Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 AM EST Sat Mar 13 2021

Valid 12Z Sat Mar 13 2021 - 12Z Tue Mar 16 2021

...Southwest to the Central Rockies and High Plains...
Days 1-2...
A major storm system will bring heavy snow to portions of the
central Rockies and the central High Plains Saturday through early
Monday.  This includes the I-25 corridor from Cheyenne to Colorado
Springs.

A deep upper low currently moving into the southwestern U.S. is
expected to move into the Four Corners region later today. 
Ongoing high elevation snow is forecast to continue across
portions of Utah and Arizona.  While widespread heavy amounts are
not expected, some areas including portions of the Mogollon Rim
and White Mountains in central to eastern Arizona may see several
inches of additional accumulation on Saturday.

Meanwhile, strengthening southerly, low-level winds will begin to
transport deeper moisture up through Texas into the central High
Plains, where a developing wave will begin to direct that moisture
back west into the high terrain.  As the upper low moves farther
east, increasing moisture and large-scale ascent will support
widespread heavy snows developing across the central Rockies and
High Plains on Saturday, continuing into Sunday.  For the Day 1
period (ending 12Z Sunday), WPC PWPF continues to shows widespread
probabilities of 50 percent or greater for snow accumulations of
8-inches or more covering much of southeastern Wyoming and central
Colorado. This includes the urban I-25
corridor from Cheyenne to Colorado Springs.  WPC PWPF shows that
heavier totals on the order of 18-inches or more are likely just
east of the corridor into the Front Range and the Laramie
Mountains.  In addition to these areas, heavy snow accumulations
are also expected across the Uinta Mountains in northern Utah and
southwestern Wyoming, as well as along the Wind River Range and
southern Big Horn Mountain in Wyoming.  Relatively lighter, but
significant accumulations will extend east into the High Plains,
with WPC PWPF indicating high probabilities for accumulations of
4-inches or more reaching out into the western Nebraska Panhandle
on Day 1.

Models show the low moving east of the central Rockies, with heavy
snow continuing across portions of the High Plains and Rockies on
Sunday.  Additional accumulations of 8-inches or more are likely
for portions of the Front Range, southeastern Wyoming, including
the Cheyenne Metro, the Nebraska Panhandle, and southwestern South
Dakota, including the Black Hills.  Snows will likely diminish
Sunday night as the low-to-mid level center moves east.  Storm
total amounts of 2-feet or more are likely from the southeastern
Wyoming I-25 corridor back into the Laramie Mountains, as well as
along the Colorado Front Range.  Storm totals of 2-feet or more
are also likely across portions of the Wind River Range and Uinta
Mountains.

...Northern Plains and Upper Midwest...
Days 2-3...
As the deep low centered over eastern Colorado Sunday evening
begins to lift farther northeast, precipitation will shift
northeast across the region, with rain changing to snow across
portions of eastern South Dakota and western Iowa, with mainly
snow developing across southwestern Minnesota Sunday night into
early Monday.  Snow is expected to continue across the region,
reaching into southern Wisconsin and northern Illinois during the
day on Monday.  Models have been trending cooler, with higher
probabilities for accumulations of 4-inches or more expanding
across southeastern South Dakota, southwestern Minnesota and
northwestern Iowa.

...Oregon and California...
Days 2-3...
Energy moving through the base of a sharp upper trough moving
across the eastern Pacific will develop a closed low west of
Oregon on Sunday, that will then drop southeast across California
into Nevada on Monday.  This will support lower elevation snows
across southeastern Oregon into northwestern California with
significant high elevation snows possible from the Klamath
Mountains and southern Cascades to the Sierra.

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

Pereira