Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
502 PM EST Fri Mar 12 2021

Valid 00Z Sat Mar 13 2021 - 00Z Tue Mar 16 2021

...Intermountain West to the western High Plains...

The 12Z models continued to show a strong signal for an impressive
late season snowstorm that will hammer much of the Front Range and
High Plains through Sunday evening.  This will include the I-25
urban corridor from Cheyenne to Colorado Springs and adjacent
portions of the western High Plains. 

An anomalous and slow moving upper level low in the southern
stream flow, currently over the Desert Southwest, will continue to
very slowly track eastward across the Four Corners region through
Saturday, and then reach western Kansas by Monday morning.  Light
to moderate mountain snow is expected to continue from the
Mogollon Rim in central Arizona to the Wasatch Range in Utah, with
some heavy patches of 6-12 inch totals possible through Saturday
afternoon, as a period of deep southerly flow ahead of the
approaching low along with favorable upper forcing helps to
increase snowfall rates. 

Surface cyclogenenis in response to the upper low will take place
across northeastern New Mexico/southeastern Colorado during the
day Saturday, and this low will likely remain nearly anchored in
place through Sunday evening.  Low level easterly upslope flow
will increase north of this low with copious moisture surging
westward toward the Front Range of the central Rockies.  A rather
pronounced TROWAL is expected to develop, and in combination with
strong 850-700mb frontogenesis, this will produce a swath of heavy
snow with snowfall rates of 2+ inches per hour possible across
portions of northeast Colorado and southeast Wyoming.  This axis
of heavy snow is expected to gradually lift northward through
Sunday and then gradually weaken by early Monday.  Two things to
monitor over the next few forecast cycles will be the evolution of
the dry slot over southeast Colorado and how far north it gets,
and also if the expected convection over western Texas and
Oklahoma ends up inhibiting the degree of moisture transport into
the region.  Right now, the potential exists for some 2-4 foot
snowfall totals for the entire three day period from the western
suburbs of Denver to the greater Casper, WY area.

...Upper Midwest states...

For the Day 3 period on Monday, the occluded surface low will
likely be over Kansas/Oklahoma, and a separate swath of moderate
snow will likely develop across portions of southern Minnesota and
northern Iowa.  This would be in response to low-mid level
frontogenesis along with a shortwave disturbance passing through
the region.  Given marginal surface temperatures, this would
likely be a wet snow with relatively lower SLRs.  The WPC
probabilities are moderate for 4 inch snow totals during this
time, and a few hundredths of an inch of ice accretion is possible
across portions of Illinois and Indiana well north of the
approaching warm front.

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

Hamrick