Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Tue Jan 02 2024

Valid 12Z Tue Jan 02 2024 - 12Z Fri Jan 05 2024

...California/Great Basin/Four Corners...
Days 1-3...

An upper level trough tracking into the West Coast Tuesday evening
will direct a conveyor belt of 850-700mb moisture into California
and the Great Basin. NAEFS shows 500mb and 200mb heights that are
below the 10th climatological percentile while aiming an IVT above
the 90th climatological percentile (300+ kg/m/s) at central and
southern California Tuesday night. Snow levels will initially
hover around 6,000ft, but in wake of a cold frontal passage, snow
levels will drop to as low as 4,000ft. Snow will fall heavily from
the Shasta/Salmon Mountains on south through the Sierra Nevada
Tuesday night, then snow will pick up farther south along the
higher peaks of the Transverse Range Wednesday morning. 700mb
moisture will spill over the Sierra Nevada and into the Great
Basin where snow is also expected from southern Oregon and Idaho
to southern Nevada on Wednesday.

By Wednesday afternoon, a vort max revolving around the base of
the 500mb upper low will track east through the Lower Colorado
Valley. At 250mb, a jet streak off the California coast will place
its diffluent left-exit region over the Four Corners states,
providing plenty of lift within the atmospheric column over the
Southwest. Add in the same conveyor belt of 850-700mb moisture
flowing into the Four Corners region, and the setup becomes
favorable for heavy snow from southwest Utah and the Mogollon Rim
of Arizona to the Southern Rockies of Colorado and northern New
Mexico. By Thursday morning, a new 500mb low beginning to form
over the Arizona/New Mexico border and causing increasingly
divergent flow withing the 250-500mb layer over the central High
Plains. This will spawn a surface low in the Texas Panhandle
Thursday afternoon, as well as a strengthening 850mb jet over the
Southern and Central Plains that funnels Gulf moisture into the
region. Temperatures will be cold enough to support snow, with
potential banding on the northern flank of the 700mb low that will
be near the Texas/Oklahoma Panhandles by Friday morning.

WPC PWPF shows high chances (>70%) for >8" of snowfall in the
Shasta/Salmon mountains of northern California, as well as much of
the Sierra Nevada above 6,000ft. The >7,000ft terrain of central
Nevada have moderate chances (40-60%) of receiving >6" of snow, as
do the Gila Mountains in eastern Arizona, the San Juans, and the
both the Nacimiento and Sangre De Cristo of northern New Mexico.
The WSSI shows Moderate Impacts in the mountains of northern
California and along the Sierra Madre, while most of the affected
ranges in the Great Basin and Southern Rockies can expect Minor
Impacts. Treacherous travel conditions are anticipated in these
regions with the Sierra Nevada likely to see the impacts that
include potential closures and disruptions on roads.

...Great Lakes...
Days 2-3...

A cold front tracking south and east out ahead of an approaching
upper level vort max will spur lake effect snow showers to develop
along the U.P. of Michigan Tuesday night. Sharpening low level
lapse rates, thanks to -12 to -15C temps at 850mb, will allow for
some weak instability for bands off Lake Superior to utilize in
producing 1"/hr snowfall rates in some bands. By Wednesday
morning, the upper level vort max will amplify as it tracks south
through the Upper Mississippi Valley. Surface winds over the
eastern Great Lakes will become increasingly convergent as the
cold front approaches the region and result in lake effect
streamers off of Lakes Erie and Ontario during the day. However,
it will be the amplification of the 500mb trough over the Great
Lakes that fosters increased diffluent flow at upper levels over
the eastern Great Lakes and the Northeast. Snow will likely be at
its heaviest, initially, over the U.P. of Michigan and the top of
Michigan's Mitten as the cold front passes through, with the
heaviest of the Lakes Erie/Ontario streamers then becoming the
heavier bands Wednesday afternoon and into Wednesday night. Latest
WPC PWPF is highlighting the Tug Hill Plateau with having
moderate-to-high chances (60-80%) of receiving >4" of snowfall.
Otherwise, portions of the Michigan U.P. and northern-most tip of
Michigan's Mitten feature moderate chances (40-60%) for >4" of
snowfall. Minor impacts, including snow covered roads and
resulting hazardous travel are possible in the most heavily
affected areas.

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

Mullinax