Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Mon Apr 19 2021

Valid 12Z Mon Apr 19 2021 - 12Z Thu Apr 22 2021

...Northern to Central Rockies and Central Plains...
Days 1-3...

An area of snow north of a cold front advancing south across
Wyoming is expected to continue today as 700 mb convergence
supports lift across the Wind River and Bighorn Ranges, continuing
south this afternoon into the Wyoming front range and Laramie
Mountains, followed by the Colorado northern front range late this
afternoon through tonight, with the strength of 700 mb convergence
tapering towards morning. These areas are where the highest WPC
probabilities for greater than 6 inches of snow exist. Total
maximum snowfall amounts may reach 12 inches across the higher
peaks in these ranges.

By this evening the 300 mb jet and upper divergence move east
across the Central Plains, accompanied by a low level
frontogenesis maxima associated with the aforementioned cold
front.  This combination will promote temperatures cooling and
ascent resulting in snowfall across the central High Plains this
afternoon into tonight. WPC probabilities for over 4 inches are
low to moderate, with the possibility of 2 separate bands of
precip and snow. The models cluster well in the distribution of
QPF and snow, so the models were equally weighted.

On Day 3, the ECMWF and NAM show a closed, broad 700 mb crossing
UT and then into CO, possibly shearing into an open wave.  Along
the track of the low, pooling moisture and 700 mb convergence
leads to snow showers across the ranges of western to central CO. 
The 21z SREF Mean, 00z GEFS mean, 00z NAM and 00z GFS show
potential for several inches of snow (although the GFS shows its
typical high bias for CO).

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

The same shortwave responsible for snowfall across the Rockies and
Central Plains today will move steadily 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,
northern IN,and possibly northwest OH. 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 to the MI
border.  Given surface temperatures will start out above freezing,
ground temperatures well above freezing, it will take high
snowfall rates to produce accumulating snowfall on roads, with
most accumulations occurring on grass and other non-paved surfaces
that cool quickly.

On Tuesday night, a 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 over northern
Indiana and the northwest Ohio, possibly into adjacent lower
Michigan. The strengthening area of low pressure and associated
precipitation is then forecast to move northeast and into the
Interior New York and northern New England 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
moderate uncertainty regarding the precip type and resultant snow
in western to northern New York and northern VT.

The 00z GFS remains the most progressive solution and thus the
farthest east with the heavy snow axis in NH/western ME, while the
00z ECMWF, NAM, and other guidance continues to be highlighting
snow further west in the NY Adirondacks and further west in
western NY State.  WPC probabilities have low chances for greater
than 4 inches of snow from western to northern New York State on
Wed.

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

Petersen