Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
531 PM EDT Tue Apr 13 2021

Valid 00Z Wed Apr 14 2021 - 00Z Sat Apr 17 2021



...Great Basin to the Central High Plains...
Days 1-3...

Areas of heavy snow are expected in the ranges from the central CA
Sierra Nevada mountains across the ranges of northeast Nevada,
northern Utah, Wyoming, and Colorado. 3-day snowfall of 12-18
inches is expected in parts of the highest terrain of WY and CO.

As the closed upper level low pivots across the Great Basin,
downstream upper divergence maxima cross northeast Nevada tonight
and northern UT int he morning, followed by western WY and CO.
Coupled upper divergence with low level convergence  aided by an
upslope component of flow in windward terrain results in several
inches of snow in the mountains from the central CA/Sierra Nevada
east across the northern Wasatch of Utah to Wind River Mountains
of WY and front range of Co/Laramie Mountains of southeast WY.

As the upper low moves east across Wyoming Day 2, mid level
deformation and upper level divergence maxim combines with return
upslope flow in the Wind River range of WY to drive moderate to
heavy snow.
The probability for 6 inches is high across this area, with 8 to
12 inches possible.

On Day 3 the upper trough crosses from WY and CO onto the central
Plains.  The 700 mb low moves from CO steadily east across the
central Plains.  The cooling temperatures across the high Plains
as the mid level low moves across leads to a period of snow across
the Plains of southwest Wyoming to western NE and eastern CO.
The departure of the 700 mb low should lead to a cessation of the
snows.

Without confidence of a low duration snow, probabilities are low
for 8 inches of snow on Day 3 in eastern WY, CO and adjacent
western NE and northwest KS. The highest probabilities for 4
inches are on the CO front range and Palmer Divide. 

...Interior New York/New England...
Day 3...

A closed 700 mb low moves from the Great Lakes across southern New
York and southern New England.  Cyclogenesis en route leads to
lowering heights/temperatures with snow developing and increasing
in intensity across the mountains of eastern New York and interior
New England.

Rapid height falls accompanying this feature will combine with
upper divergence to develop a surface low off the mid Atlantic
coast Thursday morning. This low is then progged to lift northeast
to Southeast New England while strengthening.  Warm advection
ahead of the low will spread precipitation northward Thu, but
initially will be all rain due to warm low-level temperatures.
However, as the low deepens and cooling occurs aloft,
rain is expected to change to snow, with long duration snows
expected in portions of the MA Berkshires into the Green Mountains
of VT and Monadnocks of southern NH, possibly extending north into
the White Mountains.

Strong theta-e advection along and north of the 700 mb low track
combines with upslope flow to produce heavy snow in windward
terrain of these mountains.  The Adirondacks and Catskills have
considerable uncertainty whether they will also received higher
elevation heavy snows. The 12z NAM was colder than the majority of
guidance, bringing heavy snow into valley areas of eastern New
York. Given initial temperatures well above freezing, more
weighting was given to the guidance majority that don't provide as
much cooling as the NAM to valley areas.

Consequently, higher snow probabilities are where there is more
confidence of a longer duration snow event in the Berkshires,
Green, and Monadnock Mountains.  WPC probabilities currently
indicate a 50% chance for 8 inches in portions of the
Green/northern Berkshires/Monadnocks/White Mountains. The
probabilities are lower, near 30 percent, for the Catskills and
Adirondacks of NY State.

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

Petersen