Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 PM EST Mon Mar 08 2021

Valid 00Z Tue Mar 09 2021 - 00Z Fri Mar 12 2021

Days 1 to 3...

...Western U.S. east into the Northern Plains and Upper Midwest...

A deep-layer closed low currently centered west of Vancouver
Island will drop south-southeast through midweek and advance
across California and toward the Southwest U.S. by late Thursday.
As this energy drops down along the West Coast and inland, there
will be a par of shortwave troughs that will eject east-northeast
out ahead of it across the Intermountain West and across the
central and northern Plains.

The first shortwave trough will cross the northern Rockies tonight
and then the far northern High Plains on Tuesday. Deep-layer
ascent facilitated by left-exit region upper-jet dynamics and low
to mid-level frontogenesis will combine with an axis of mid-level
Pacific moisture for a swath of light to moderate snowfall across
areas of the northern Rockies and also the high plains of western
and northern Montana. Portions of the Bitterroots and the Tetons
are expected to see locally over 6 inches of new snow going
through Tuesday night with a few inches possible across areas of
western and northern Montana east of the Continental Divide. The
energy will then quickly exit across southern Canada by early
Wednesday.

The second shortwave trough ejects inland across central
California by Tuesday evening and then quickly traverses the
southern Great Basin and the central Rockies going through early
Wednesday. A combination of favorable upper-level jet support,
frontogenetical forcing, and orographics will favor areas of heavy
snow across the Uinta range of northeast Utah, the high country of
northwest Colorado, and spreading east-northeast across large
areas of central and eastern Wyoming including the Wind River
range, the Big Horns, the Laramie Range, and the Black Hills. All
of these areas are expected to see locally a foot or more of new
snow going through late Wednesday.

This energy is then expected to eject up across the northern
Plains and the upper Midwest Wednesday night into early Thursday.
This will allow an area of deepening surface low pressure to ride
northeast from the central Plains to the U.P. of Michigan and
favor an increase in low-level moisture transport and warm-air
advection for an expanding swath of at least moderately heavy
precipitation. Right now, it appears the thermal profiles will be
largely conducive for an area of accumulating snow that will
streak out across areas of southwest South Dakota and northwest
Nebraska up through much of west-central to northeast Minnesota
where there should be an axis of favorable frontogenetical forcing
and track of a mid-level deformation axis. Thus, expecting locally
at least a few inches of new snowfall with this and especially
over northeast Minnesota as the low to mid-level low centers
associated with this system deepen with time. In addition to the
snow, a narrow transition stripe of at least some light freezing
rain and sleet will also be possible across central Minnesota and
northwest Wisconsin, but the icing threat is expected to be rather
minimal at this time.

Aside from the shortwave impulses, as the aforementioned closed
low advances down across California by Wednesday and Thursday,
there should be a notable increase in deep-layer Pacific moisture
transport along with a lowering of snow levels as the height falls
arrive. This coupled with rather strong dynamic ascent and
orographic forcing will favor heavy snow across the length of the
Sierra-Nevada where locally 1 to 2+ feet of new snow is expected
going through Thursday. Moisture and energy will spread down
through the mountains of southern California too which will favor
locally as much as 6 to 12 inches, and especially for the higher
terrain of the San Bernadino mountains. Farther inland, the influx
of Pacific moisture and good orographic forcing will yield
accumulating snow for the interior of the southern Great Basin,
the Wasatch of Utah and also gradually the western Colorado high
country again where several inches of new snow is expected on
Thursday with the approach of the upper low.

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

Orrison