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

Valid 12Z Sun Dec 22 2024 - 12Z Wed Dec 25 2024


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

A pair of atmospheric rivers (AR) are going to make for an active
multi-day stretch of weather from California on north through the
Northwest and into the Rockies. The first AR arrives Sunday
afternoon, which NAEFS shows will top 750 kg/m/s off the Oregon
coast Sunday morning, will weaken on approach but still deliver
>90th climatological percentile precipitable water values into the
northwestern U.S.. 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 will initially be
as low as 4,000ft in the Cascades and Olympics, but rise to above
5,000ft by Sunday night as WAA increases aloft. This same streams
of moisture will advance inland through the Northern Rockies Sunday
night and into Monday with locally heavier snowfall possible in the
>5,000ft peaks of the Blue Mountains, above 7,000ft in the Salmon
River and Sawtooth Mountains of Idaho, and the Tetons in Wyoming.
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 is already getting ready to strike
the Pacific Northwest coastline, but this AR is noticeably stronger
than the one arriving late Sunday. 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 Sunday'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.


...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 Midwest today, the Great Lakes on Monday, and the
Northeast Monday night into Tuesday morning. The initial wave of
low pressure will be quite weak on Sunday, but 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 >0C.
This is an icy wintry mix setup from northeast Montana through
central North Dakota and into central Minnesota Sunday and into
Sunday night. WPC probabilities shows moderate-to-high chances for
ice accumulations >0.01" for much of 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 souther 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. Latest WPC
probabilities show moderate-to-high chances (50-70%) 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 do 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.


Mullinax