Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
510 PM EDT Sat Apr 16 2022

Valid 00Z Sun Apr 17 2022 - 00Z Wed Apr 20 2022

...West Coast to the Northern Rockies...
Days 1 and 3...
A mid to upper level low spinning into the Northwest this
afternoon is expected to transition to an open wave, with the
remnant shortwave forecast to track quickly eastward along the
Canada-U.S. border, across the Rockies and into the High Plains
overnight.  Upslope flow on the northwest side of a developing low
level wave, along with favorable upper jet forcing, is expected to
support locally heavy amounts across the northern Idaho and
northwestern Montana ranges.  WPC probabilities indicate snowfall
accumulations of 6 inches or more are likely across portions of
the higher terrain.

Shortwave ridging will support dry conditions across much of the
area Sunday night before another upper low begins to impact the
region on Monday.  Widespread forcing for ascent and plentiful
moisture will yield precipitation including heavy mountain/terrain
snow, developing initially across the Olympics and northern
Cascades on Monday, before spreading south into the southern
Cascades, Northwest California mountains, and the northern Sierra
late Monday into Tuesday as the associated front moves inland. 

...Northern Plains and Upper Midwest...
Days 1-2...
As the previously noted shortwave in the Northwest moves east
overnight, latest guidance continues to show significant potential
for at least a few inches of snow developing on the north to
northwest side of the associated surface low as it tracks
northeast from Wyoming into the Dakotas.  Favorable upper jet
coupling overlapping low-to-mid level frontogenesis is expected to
help support some heavier snowfall rates, especially during the
overnight/early morning hours.  WPC probabilities for snowfall
accumulations of 4 inches or more are above 70 percent from the
northeastern corner of Montana across much of northern North
Dakota.  Probabilities for 8 inches or more are in the 10-30
percent range across this area.

As the system progresses eastward towards the Great Lakes on
Sunday, the potential for heavy snow is expected to decrease along
with diminishing forcing and moisture advection.  From northern
Minnesota to northern Michigan, accumulations of an inch or two
are expected.  WPC probabilities for snowfall accumulations of 4
inches or more generally less than 30 percent across the region. 
The North Shore of Lake Superior into the Iron Ranges remains an
exception, where moist onshore flow will likely enhance totals.

...Eastern Great Lakes and Central Appalachians to the Northeast...
Days 1-3...

A broad upper trough centered over the Great Lakes this afternoon
is expected to amplify as it moves farther east this evening. 
Along the associated cold front, an area of low pressure is
expected to quickly intensify and track northeast along the
northern New England coast overnight.  This will support rain
changing to snow, with mostly minor accumulations expected across
northern New England.  Northwesterly winds are also expected to
support some lake enhanced snow showers, producing some light
accumulations in the lee of the eastern Great Lakes as well.

High pressure and dry conditions are forecast across much of the
region beginning late Sunday and continuing into early Monday.  
By Monday afternoon however, the threat for snow is expected to
return initially to the eastern Lakes.  Models show the upper
trough beginning to assume a negative tilt, with left-exit region
upper jet forcing helping to enhance precipitation rates on the
north side of a developing wave lifting north from the Ohio Valley
into Great Lakes.  Rain changing to snow in this area may produce
an inch or two of snow across portions of northern Ohio and
northwestern Pennsylvania before the low lifts into Ontario late
Monday.

As the trough axis continues to sharpen, low pressure originating
out of the Southeast and Mid-Atlantic is forecast to deepen
rapidly and track along the Mid Atlantic coast Monday night before
moving inland over the Northeast on Tuesday.  The favorable
forcing for ascent aided by a coupled jet streak over the region
will lead to a widespread area of precipitation northwest of the
low track. Temperatures are expected to be marginally supportive
of wet snow, generally in the lower 30s, and an elevation driven
snow event is increasingly likely. The system will be moisture
rich though should move through fairly quickly Monday night into
Tuesday. Higher elevations of the Catskills, Adirondacks,
Berkshires, Green, and White Mountains have best chance for
several inches of wet snow.  Latest WPC probabilities show high
probabilities for accumulations of 4 inches or more across the
higher terrain. The potential lift/forcing could overcome the
marginal temperatures, especially given the timing during the
overnight into early morning hours and there remains quite a bit
of spread in the model guidance both in QPF and thermals.  At
least some minor impacts due to snow are possible.

For Days 1-3, the probability of 0.10 inch or more of ice is less
than 10 percent.

Pereira/Taylor