Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
446 PM EDT Sun Apr 18 2021

Valid 00Z Mon Apr 19 2021 - 00Z Thu Apr 22 2021

...Northern to Central Rockies and Central Plains...
Days 1/2...

A vigorous shortwave digging south from Canada along with a strong
cold front will dive south across the Northern Rockies tonight and
reach the Central Rockies/Plains on Monday. Meanwhile, upper level
divergence in conjunction with the left exit region of a 300 mb
jet streak will promote enhanced lift between tonight and Monday
morning from western Montana to eastern Wyoming. Upslope
enhancement will also add to snowfall accumulations across the
windward terrain over the Northern and Central Rockies from
western MT, the Absarokas, Wind Rivers, Big Horns, Black Hills,
and into the Front Range of CO. This is also where the highest WPC
probabilities for greater than 6 inches of snow exist. Total
maximum snowfall amounts may exceed 12 inches across the higher
peaks in these ranges. By Monday evening the jet and upper
divergence move east across the Central Plains, which will remove
necessary forcing and leave only light snow across the central
High Plains by Tuesday morning. Nonetheless, strong 700 mb
frontogenesis associated with the aforementioned cold front will
promote snowfall across the central High Plains Monday night. The
timing is important given it is now the second half of April, as
much of the snow across eastern Colorado and Kansas will occur at
night. WPC probabilities for over 4 inches are fairly low (between
10 and 20 percent), likely because the precipitation will be fast
moving and snowfall rates will struggle to top 1 inch/hour. The
peak highest Day 1 plus Day 2 totals are over the WY Bighorns and
north-central Colorado Rockies, as northwest flow leads to well
defined and persistent, long lasting upslope flow. The models
cluster well in the distribution of QPF and snow, so the models
were equally weighted.

...Mid MS Valley to Midwest/Lower Great Lakes...
Days 2/3...

The same shortwave responsible for snowfall across the Rockies and
Central Plains on D1/Monday will race eastward and begin to
amplify on D2/Tue as it crosses the mid MS Valley toward the
Midwest and Lower Great lakes. This feature will be accompanied by
an intensifying jet streak crossing IA, WI, and MI.  The
combination of divergence aloft in the right jet entrance region
and convergence along the 700 mb front leads to ascent and snow
across northern Missouri to northern IL, southern lower MI, and
northern IN. An intensifying area of low pressure developing along
the cold front leads to a possibility of an area of several inches
of snow across northern IN and lower MI. Given surface
temperatures will start out above freezing, ground temperatures
well above freezing, and an increasing sun angle, it will take
intense snowfall rates occurring during nighttime hours to produce
hazardous snowfall.

By Tuesday night, a potent band of 850-700 mb frontogenesis will
help promote enhanced ascent and dynamically cool the column and
lead to snowfall rates that result in snow accumulations between
northern Indiana and lower Michigan. The strengthening area of low
pressure and associated precipitation is then forecast to move
northeast and into the Interior Northeast by D3/Wednesday. Snow
will remain possible along the axis of strongest frontogenesis and
to the northwest of the low pressure system, but there remains
high uncertainty regarding the position of the highest snowfall on
D3. Recent model trends have shifted this axis of potentially
heavy snow southeastward, while lowering amounts slightly as well.
The 12z GFS remains the most progressive solution and thus the
farthest southeast with the heavy snow axis, while the 12z ECMWF,
NAM, and other guidance continues to be highlighting an area
toward the northwest. WPC probabilities have low chances for
greater than 4 inches of snow from northern Indiana to lower
Michigan between Tuesday night and Wednesday morning, which
extends into far western New York State by the end of D3.

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

Snell