Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EST Wed Jan 03 2024

Valid 12Z Wed Jan 03 2024 - 12Z Sat Jan 06 2024

...California/Great Basin/Four Corners...
Days 1-2...

An upper low tracking through California this morning will make
its way southeast into the Lower Colorado River Valley Wednesday
night and head for the Four Corners region on Thursday. As the
250mb trough makes this trip through the Southwest, so will its
diffluent left-exit region that will be ideally placed over the
myriad of mountain ranges from southern California to the southern
Rockies. This upper trough will also direct a healthy supply of
850-700mb moisture flux into the region while southwesterly flow
aloft also fosters favorable upslope flow into orthogonally
favored ranges. These ingredients all support heavy snow at
elevations >5,000ft in the Sierra Nevada, >7,000ft in the Great
Basin, southern Utah, and the higher terrain of central/northern
Arizona, and >8,000ft in the Southern Rockies of Colorado and New
Mexico. Snowfall rates at their peak will range between 1-2"/hr in
all of these ranges. WPC PWPF shows moderate-to-high chances
(50-80%) for >6" of snow in the central Great Basin, southwest
Utah, along the Mogollon Rim and Gila Mountains of Arizona, the
San Juans, and both the Nacimiento and Sangre De Cristo of
northern New Mexico. Heavy snow could even be observed as far
south as the Sacramento Mountains where WPC PWPF shows
moderate-to-high chances (50-80%) for snowfall accumulations >6".
The WSSI shows Moderate to Major Impacts in the central and
southern Sierra Nevada where more significant impacts to travel
and infrastructure could be felt. Elsewhere, the remainder of the
Great Basin and Southwest mountains ranges can generally expect
Minor Impacts with Moderate Impacts in the tallest peaks.

...Great Lakes & Northern New England...
Days 1-2...

A surface low an associated cold front swinging through the Great
Lakes will spark lake effect snows downwind of Lakes Superior,
Erie, and Ontario. 850mb temperatures will range comfortably
between -10C to -15C, which combined with convergent low level
winds will spur the development of multi-band streamers taking
advantage of steepening low level lapse rates. The area most
favored to pick up >4" of snowfall is the Tug Hill Plateau where
WPC PWPF show high chances overall (>70%). In fact, there are even
some even moderate probabilities (40-60%) for >6" on the western
upslope side of the Tug. Meanwhile, positive vorticity advection
over northern New England along with a narrow ribbon of 850-700mb
moisture will allow for snow to envelop the region Wednesday night
and into Thursday morning. WPC PWPF shows moderate-to-high chances
(50-70%) for >2" of snow in the central U.P. of Michigan, in
western New York downwind of Lake Erie, and in the Adirondacks.
Lesser chances (20-40%) for >2" are present in northern Vermont,
far northern New Hampshire, and along the western Canadian border
of Maine. In terms of impacts, the WSSI depicts the Tug Hill
Plateau and select ares of the Michigan U.P. with potentially
dealing with Minor Impacts, suggesting a few inconveniences and
disruptions to daily life are possible on Wednesday. Otherwise,
other areas with lesser snowfall totals could contend with reduced
visibilities and snow covered roads.

...Southern to Central High Plains...
Days 2-3...

An exceptional upper low (below the 2.5 climatological percentile
at 200-500-700mb according to NAEFS 12Z Thursday) tracking into
New Mexico and an increasingly negative tilted trough at 250mb
will foster excellent divergent flow atop the atmosphere Thursday
afternoon. This will result in a couple of key atmospheric
responses in the southern Plains. First is the tightening pressure
gradient that causes a strengthening LLJ to usher in a fetch of
850-700mb moisture originating out of the western Gulf of Mexico.
The second is a deepening 850-700mb low over eastern New Mexico
aiding in the development of strong 300-700mb layer-averaged
Q-vector convergence field over northeast New Mexico Thursday
afternoon. This convergence zone then advances east over the TX/OK
Panhandles and southwest Kansas by early Thursday evening. With
the warm conveyor belt of moisture from the south ascending around
the northern flank of the 700mb low, there is the potential for
heavy snow banding over northeast New Mexico that could reach into
the OK/TX Panhandles Thursday afternoon and evening. The WPC
Snowband Probability Tracker Prototype showed the potential for
2"/hr snowfall rates in northeast New Mexico that could then make
its way into the Oklahoma and northern Texas Panhandles around 00Z
Thurs. These kind of rates could make their way as far north as
southwest Kansas, but chances are more uncertain as it is a little
farther away from the strongest forcing located to their south.

WPC PWPF shows moderate odds (40-70%) for snowfall totals >6" in
northeast New Mexico while there are also low but respectable
chances (20-40%) for snowfall totals >8". In southeast Colorado,
the northwestern corner of the Texas Panhandle, and the far
western Oklahoma Panhandle sport Moderate chances (40-60%, locally
up to 70% in southeast Colorado) for >4" of snowfall. The WSSI
does depict Moderate Impacts in northeast New Mexico and southwest
Colorado, implying these areas could see the more hazardous travel
conditions due to a combination of Snow Amount and Blowing Snow.
Meanwhile, the Probabilistic WSSI show moderate-to-high chances
(60-80&) for Minor Impacts in southwest Kansas, the Oklahoma
Panhandle, and the northwestern most corner of the Texas
Panhandle. Similar to their neighbors a little farther west, Snow
Amount is the primary driver in impacts but some Blowing Snow
related impacts could occur as well.

...Northwest...
Days 2-3...

A powerful storm system over the Aleutians will direct a series of
Pacific disturbances at the Pacific Northwest and British
Columbia. The first and weaker of the two disturbances arrives
early Thursday morning with heavy snow occurring above 5,000ft in
the Olympics and Cascade Range. The surge of 850-700mb moisture is
relatively short lived, but prolonged westerly 850-700mb winds
Thursday night into Friday morning will support upslope flow into
the Cascades, keeping some periodic snowfall in the forecast. The
second and stronger shortwave trough arrives Friday evening with
sharper height falls and a stronger cold front. This system will
also feature a healthier 850-700mb moisture fetch, which combined
with the colder temps aloft, will force snow levels to drop to as
low as 2,000ft. The Washington Cascades and Olympics at elevations
>3,000ft will be most preferred for heavy snowfall >6",
highlighted by the WPC PWPF which shows high chances in those
ranges (>70%). There is a more impactful potential that exists
with WPC PWPF even sowing moderate chances (40-60%) for >12" late
Friday into Sunday morning. WPC's Probabilistic WSSI shows
moderate chances (50-60%) for Moderate Impacts in the Olympics
above 3,000ft through early Saturday morning. Expect potentially
treacherous travel conditions in the Washington Cascades in
Olympics, particularly any passes that are above 3,000ft in
elevation.

The probability of significant ice (> 0.25 inch) across the CONUS
is less than 10%.

Mullinax