FOUS11 KWBC 070727
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 AM EST Sat Dec 7 2024

Valid 12Z Sat Dec 07 2024 - 12Z Tue Dec 10 2024

...Great Lakes and Northeast...
Day 1...

Shortwave on the southwest side of the expansive eastern North
American trough will zip quickly across the Great Lakes and
into/through the Northeast later today. With a surface low track
through southern Ontario, WAA-driven snow will spread across the
Northeast and end early Sunday. With a general westerly wind
component, upslope into N-S terrain (e.g., the Green and White
Mountains) will promote moderate amounts of snow with lighter
amounts in lower elevations. Some additional lake enhancement is
also likely off Lake Ontario into the Tug Hill. Quick movement of
the system should limit amounts, and WPC probabilities for at least
4 inches of snow are highest (>50%) north of I-90 in NY (Tug
Hill/Adirondacks) and into the Green and White Mountains. By later
Saturday night into early Sunday, as the system departs, could see
some late enhancement over eastern Maine where WPC probs for 4
inches are also near 50%.


...Pacific Northwest/Northern Rockies...
Days 1-3...

An upper low in the northeast Pacific heading into British
Columbia will direct two shortwave troughs into the Pacific
Northwest this weekend. Snow levels will initially start out
between 8,000-9,000ft this morning, but plummet to as low as
3,000ft by early Sunday morning. Rain at most locations will change
to snow at Stevens/Snoqualmie Passes as the first wave of
precipitation wanes. East of the Cascades, lingering sub-freezing
temperatures within the boundary layer will be trapped in the
Columbia Basin as WAA and overrunning moisture aloft provide a
favorable setup for an icy wintry mix in central and northeastern
Washington today and even into parts of southeastern
OR/southwestern ID tonight. Some minor icing accumulations are
possible up to about a tenth of an inch.

The second shortwave trough makes its way through the Pacific
Northwest tonight and into Sunday. Lower snow levels support
moderate snow at pass level (esp Stevens) with this second wave of
moisture. WPC probabilities for at least 8 inches of snow are
highest above about 3500ft.

These atmospheric disturbances will push their respective plumes
of moisture east into the Northern Rockies as well, with the
heaviest snow tonight and into Sunday as snow levels decrease, SLRs
increase, and northerly low level winds aid in upslope enhancement
for snowfall rates. Highest probabilities for >6" snow are over
the terrain including the Bitterroots, Lewis Range, Blue Mountains,
and into the Little Belt Mountains in central Montana, the
Absarokas in southern Montana, and the Tetons of western Wyoming.


...Northern Minnesota and northeastern North Dakota...
Days 2-3...

The first upper level shortwave trough that brought snow to parts
of the Pacific Northwest mountain ranges will spawn low pressure
over the Canadian Prairies Sunday morning. As its warm front moves
east, WAA across the Northern Plains will result in a shield of
precipitation that passes over a sub-freezing boundary layer. While
precipitation may start out as snow along the Canadian border,
mid/low-level WAA may promote an area of freezing rain for portions
of eastern ND into MN late tonight into early Sunday. Depending on
the evolution of the main low along the Canadian border and other
frontal waviness over the Corn Belt moving up toward the MN
Arrowhead, snow could be maximized over northeastern MN from north
of Duluth to the border. WPC probabilities for at least 4 inches of
snow are >50% from around the Northwest Angle southeastward to
Embarrass and points northeast.


...CO Rockies/Sangre de Cristos...
Day 3...

Height falls associated with the second wave through the Pacific
Northwest will push through the central Rockies, spreading light
snow across the region on Monday into early Tuesday. Quick movement
and general lack of moisture will limit snowfall, with WPC
probabilities of at least 4 inches >30% limited to the Sangre de
Cristos in CO and just into NM.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Fracasso/Mullinax
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