Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 AM EST Sat Feb 22 2020

Valid 12Z Sat Feb 22 2020 - 12Z Tue Feb 25 2020


...Southern Great Basin through Central High Plains...
Days 1-3...

On Saturday a 700 mb low crosses southern CA and NV and then
southern UT, reaching CO by Sun morning.
Low mid level convergence and moisture advection maximizes along
and near the low track, bringing a period of heavy snow to the
ranges of southern NV and across the central to southern UT
Wasatch Mountains.
WPC probabilities indicate snow accumulations potentially reaching
18" in higher terrain of the Wasatch, with 12-18 inches possible
in the ranges of central Co.

The forecast snow becomes more challenging Sunday as lee
cyclogenesis occurs in CO and then sinks E/SE into the Southern
Plains.  There remains considerable spread with the strength and
phasing of the 700-500 mb low, resulting in differences in both
QPF and whether it is cold enough for snow.  The stronger NAM
solution produces both more QPF and more rapid cooling, producing
heavier snow than the preferred NAM Conus Nest, which aligns well
with the intensity of the low depicted in the UKMET, GFS, and
ECMWF.  Several inches of snow are possible in the CO Front Range,
Sangre de Cristo mountains, and Palmer Ridge.

On Monday, the low weakens into an open wave and is forecast to
eject northeast across the mid MS Valley.
A stripe of light snow or sleet/freezing rain is possible from
central KS northeast across Iowa into northern IL.


...Pacific Northwest/Northern Rockies/Northern High Plains...
Days 2-3...

Surface low pressure will move onshore British Columbia Sunday
beneath a potent upper shortwave digging southwest from the Gulf
of Alaska.  A Pacific Jet streak, mid-level moisture convergence,
and PWATs to +0.5 standard deviations above the climo mean support
widespread locally heavy precip.  The heaviest amounts are
expected in the Olympics and Cascades on D2, continuing across the
ranges of central to northern ID and northwest MT.

In the Washington Cascades total snowfall is expected to be 2 to 3
feet at higher elevations. Heavy snow will spread into the ranges
of western to south central MT, ID, and northern WY, with a foot
of snow expected.  As the 700 mb wave crosses the northern Plains,
the frontal wave is expected to produce a band of light snow.
There are west-east differences on the axis of snow near the front
to be resolved in later forecasts.

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

Petersen