Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
345 PM EST Thu Jan 14 2021

Valid 00Z Fri Jan 15 2021 - 00Z Mon Jan 18 2021

...Minnesota and Iowa/nearby Midwest...
Day 1...

A closed low-mid level circulation drifts south from MN into IA
tonight into early Friday before moving slowly east and
dissipating Friday night.
Low-mid level convergence and warm advection wraps around the low
across southern MN and Iowa, leading to periods of snow. The slow
movement of the circulation should allow bands of snow to persist
for several hours overnight tonight, leading to accumulations of
several inches of snow.

The latest WPC PWPF shows high probabilities for snow
accumulations of 4 inches or more covering a large portion of
southern Minnesota into central Iowa.
The peak probability for 8 inches is 40-50 percent near the
central MN/IA border.  The snow, combined with blowing and
drifting due to windy conditions, has lead to the issuance of
blizzard warnings. The system is expected to gradually weaken and
move east on Friday, leading to snow fall coverage and intensity
waning as the day progresses. 


...Central Appalachians and Northeast...
Days 1-3...

The closed low from the Midwest will lift east/northeast Friday
into Saturday, with widespread precipitation expected east of the
low within the broad low-mid level warm/moisture advection coming
into the Central Appalachians Fri on the nose of the 700 mb jet,
then extending up into New York and New England Fri night to
Saturday. Ascent should be strong, given the nicely coupled jet
structure at 300 mb leads to a well defined divergence maxima in
eastern NY/western New England.
Initial temperatures are below freezing through a deep layer in
upstate NY and interior New England, allowing the initial burst of
precipitation to start as snow.
Favorable upslope conditions should lead to locally heavy snow on
the eastern slopes of the Adirondacks, and the Green Mountains of
southern VT.
The low-mid level warming results in the possibility of a change
over to mixed precip types in the Catskills to the Berkshires,
southern Green and White Mountains, so precip type uncertainty is
in place.

A triple point sfc low develops and moves north across New York. 
West of the low track has a better chance of remaining all snow. 
By Day 3, the low departs north into adjacent Canada, with the
slow moving circulation resulting in moisture wrapping back around
the low into the mountains of western Maine, the White, Green, and
Adirondack Mountains, with several inches of snow possible in
favored terrain. Strong cold advection across lake Erie and
Ontario should results in favorable lapse rates and development of
snow showers and bands of snow in lee shore convergence areas. 
Several inches of snow is possible along the eastern shore of Lake
Erie and the eastern shore of Lake Ontario, including in upslope
areas of the Tug Hill, where up a foot of snow is possible.


Pacific Northwest to Northern Rockies Day 3...

The mid level ridge retreats inland Sat night and the next low-mid
level front approaches the Pacific Northwest by Sun morning.  The
low-mid level moisture advection and convergence leads to valley
rain and mountains snow for the WA/OR Cascades, spreading into the
northern Rockies by the end of the Day Sunday.  Higher amounts are
expected in the WA Cascades due to closer proximity to the
moisture source and the longer duration snows. This is supported
by multiple models and SREF and GEFS Mean snowfall forecasts. 
Several inches of snow is expected in the WA Cascades.

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

Petersen