Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
444 AM EDT Tue Apr 20 2021

Valid 12Z Tue Apr 20 2021 - 12Z Fri Apr 23 2021


...Mid Mississippi Valley/Great Lakes/Northeast...
Days 1-3...

Snow is progressing along a low-mid level frontal zone across
Kansas and northern MO. This is expected to be a fast-moving
system, with low-to-mid level frontogenesis and convergence within
that band expected to produce snow today from northern MO to
central IL, extending into northern IN, northern OH and far
western NY near the shores of Lake Erie tonight. Moderate
probabilities exist for 4-inches or more across northeast IN to
far southeast MI and northern Ohio.

Probabilities for heavier amounts increase some as the system
lifts into northern NY into interior northern New England.  An 850
mb low is forecast to progress from western to northern New York
and then near the VT border with Quebec and the Maine border with
Quebec. Strong warm/moisture advection along the path of the 850
mb low brings a period of snow to the NY Adirondacks and the
northern Green Mountains of VT before moving downstream to western
Maine Wed.
East of the low, precip should change to rain. 

Then as the low passes late Wed night to early Thu, mid level
deformation wrapping around the low allows moisture to circulate
back into northwest Maine, along with a return of colder air. 
Several additional inches of snow are possible near the Quebec
border before the system departs Thu.

Accumulations of 4-inches or more are likely across portions of
western New York, the Tug Hill, the Adirondacks and the northern
New England mountains.  Locally heavier amounts are likely within
this area, with WPC PWPF showing moderate to high probabilities
for storm total amounts of 8-inches more for portions of the
northern Adirondacks, northern Green Mountains of VT, and far
western Maine.

...Northern Rockies and Northern High Plains Day 3...

The next low-mid level frontogenesis maxima and enhanced pooling
of moisture occurs with the next front moving southeast out of
Alberta across Montana/Idaho on Day 3.
Post-front northeast winds turn upslope in windward locations of
the front range/Glacier National Park south to the Beartooth
Mountains of MT on Wed/Wed night.
Slightly weaker frontogenesis maxima and more progressive 700 mb
convergence maxima cross the ID Clearwater Mountains, so most
solutions don't have as much snow potential in ID.
With the progressive front, probabilities for 8 inches of snow are
low.

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

Petersen