becoming better
organized across parts of Colorado and New Mexico.  Deepening
moisture and large-scale ascent near the track of a low-to-mid
level low moving east across Arizona into New Mexico is expected
to support some light to moderate accumulations through the
overnight from northern and central Arizona into western New
Mexico and southwestern Colorado.  Meanwhile farther east,
post-frontal upslope flow will help develop heavier amounts along
and east of the Sangre de Cristo and Sandia Manzano mountains. 
Heavy snows are expected to develop initially during the evening
hours along and east of the Sangre de Cristo mountains before
shifting farther south overnight into early Wednesday. 
Probabilities still remain high for snowfall accumulations of
8-inches or more for the eastern slopes of the central and
southern Sangre de Cristo mountains and for areas further east
along the Raton Mesa.

Beginning late Wednesday, drier weather is expected as the initial
shortwave lifts out through the Plains.  Dry weather will continue
into late Thursday when another shortwave trough is expected to
dive into the Southwest.  This will bring high elevation snow back
into Utah and Arizona, with some potential for locally heavy
accumulations late Thursday into Friday.  A greater threat for
heavy accumulations is farther east, where a period of
southwesterly inflow ahead of the approaching trough is expected
to support heavy snows developing across the San Juan Mountains.

...Pacific Northwest to the Northern Rockies and Great Basin...
Days 1-3...
A shortwave trough diving south into the northwestern U.S. is
expected to bring mountain snows back into the Northwest and
northern Rockies by early Wednesday.  As the upper system
continues to dig south and amplify, snow will spread farther south
into the Great Basin and along the Rockies on Wednesday and
Thursday.  While some areas, including portions of the northern
Cascades and Rockies will likely see storm total amounts of
8-inches or more, widespread heavy accumulations are not expected.

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

Pereira