Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
258 PM EST Sun Dec 8 2024

Valid 00Z Mon Dec 09 2024 - 00Z Thu Dec 12 2024

...Northeast...
Days 1-3...

As the snow today in parts of the Northeast winds down, focus
shifts to the next wintry setup that to affect the region beginning
midday Monday, peaking in intensity Monday evening, and concluding
Tuesday morning. An upper level disturbance in the Ohio Valley
will direct a plume of rich Gulf of Mexico moisture towards the
region Monday afternoon. NAEFS shows IVT values of 300-400 kg/m/s
approaching the Northeast at 18Z. These IVT values are above the
90th climatological percentile according to NAEFS, highlighting the
anomalous nature of this moisture plume and advection. Meanwhile,
a storm system in the northern Great Lakes will work in tandem with
the Ohio Valley disturbance to produce brisk 700-300mb WSWrly
winds aloft. There is weak high pressure located over Quebec that
will help to lock in some low-level cold air and allow for
precipitation to fall in the form of snow Monday afternoon over
northern New England. However, the strong WSW flow aloft will force
a >0C warm nose to ensue at low levels and cause precipitation to
transition to an icy wintry mix for most areas.

By Monday evening, a weak coastal low near the tip of Long Island
that will then track towards the Massachusetts Capes. This low is
expected to introduce additional low-level moisture into Maine late
Monday night and into Tuesday. This could make for a more prolonged
period of snow or icy wintry mix, with the latter increasingly
more likely early Tuesday morning due to the dryslot aloft making
it increasingly difficult to favor dendrites. By the time the
storm departs for Nova Scotia around midday and precipitation
tapers off, snowfall amounts will generally range in the 1-3" range
for the Adirondacks, Green and White Mountains above 2,000ft.
Minor ice accumulations (generally topping out around 0.1") are
likely in many of these mountain ranges, as well as near the Maine
coast and as far south as the Poconos and Catskills.

By Wednesday morning a far more amplified jet stream pattern will
bring about the next chance for measurable snow, but there remains
a good deal of uncertainty in its evolution. NAEFS and ECMWF are in
good agreement on an anomalous upper level trough over the
Mississippi Valley 12Z Wednesday, but depth/tilt of the primary
500mb shortwave at the base of the upper level trough differs by
guidance members. The ECMWF ENS is a bit deeper with this southern
disturbance while the GEFS is stronger with the upper low
traversing southern Canada. This can have implications on the
transition from rain to snow throughout the Northeast and how
quickly rain can changeover to snow in the central Appalachians.
There are some aspects ensembles do agree upon; a lack of
sufficiently colder air (albeith enough to where parts of northern
Maine could witness up to a tenth of an inch of ice), a storm
track over the I-95 corridor, and lower SLRs that would support a
heavy/wet snow type. WPC PWPF does show the potential for some ice
accumulations This kind of setup typically favors the Appalachians
for potential heavy snowfall, but when this transition to snow
occurs will be key in determining amounts and impacts in subsequent
forecast cycles.


...Northern Rockies & Northern High Plains...
Day 1...

Snow will taper off later this evening in the Olympics and Cascades
as the upper level trough responsible for the disruptive snowfall
amounts heads east. Pacific moisture associated with the upper
trough will continue east of the Cascades across to the Rockies as
the cold front advances through the High Plains and Central
Rockies. WPC probabilities show moderate-to-high chances (50-70%)
for
snowfall >4" of snow are over the higher terrain including the
Bitterroots, Lewis Range, Blue Mountains, and into the Little Belt
and Big Snowy Mountains in central Montana, the Absarokas in
southern Montana, and the Big Horns in northern Wyoming.

As a storm system tracks east along the US/Canada border, a
deformation band of snow will pivot over eastern Montana and
western North Dakota this evening. The storm's progressive movement
and strong winds fracturing dendrites will put a cap on snowfall
totals, but those same winds are likely to cause blizzard
conditions in parts of eastern Montana. The WSSI does suggest
Moderate Impacts that are primarily driven by wind gusts that could
approach 55 mph, leading to whiteout conditions and snow drifts.
Strong NWrly winds also favor upsloping flow into the Black Hills
where WPC PWPF shows moderate-to-high chances for snow totals >4"
above 6,000ft. The strong upper level ascent and steep lapse rates
could support possible snow squalls tonight and into Monday
morning as well across the western Dakotas. Snow should taper off
throughout the region by Monday afternoon.


...Northern Minnesota and northeastern North Dakota...
Day 1...

Low pressure over southern Saskatchewan will move toward northern
ND/MN tonight, lifting its warm front through the region and acting
as the primary source of lift for precipitation throughout the
region. While precipitation will start out as snow along the
Canadian border, persistent mid/low-level WAA may cause snow to
changeover to freezing rain for portions of eastern North Dakota
and north-central Minnesota this morning, with several hundredths
of an inch of ice possible in parts of north-central Minnesota and
as far east as northern Wisconsin. The heaviest snow is set to
occur along the Minnesota Arrowhead where the initial snowfall via
WAA aloft will stay snow the longest, and snowfall rates tonight
will be enhanced by easterly flow off Lake Superior. WPC
probabilities for at least 6 inches of snow are >50% along the
northern coast of the Minnesota Arrowhead. In fact, the very tip of
the Arrowhead sports moderate chances (40-60%) for snowfall totals
over 12" through Monday afternoon.


...CO Rockies/Sangre de Cristos...
Days 1-2...

Height falls at the southwest side of a large positively-tilted
longwave trough over the Heartland will swing through the central
Rockies on Monday, bringing with it a plume of residual Pacific
moisture. High pressure building in from the north in the wake of a
cold frontal passage Monday morning will result in modest upslope
flow into Colorado's Front Range during the day. This upslope
component will extend as far south as the Sangre de Cristos in
northern New Mexico by Monday night. Periods of snow across the
region are expected as the system moves quickly through the region
and heights build in by Tuesday. WPC probabilities have increased
over the past 24 hours withj high chance probabilities (>70%) for
snowfall >4" that now stretchs from parts of the Palmer Divide all
the way to Raton Pass. The highest and more remote elevations could
see localized amounts approach 8" by the time the snow comes to an
end Tuesday morning, and this does include the Raton Pass where
snow could cause hazardous travel conditions.

...Great Lakes...
Day 3...

As a robust 500mb low dives south Wednesday morning, an Arctic
front and blusterly NWrly winds will race over the Great Lakes
starting Wednesday morning over Lake Superior, then over Lake
Michigan by Wednesday evening. NAEFS and ECMWF SATs agree that
850mb temps rushing over Lake Superior and Michigan will be below
the 10th climatological percentile. The sharp contrast between the
Lakes water temperatures and frigid low-level temperatures advected
by brisk NW winds will kick-start the lake-effect snow (LES)
machine into high gear yet again by Wednesday night. This same cold
air-mass will race over the eastern Great Lakes Wednesday night
and lead to more heavy snowfall into Thursday. For the scope of
this discussion (ending 00Z Thurs), WPC PWPF shows high chance
probabilities (>70%) for snowfall totals >6" along the northern
coasts of the Michigan U.P. and along the western coast of
Michigan's Mitten. Look for some snowfall rates within the more
robust bands to top >2"/hr Wednesday night in these areas.


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


Mullinax