Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EDT Sun Apr 17 2022

Valid 12Z Sun Apr 17 2022 - 12Z Wed Apr 20 2022

...West Coast to the Northern Rockies...
Days 2-3...
Another upper low approaches the region late Monday through
Tuesday bringing renewed unsettled weather. Left exit jet dynamics
coupled with a plume of moisture moving onshore will bring
widespread precipitation to the region, particularly the coastal
regions on day 2 then spreading inland/eastward on day 3. The
heaviest snow totals will be found across the Olympics and WA
Cascades with lesser, but still significant snow, for the OR
Cascades, Northwest California mountains, and northern Sierra.
Through Day 3, heavier snow continues for the Cascades and Sierra,
but also spreads inland toward the ID Panhandle and northwest MT.
These regions have moderate (40-50 percent) probabilities of
totals exceeding 6 inches per the latest WPC PWPF.


...Northern Plains and Upper Midwest...
Day 1...
Shortwave energy tracking across the Northern Rockies reaches the
Northern Plains later today while the associated surface low moves
east through the Dakotas. Strong isentropic lift ahead of the
frontal boundary along with focused banding north/northwest of the
low center will bring widespread precipitation to the region with
the potential for several inches of snow, particularly across
northern North Dakota. Snow rates this morning should top 1"/hr at
times and totals through 12Z Tuesday are likely to exceed 6" well
north of I-94. The latest WPC probabilities show 60 to near 80
percent probabilities for 6" or more with isolated amounts between
8-10" possible.

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
with a localized/narrow potential of 6" or more, as shown by the
WPC probabilities that reach 30 percent during the day 1 period.


...Eastern Great Lakes and Central Appalachians to the Northeast...
Days 2-3...
Phasing southern and northern stream shortwave energies over the
central U.S. to Great Lakes will lead to a negatively titled
trough closing off by Tuesday morning. An initial wave of low
pressure will track up the through the Ohio Valley, Appalachians
toward the eastern Great Lakes followed by a coastal low off the
Mid-Atlantic that then tracks near or just offshore the Northeast
through Tuesday. As this occurs, colder air in place combined with
cold advection behind the departing wave will lead to rain
changing over to snow across portions of Ohio, Michigan and
western/northwest PA Monday. A few inches of snow will be possible
with the possibility of some overachieving meso bands where
dynamic cooling brings down heavier snow rates possibly up to
1"/hr as shown by the 00Z HREF across northern OH, northwest PA,
and far southeast MI. The latest WPC probabilities show slight
percentages (20-30) for 4 inches or more.

As the coastal low deepens, it will tap into much higher moisture
content along with favorable strong forcing for ascent. There
remains some uncertainty in the storm track, particularly how far
inland it reaches late Monday night through Tuesday. The NAM has
been the furthest west/inland solution, but has flip-flopped run
to run while ECMWF/GFS/CMC have been more consistently near/just
offshore with its storm track. Ptype/thermals vary between the
models but in general, boundary layer temperatures and elevation
will be the main driver for ptype and possible accumulations.
North/northwest of the low center where forcing and meso-bands are
most likely, dynamic cooling could lead to some overachieving snow
totals given the high water/moisture content, particularly for
elevations above 1500-2000 ft. It's a lower confidence, but
potentially higher impact late season snow event for the higher
elevations of the Catskills, Adirondacks, Berkshires, Green, and
White Mountains which stand the best chance for several inches.
The latest WPC PWPF percentages for 4" or more are high and for 8"
or more, reach 40-50 percent for the highest elevations of the
Adirondacks and the Greens. Some localized 12"+ totals cannot be
ruled out at this time. The heavy, wet nature of the snow in this
late season storm could result in more impacts than typical
including downed trees limbs/branches, power outages, and more
difficulty in shoveling in addition to hazardous road conditions
due to the slushy, wet snow.


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

Taylor