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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Sun Dec 22 2024

Valid 00Z Mon Dec 23 2024 - 00Z Thu Dec 26 2024


...California, Pacific Northwest, & Intermountain West...
Days 1-3...

As the first of a pair of atmospheric rivers (AR) causing active
weather continues moving ashore in northern CA, OR, and WA,
significant precipitation associated therewith should diminish into
tonight as the moisture plume shears apart over the interior
Pacific Northwest. GEFS integrated vapor transport (IVT) analysis 
shows this first AR has topped at about 600 kg/m/s off the Oregon
coast this morning. This AR will neither be accompanied by an
unusually cold air-mass, nor will it be directed at a continental
polar air-mass over the Northwest. Snow levels are currently over
7,000ft as per 18Z NBM analysis from the Cascades west. As the
precipitation moves inland, snow levels will remain above 6,000 ft
into Idaho and the interior Northwest. These very high snow levels
will confine any significant snowfall through Monday evening to the
highest peaks of the northern Washington Cascades and the Blue,
Salmon River, Tetons, and Sawtooth Ranges. The peaks of the Tetons
above 8,000ft sport moderate- to- high chances (50-70%) for
snowfall totals >6", while the Blue, Salmon River, and Sawtooth
show similar probabilities for >4" through Monday evening.

By Monday afternoon, the next AR will be moving into the Pacific
Northwest coastline, especially northern California. This AR is
noticeably stronger than the one moving ashore now. NAEFS shows a
larger 750 kg/m/s fetch (above the 99th climatological percentile
off the northern CA coast) with origens stemming out of the
subtropical Pacific. The initial round of precipitation arriving
Monday evening will be primarily rain in western OR and northern CA
(snow levels as high as 7,000ft), but snow will be most likely in
the Cascades and Blue Mountains. By Tuesday morning, sharper height
falls from the approaching upper trough will force snow levels to
drop to as low as 4,000ft in the Cascades, Olympics, and Blue,
while farther south, snow levels fall to ~5,000ft in the CA ranges
and northern Great Basin. Unlike today's AR, Tuesday's AR will
have a higher longevity with >90th climatological percentile IVTs
extending as far inland as the Wasatch and on south to the Mogollon
Rim Tuesday night. Mountain snow will be common throughout much of
the Intermountain West Tuesday night with heavy snow advancing
into the Tetons, Uinta, and Wasatch by early Wednesday morning.

WPC probabilities depict high chances (>70%) for snowfall >6" for
the central Sierra Nevada above 7,000ft. Farther north, moderate
chances (40-60%) for snowfall >4" are shown in the higher terrain
of the Blue, Salmon River, and Sawtooth ranges. Lastly, the Wasatch
and Tetons show low chances (10-30%) for snowfall >4" above
8,000ft through 12Z Wednesday, but additional snowfall is possible
Wednesday afternoon. In total through 12Z Wednesday, there are high
chances (>70%) for snowfall totals >12" for elevations above
5,000ft in the Cascades, while similar high chance probabilities
for >8" of snow are present in the Blue, Salmon River, Sawtooth,
Tetons, and central Sierra Nevada. Snow will continue into Utah and
Colorado through the day on Wednesday with a 30-50% chance of 3-6
inches of snow through Wednesday night.


...Northern Plains, Great Lakes, to Northeast...
Days 1-3...

A progressive 500mb shortwave trough over western Montana this
morning will be the catalyst for the next winter storm to track
through the Great Lakes on Monday and the Northeast Monday night
into Tuesday morning. The initial wave of low pressure is quite
weak, with modest upper level ascent aided by a 110kt 250mb jet
streak's left-exit region over the Northern Plains. In addition,
the moisture source and DGZ are marginal while low level
temperatures above the surface are above freezing. This is an icy
wintry mix setup from northeast Montana through central North
Dakota and into central Minnesota into tonight. WPC probabilities
shows moderate-to-high chances for ice accumulations >0.01" for
much of west-central North Dakota. The potential for light icing
extends as far east as eastern Wisconsin, northern Illinois,
southern Michigan, and northern Indiana by Monday morning.

Monday morning is when the storm system tracking through the Great
Lakes becomes more organized, while at the same time generating a
>300 kg/m/s IVT (>90th climatological percentile values via NAEFS)
oriented at Michigan and the eastern Great Lakes. An expansive
area of 850-700mb WAA over a boundary layer air-mass that remains
quite cold and dry allows wet- bulb temperatures to support snow
being the primary precipitation type for much of Michigan's Mitten
and the eastern Michigan U.P.. Combined with excellent upper level
divergence courtesy of a broad 250-500mb trough to the west, and
there is likely to be a swath of heavy snow over northern Michigan
and the far eastern Michigan U.P.. Residents in southern Michigan
should anticipate some wintry precipitation, although it remains
unclear if snow or a wintry mix (causing light ice accumulations
in the process) will be dominant precipitation types. The latest
WPC probabilities show high chances (70-90%) for snowfall totals
>4" in northern Michigan and over the thumb of Michigan, while some
parts of northern Michigan also have moderate chances (40-60%) for
localized totals surpassing 6" through Monday afternoon.

This same storm system will direct its anomalous moisture source
and modest low-level WAA into the Northeast Monday night.
Sufficient upper-level divergence provides support for the healthy
shield of precipitation throughout the region, while the air-mass
remains quite cold and dry ahead of the storm (temps in the teens,
dew points in the single digits). Higher SLRs of 12-16:1 are
anticipated in these areas and the ground is very cold, which
combined with the event occurring at night, will maximize snowfall
accumulation potential. The storm races off over Nova Scotia by
midday Tuesday and snowfall should taper off by Tuesday afternoon.
WPC probabilities show moderate-chance probabilities (40-60%) for
snowfall >4" in parts of northern NY (including the Tug Hill) and
in the tallest peaks of the Green mountains. Should guidance
continue to trend wetter, it is possible for an increase in
snowfall amounts in subsequent forecast. Downeast Maine also sports
similar moderate-chance probabilities for >4" of snowfall through
Tuesday morning. For those affected areas listed above, the WSSI-P
does sport moderate-chance probabilities (40-60%) for Minor Impacts
from northern Michigan and northern NY to Downeast Maine.


Wegman/Mullinax