Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
441 PM EST Thu Mar 11 2021

Valid 00Z Fri Mar 12 2021 - 00Z Mon Mar 15 2021


...Great Basin/Southwest to the Central Rockies and High Plains...
Days 1-3...
...Major Snow Storm Coming to Colorado and Wyoming...

Confidence is high that a major winter storm will bring heavy snow
to portions of the central Rockies and High Plains beginning on
Friday and continuing through Sunday.  This includes the I-25
corridor from Cheyenne through Denver.  Multiple feet of snow are
possible in the foothills and front range of northern CO and
southeast WY.

This storm is associated with a deep upper trough/low currently
centered over Nevada. On Day 1, the primary snow is expected over
the UT Wasatch, mountains of southern NV, and Mogollon Rim of AZ. 
The 700 mb circulation advects moisture north across AZ in NV and
UT, with lift occurring to produce the snow tonight starting in
the Mogollon Rim of AZ up to the UT Wasatch late tonight through
Fri.  WPC PWPF indicates a moderate probability of 4 inches and
low probability of 8 inches most areas, with one max of moderate
probabilities for 8 inches in the Mogollon Rim.   

On Day 2, as the upper low begins to slowly move east, snow is
expected to continue across many of these same areas through
Friday into early Saturday, with additional light to moderate
accumulations likely, especially along the Mogollon Rim and the
Utah Mountains.

By early Saturday, strengthening warm/moisture advection on the
high Plains will produce increasing integrated vapor transport
across the high Plains and into the foothills and front range of
northern CO and WY.  The strongest persistent  700 mb jet develops
in NE coming into WY. Increasing moisture and large-scale ascent
will support widespread heavy snows developing across the central
Rockies and High Plains on Saturday, continuing into Sunday. The
strong low level jet leads to anomalously strong upslope
conditions in northeast CO and southeast WY, with good multi-model
agreement on heavy snow. 

For the Day 3 period, WPC PWPF shows widespread probabilities of
50 percent or greater for snow accumulations of 12 inches covering
much of southeastern Wyoming into the foothills and front range of
northern CO.  Heavy snow is expected in the urban I-25 corridor
from Cheyenne through Denver.  The 72 hour WPC PWPF shows that the
highest probability for 2 to 3 feet of snow is in the foothills
into the Front Range of northern CO/southern WY.

One major component of uncertainty is are how far north the
QPF/snow extends out of NE into and across southern SD, with
remaining large spread in solutions based on the 850-700 mb low
track north-south differences.  Another component of uncertainty
is further east on the CO Plains and NE Plains, temperatures are
initially too warm for snow.  As the low drifts east, the
rain/snow transition line drifts east across the NE panhandle and
CO Plains.  The operational NAM produces dynamical cooling and a
faster transition than other models.  The NAM is offset by other
models plus the NAM COnus Nest, which remains warmer longer than
the parent NAM, and thus has  shorter duration snow in eastern CO
and western NE. 

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

Petersen