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

Valid 12Z Tue Mar 23 2021 - 12Z Fri Mar 26 2021



...Great Basin and Southwest to the Central and Southern Rockies...
Days 1-3...
A well-defined deep trough develops over the Southwest late today
into early Wednesday.  This will bring snow across Utah and
northern Arizona and through the Four Corners region today. 
Locally heavy accumulations are possible along the central
southern Utah mountains and along the northern Arizona high
terrain such as the Mogollon Rim.

As a broad 700 mb low and area of increasing ascent associated
with the left-exit region of the upper jet interact with an area
of deepening moisture, the potential for heavy snow is expected to
increase from the eastern Mogollon Rim and White Mountains of
eastern Arizona into western New Mexico and the San Juan Mountains
of both Co and NM.  Meanwhile, post-frontal, northeasterly winds
along with the supportive upper level dynamics will encourage
heavy snows developing farther east along the Sangre de Cristo
Mountains and the adjacent high plains of southeastern Colorado
and northeastern New Mexico Tuesday night into Wednesday.  Snow is
expected to continue into late Wednesday before diminishing as the
leading shortwave ejects northeast into the central Plains. 
Heaviest storm totals are expected along the eastern slopes of the
Sangre de Cristo and the adjacent mesas and plains, with WPC PWPF
showing high probabilities for snow accumulations of a foot or
more.

On Day 3, Thu., the next 700 mb wave crossing NV and the CO River
Valley continues east across UT and then CO, enhance moisture
pools in advance of the 700 mb front.  Combined synoptic ascent
with the front and orographic enhancement in windward terrain of
the UT Wasatch/Uinta Mountains and then into the CO San Juan
Mountains should lead to enhanced snow totals in these ranges. The
highest probability of 4 and 8 inches of snow on Day 3/Thu is in
the CO San Juan mountains.  The highest Days 1-3 snow totals are
forecast across the San Juans of CO/NM and Sangre DeCristo
Mountains of NM to adjacent southeast CO, where 1 to 2 feet are
expected over the next 3 days.


...Pacific Northwest through the Northern Rockies...
Days 1-3...
The upper trough moving east across MT and WY today will result in
a cold front moving south through the northern Rockies. 
Post-frontal, low-to-mid level, northeasterly winds will promote
snow showers east of the Divide, with some locally heavy
accumulations possible along the Absaroka Range, Beartooth
Mountains and Bighorn Mountains.  Another shortwave trough diving
south into the Pacific Northwest will bring mountain snows back
into the WA/OR Cascades on Wednesday, continuing inland to the OR
Blue Mountains and then the ranges of central to northern ID and
northwest MT.  On Day 3/Thu, the upper trough drifts east across
MT and WY. Sporadic 300 mb divergence maxima occur over the ranges
of south central MT to northern WY and southeast ID, bringing
periods of snow, with several inches expected in the Absaroka
Mountains to the Beartooth and Bighorn Ranges. The vertical
motions are modest but the slow movement of the broad 700 mb
circulation should lead to extended snow showers in these ranges.

...Upper Mississippi Valley/Minnesota...
Day 2...
An upper trough/low currently is expected to produce mainly rain
as it tracks farther northeast from the central Plains on Tuesday
and into the upper Mississippi Valley Tuesday night.  In the mid
level deformation zone, dynamic cooling should produce a brief
period of snow across central NE, and then eastern SD. As the
circulation crosses MN, a northern stream trough helps to bring
colder air in behind the system, increasing the potential for
accumulating snow across northeastern Minnesota Wednesday morning,
with at least a few inches likely across portions of the
Arrowhead. Up to half an inch liquid equivalent in the form of
snow is shown in the NAM and SREF, indicating potential for 4 to 6
inches of snow.  The event winds down Thu as the circulation moves
across the border into Canada.

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

Petersen