Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
258 PM EST Tue Dec 31 2024

Valid 12Z Tue Dec 31 2024 - 12Z Fri Jan 03 2025

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

A 500mb vorticity maximum shearing off from an upper low over the
North Pacific will escort a slug of moisture into the Pacific
Northwest tonight. This elongated area of upper-level divergence
aloft paired with a couple frontal systems that will provide a
source of lift at low levels as well. Snow levels as low as 2,000ft
are expected initially over western Washington, but as the warm
front approaches the northern California and southern Oregon coast
Wednesday evening, snow levels in the southern Oregon Cascades
will rise to as high as 7,000ft by Wednesday night. This milder
air-mass remains in place ahead of the next storm system that
arrives on Thursday with rain being the primary precipitation type
south of the Columbia River. The Cascades and Olympics will be the
mountain ranges likely to remain mostly snow above 3,000ft with
locally heavier totals above 4,000ft. Meanwhile, sub-freezing wet-
bulb temperatures may become more prominent in the Columbia Basin
and along the leeward slopes of the Cascades Thursday afternoon.
This could result in some minor ice accumulations in these areas
through Thursday night. The active Pacific pattern continues into
Friday as another plume of moisture surges into northern California
and the Oregon Cascades. Steeper 700-500mb height falls associated
with this system means plummeting snow levels to where parts of the
Trinity/Shasta and Oregon Cascades above 5,000ft could see locally
heavy snowfall. In total over the next three days, the Oregon
Cascades have high chances (>70%) for snowfall totals >8" through
Wednesday afternoon. After the brief lull and rising snow levels
Wednesday night, the Thursday system then gives the Washington
Cascades moderate-to-high chances (50-70%) for >6" of snowfall at
elevations above 4,000ft.

Farther east, the Northern Rockies best chances for accumulating
snowfall arrives New Year's Day and into early Thursday (Jan. 2)
when the first 500mb trough pushing through the Pacific Northwest
Wednesday morning provides modest upper level divergence aloft and
directs the same plume of Pacific moisture towards the higher
elevations of the northern Great Basin, the central Idaho ranges,
and into both the Bear River and Teton Ranges. The two latter
ranges above 8,000ft sport high chance probabilities (>70%) for
snowfall >8" through Thursday morning. As far east as the >9,000ft
peaks of the northern Colorado Rockies have similar high chance
probabilities for >8" of snowfall between Wednesday morning and
Thursday morning.

...Great Lakes, Central Appalachians, Northeast...
Days 1-3...

A dynamic and complicated storm evolution will make for a
synoptic-scale snowfall event through Wednesday evening, then
transition into a lake-effect snow (LES) event Wednesday night and
through the remainder of the week. Focusing on the synoptic-scale
event, an occluded low tracking through northern Ohio is rather
anomalous with NAEFS showing MSLP values below thew 1st
climatological percentile over the upper Ohio Valley. The occluded
front will spawn a new wave of low pressure along the triple point
located along the New Jersey coast tonight while, at the same time,
the 250-500mb trough aloft maintains a negative tilt over
Pennsylvania. The excellent divergence atop the atmosphere over
the Northeast will lead to a steady deepening of the surface
cyclone throughout New Year's Day. As the primary occluded low
weakens, it will effectively turn into a TROWAL over the eastern
Great Lakes late tonight and into New Year's morning. This TROWAL
will work in tandem with accelerating low-level NWrly flow over
Lakes Huron and Erie to support lake-enhanced snowfall rates over
northeast Ohio, northwest Pennsylvania, and western New York. WPC's
Snowband Probability Tracker suggested that a couple HREF members
showed >1"/hr snowfall rates in these areas Wednesday morning.

Farther east, there is growing consensus on an impressive 850-700mb
FGEN signal over northern New England that is paired with a
southeasterly IVT that tops the 90th climatological percentile.
With no shortage of moisture and impressive mesoscale forcing,
strong dynamic cooling over the Whites and northern Maine will
result in heavy-banded snowfall Wednesday morning and into
Wednesday afternoon. By Wednesday evening, the steadily deepening
cyclone over the Gulf of Maine will wrap Atlantic moisture around
the western flank of the low and lead to a deformation zone along
the Maine/Canada border while wrap around moisture and upslope
ascent via NWrly flow strikes the White Mountains. Moisture
associated with the TROWAL will also support prolonged lift, with
assistance from upslope enhancement, in the Adirondacks, Greens,
and as far south as the Berkshires Wednesday night into Thursday
morning. Cyclonic flow will persist through Thursday night and into
Friday as the cyclone has effectively no where to go over Quebec
thanks to a strong -NAO block over Greenland and the Davis Strait.

The prolonged cyclonic flow will be responsible for a multi-day
LES setup as far west as the Michigan U.P. and as far east as the
Tug Hill and Finger Lakes of central New York. The heaviest
snowfall via LES bands begins in the Michigan U.P. starting
Wednesday morning, while LES really kicks in late Wednesday night
or Thursday morning for the eastern Great Lakes. WPC probabilities
show moderate-to-high chances (50-70%) for snowfall >18" in parts
of the Tug Hill, along the Chautauqua Ridge, and in some select
parts of the eastern Michigan U.P.. Through the three-day span, the
WSSI shows Major Impacts along the Chautauqua Ridge (including
some portions of I-90) and the Tug Hill. These areas can expect
considerable disruptions to daily life with dangerous to near
impossible travel the most notable impact. Moderate Impacts
(hazardous travel conditions) are possible in areas just east of
Cleveland and within the Syracuse metro area. This same cyclonic
flow regime will be the catalyst for heavy mountain snow in the
central Appalachians between early Wednesday morning and Thursday
morning. WPC probabilities show moderate-to-high chances (50-70%)
for snowfall totals >6" above 2,500ft in the Laurel Highlands on
south into the east-central West Virginia Appalachians.

...Midwest...
Day 2...

The sheared off 500mb vorticity max that tracked through the
Pacific Northwest on Wednesday will head for the Central Plains
Wednesday night. Surface-850mb frontogenesis over Nebraska will
help to spawn low pressure along the front and track east towards
the Missouri River late Wednesday night and into Thursday morning.
The storm system will be a small and compact one, but the presence
of strong 850mb FGEN and an air-mass plenty cold enough to support
snow could generate a narrow band of heavy snow from eastern
Nebraska to central Iowa. As 850mb WAA wains Wednesday afternoon,
the mesoscale lift driving the snow across parts of eastern
Nebraska and central Iowa will gradually taper off as it moves east
into Illinois and Indiana. WPC probabilities show low chances
(10-30%) for >4" amounts in west-central Iowa, but this event is
more likely to support a general 2-4" event with moderate
probabilities (40-60%) for >2" of snow from northeast Nebraska to
south-central Iowa Thursday morning.


Mullinax