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

Valid 00Z Tue Apr 20 2021 - 00Z Fri Apr 23 2021

...Central Rockies/Central Plains/Mid Mississippi Valley/Great
Lakes/Northeast...
Days 1-3...
An amplifying shortwave trough will usher in well-below normal
temperatures across the central and eastern U.S., with
accumulating snows likely from the central Rockies to the
Northeast.  Heavy amounts are possible across portions of the
central Rockies, as well as parts of northern New York and New
England.

A shortwave trough currently digging south across the northern
Rockies is expected to drop into the central Rockies Monday
evening.  Right-entrance region upper jet forcing along with the
low-to-mid level frontogenesis, and low level upslope flow are
expected to help support moderate to heavy snows developing late
Monday and continuing into the overnight.  Latest WPC PWPF shows
high probabilities for snowfall accumulations of 4-inches or more
extending from the Colorado Front Range eastward into the I-25
Corridor, including the Denver Metro.  Heaviest amounts are
expected to fall along the Front Range, where the PWPF indicates
that local amounts of 8-inches or more are likely. 

This is expected to be a fast-moving system, with the better upper
forcing moving quickly east into the central Plains overnight. 
Here too, favorable forcing aloft, along with low-to-mid level
frontogenesis, are expected to contribute to a period of
potentially heavy snowfall.  Accumulating snows appear likely
across a large portion of southern Nebraska, and northern and
central Kansas into northwestern Missouri overnight.  Strong
low-to-mid level frontogenesis centered over central to
northeastern Kansas overnight is helping to support an increase in
probabilties for accumulations of 4-inches or more across the
region.

This system will continue to move progressively east on Tuesday,
with accumulating snow possible from the lower Missouri and mid
Missippi valleys to Lower Michigan.  However, the mid April sun
angle should help limit the threat for widespread heavy
accumulations. 

Probabilties for heavier amounts increase some as the system lifts
into the lower Great Lakes region Tuesday night.  Then by late
Wednesday, the system is expected to begin to slow, with a closed
low developing over the Northeast by early Thursday.  This is
expected to bring a more prolonged period of snow to Upstate New
York and northern New England Wednesday and Thursday. 

Accumulations of 4-inches or more are likely across portions of
western New York, the Finger Lakes, and the Tug Hill -- where lake
effect and upslope snow showers following the synoptic snows will
likely bolster amounts.  Probabilities for accumulations of
4-inches or more are also high across the Adirondacks and the
northern New England mountains.  Locally heavier amounts are
likely within this area, with WPC PWPF showing high probabilies
for storm total amounts of 8-inches more for portions of the
northern Adirondacks and far northern Maine.

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

Pereira