Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
236 PM EST Fri Dec 6 2024

Valid 12Z Fri Dec 06 2024 - 12Z Mon Dec 09 2024

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

Lake-effect snow (LES) bands will continue throughout portions
of the northern and eastern Great Lakes today as high pressure to
the south and a large area of low pressure over Atlantic Canada act
to sustain cyclonic flow over the Lakes. By this evening, LES snow
in northern Michigan and the Michigan U.P. will taper off while
LES bands will become reinvigorated down-wind of Lakes Erie and
Ontario. This is due to an approaching shortwave trough that both
backs and accelerates westerly low-level winds over these Lakes
overnight. These bands will pack a punch as 1-3"/hr snowfall rates
are likely in these single bands that are expected to produce
another 12-18" (locally up to 24" possible) from the eastern shores
of Lake Ontario on east to the I-81 corridor between Watertown and
Syracuse and finally to the Tug Hill. Farther south, it will be
areas along I-80 in northwest PA and ENE along the Chautauqua Ride
that could receive 8-12" of snow with localized amounts
approaching 18". LES snow bands should gradually taper off
Saturday morning as WAA and WSWrly winds pick up.

While these LES bands diminish, another strong system will race
east across southern Canadian Provinces Saturday morning with
sufficient 850-700mb WAA along the storm's warm front leading to a
progressive shield of snow moving east across the northern Great
Lakes. By Saturday evening, the warm front advances through the
Northeast with periods of snow enveloping parts of central and
northern NY to start, then into northern New England Saturday night
and into Sunday morning. The storm is rather progressive and a dry
slot will quickly dry out the column by late Sunday morning.
However, some minor accumulations are possible, especially in the
Adirondacks, Green, and White Mountains. WPC PWPF shows moderate
chances (40-60%) for snowfall totals topping 4" in these
mountains. One area to monitor is Downeast Maine, for if the storm
can quickly wrap around a comma-head of precipitation Sunday
afternoon, there could be localized snowfall totals that approach
4" in some spots. The WSSI-P does show similar moderate
probabilities (40-60%) for Minor Impacts in Downeast Maine on
Sunday.


...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 Saturday morning, but plummet to as low as 3,000ft by
early Sunday morning. This will result in rain being the primary
precip type a the onset Saturday, but as the cold front moves
through and snow level drop, passes should see the changeover to
snow occur Saturday night. 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 on Saturday. Some minor icing accumulations are possible
with areas west of I-97 in northern Washington having the better
odds for ice accumulations up to a tenth of an inch. Slick travel
conditions are possible in these areas on Saturday.

The second shortwave trough makes its way through the Pacific
Northwest Saturday night and into Sunday. Unlike the first event,
snow levels are now hovering at or just below pass level, making
this event the more concerning for potential travel impacts. The
good news is it is a quick moving system and snow should taper off
by Sunday night as high pressure builds in by the start of next
week. WPC PWPF does show moderate-ot-high probabilities (50-70%)
for >6" of snow for the duration of this weekend event in the
Snoqualmie Pass, while there are high chances (>70%) for snowfall
totals >12" at elevations above 5,000ft in the Cascades and
Olympics.

These atmospheric disturbances will push their respective plumes
of moisture east into the Northern Rockies as well, with the
heaviest snow ensuing Saturday night and into Sunday as snow levels
decrease, SLRs increase, and northerly low level winds aid in
upslope enhancement for snowfall rates. For northern Idaho snowfall
totals of 6-12" are possible above 5,000ft in the Bitterroots. The
>6,000ft elevations of the Lewis Range, Blue, and Bitterroot
Mountains in central Idaho will be favored for similar totals.
Lastly, parts of the higher terrain of the Little Belt Mountains in
central Montana, the Absaroka in southern Montana, and the Tetons
of western Wyoming could also see 6-12" of snow, particularly above
7,000ft.

...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 through as the
atmospheric column cools to a sub-freezing wet-bulb temperature
profile, the strong WAA will cause low level temps (850-750mb) to
rise above freezing and thus cause snow to change over to a
sleet/freezing rain mix from the Red River of the North on east
into northern Minnesota. It should be noted that this can still
change, for by Sunday night, the primary occluded low tracking
through southern Saskatchewan and Manitoba will weaken as a new low
forms along the triple point in the Upper Mississippi Valley.
Should this low form sooner and or track farther south, it could
mean snow occurs for a longer duration across northern Minnesota
either due to the storm track, or a deeper low Sunday night that
allows for a more defined deformation axis on its northern flank.
These solutions are depicted in some members of the WPC super
ensemble and are largely why WPC PWPF shows moderate chances
(40-60%) for snowfall greater than 4" along the northern Minnesota
border and into the northern most section of Minnesota's
Arrowhead. Residents in North Dakota and northern Minnesota should
follow the forecast closely in the coming days as the forecast
comes into better focus over the next 24-36 hours.


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

Mullinax