Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Sat Apr 17 2021

Valid 12Z Sat Apr 17 2021 - 12Z Tue Apr 20 2021


...CO/NM Rockies...
Days 1-2...

Water vapor imagery this morning shows an upper low over southwest
Utah that is adverting high level moisture towards southern CO and
northern NM.
The NAM forecasts a 700 mb low to drift east across southeast UT
into southern CO today, and drift south into NM tonight.
Bands of 700 mb convergence rotate around the low and combine with
difluent flow aloft to produce ascent.
QPF and snow maxima re expected in the San Juans of southwest
CO/northern NM, and also further east in the Sangre De Cristos.
WPC probabilities on D1 are high for 6 inches in the San Juans and
Sangre De Cristos, with locally 12 inches likely in the best
upslope regions.
On Sunday, the weak 700 mb low tracks across eastern AZ southward
to the Mexican border, with modest convergence downstream from the
low supporting snow showers as the convergence maxima cross the
mountains of eastern AZ and western NM. Several inches are
possible before the wave departs and ascent decays Sun night.

...MT/WY Rockies Day 2 and WY/CO Rockies and Central Plains Day
3...

A potent cold front will drop southward across the Northern Plains
and then into the Central Plains. Frontal convergence/pooling of
moisture and upslope flow will be the primary mechanisms for
ascent, combined with divergence maxima aloft as the jet moves
south across Montana into Wyoming Day 2. WPC probabilities on D2
are high for 6 inches in the Rocky Mountain Front/Glacier National
Park, Absarokas, and Big Horn Mountains. The heaviest snowfall is
likely in the Big Horns where more than 12 inches is possible.

On Day 3, Tue., the upper jet continues to build south, crossing
southern Wyoming and northern CO before moving east out on the
central Plains. Upper divergence maxima combine with 700 mb
convergence to produce ascent, starting in the WY Wind River,
Snowy and Laramie ranges and then building south across the
foothills and front range of CO Monday to Monday evening.  Several
inches are likely in these areas, with isolated totals up to a
foot possible. The highest 8 inch probabilities are moderate in
the front range of northern CO 12z Mon-12z Tue.

As the front moves south steadily across the central Plains,
precipitation occurs in association with the front and enhanced by
300 mb divergence maxima in the right entrance of the 300 mb jet
crossing Nebraska.  This favors post-frontal snow for several
hours centered on southern Nebraska and northern KS.
Probabilities are low for 4 inches of snow in this area.

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

Petersen