Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Wed Apr 21 2021

Valid 12Z Wed Apr 21 2021 - 12Z Sat Apr 24 2021

...Great Lakes/Northeast...
Days 1-2...

An area of low pressure developing along the cold front in the mid
Appalachians is forecast to move into eastern New York and New
England, intensifying along the way. 
Snow is occurring in northwest PA and western NY and this snow
area should move northeast today in the low-to-mid level
frontogenesis maxima west of the deepening surface low.
Locally heavy snow is possible in the NY Adirondacks, northern
Green Mountains, White Mountains, and ranges of western Maine.
East of the low track, snow accumulations get reduced due to warm
advection and a short duration of snow. Lake effect snow follows
the system as northwest winds cross Lake Ontario with less shore
convergence leading to snow showers from Rochester to Syracuse and
along the southeast shore of the lake. Several inches of snow are
expected in these areas.

Low pressure exists Maine on Thursday, with continuing mid level
deformation/frontogenesis supporting ascent and potential for
several more inches of snow in northwest Maine.
Overall, the heaviest snowfall totals are expected to occur across
the Adirondacks and the New York North Country, as well as far
northern Vermont to Maine, where day 1 plus day 2 accumulations of
8 inches or more are likely.

...Northern Rockies and Northern High Plains Day 2/3...

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 2. Post-front northeast winds
turn upslope in windward locations of the front range/Glacier
National Park south to the Beartooth Mountains of MT. Several
inches of snow are expected in favored windward terrain, with a
low probability of 8 inches forecast. 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.  
On Day 3, the weak front progresses steadily across the Bighorn
Mountains, with enhanced relative humidity near 90 percent Fri as
the front passes.  Lift is provide from the front and also upslope
flow in windward terrain. A modest 4-6 additional inches of snow
are possible there before the front passes to the south and drying
aloft occurs.

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

Petersen