Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Thu Jan 19 2023

Valid 12Z Thu Jan 19 2023 - 12Z Sun Jan 22 2023

...Upper Midwest, Northern Great Lakes, and Northeast...
Days 1-2...

An expansive winter storm stretching from the Upper Midwest to the
Northeast will produce a variety of impactful winter weather
precipitation types through the end of the week.

Early this morning, the winter storm will be making its way out of
the Central Plains and head for the Great Lakes. Heavy snow bands
along the northern and northwest flanks of the 850mb low will
traverse northern Wisconsin, northern Michigan, and the U.P. of
Michigan this morning. This area resides favorably beneath the
left exit region of a 150kt jet streak positioned over the
Mid-South with supportive 300-700mb Q-vector convergence over the
northern Great Lakes This area of strong vertical motion and
strong frontogentical forcing also resides within a well saturated
layer aloft, particularly in the 700-500mb layer where NAEFS shows
mean specific humidity values >90th climatological percentile
values. With such a saturated profile around 600-550mb (which on
average is where the DGZ is located) and plenty of both synoptic
and mesoscale lift present, snow will comes down heavy at times
within the TROWAL, which according to the Snowband Probability
Tracker (SPT) does show members of the HREF suggesting 1-2"/hr
rates are expected, and could even be heavier than 2"/hr at times.
The experimental PWSSI does show a large area of >60% for Moderate
Overall Impacts from northern Wisconsin to northern Michigan
between 12Z Thurs - 12Z Fri. It is worth noting that "Snow Rate"
is what is driving the rather high probabilities, which does speak
to the intense nature of these snowbands. As the strongest forcing
moves east Thursday evening, residual snow showers will stick
around across Michigan's U.P. Thursday evening thanks for cyclonic
flow atop the northern Great Lakes. That said, snow showers will
gradually diminish and eventually dissipate by Friday morning.

Farther east, vertical ascent will be maximized aloft from
favorably positioned jet streaks over northern Maine and in the
Tennessee Valley, placing their respective diffluent left exit
(Tennessee Valley jet streak) and right entrance region (northern
Maine) directly over the Interior Northeast. Strong southerly flow
associated with 290K isentropic glide and resulting 850mb
frontogenesis in the northern Mid-Atlantic will generate periods
of moderate-to-heavy precipitation Thursday morning. As the 850mb
front advances north and east, look for periods of disruptive
wintry weather to advance north into the heart of New England. It
is a messy precipitation forecast in north-central Pennsylvania
and into the southern tier of New York. It is in these areas
where, after a brief start as snow, the warm nose aloft leads to a
>0C temperature profile with sub-freezing temperatures hanging on
to allow for several hours worth of icy freezing rain and sleet.
Latest WPC PWPF freezing rain >0.1" probabilities were the highest
(50-70%) in the Catskills, Berkshires, and into the southern
Adirondacks. Farther north, the boundary layer will stay colder
longer and allow for snow to be the dominant precipitation type.
Latest WPC PWPF shows 70-90% probabilities for >6" of snowfall
from the northern Adirondacks and through the vast majority of
Vermont and New Hampshire to southern Maine. The WSSI does depict
Moderate Impacts in southern New Hampshire, far southeast Maine,
and portions of northern Massachusetts through Thursday night. By
Friday morning, a coastal low will form off the Massachusetts
Capes and the 850mb low will direct Atlantic moisture via easterly
850mb flow back into New England. This will allow for additional
periods of snow to stick around through Friday morning, but rates
will diminish throughout the day as the best forcing moves
offshore Friday afternoon.

...Western U.S./Great Basin/Four Corners..
Days 1-3...

The next winter storm set begins to unfold as a sharpening upper
level vorticity maximum amplifies and cuts off into an upper low
over the Great Basin Thursday night and tracks into northern
Arizona on Friday. Broad synoptic-scale ascent over northern
Nevada and into both southern Utah and northern Arizona, along
with crashing snow levels will result in periods of snow
throughout these regions. While moisture is lacking initially, it
will be the colder temperatures in the 500-700mb layers that help
to cause SLRs to come in higher throughout the day on Friday. By
Friday afternoon, the upper low will begin interacting with the
subtropical jet and both WAA in the 850-700mb layer, as well as
positive vorticity advection out ahead of the upper low, will
increase and be directed at the southern High Plains.

As the synoptic scale ascent strengthens, the low level jet in
western Texas will also strengthen Friday evening. All global and
ensemble guidance is in agreement on a steady injection of 850mb
moisture flux originating out of the western Gulf of Mexico. By
00Z Sat, the 850-700mb low is emerging over northern New Mexico
and the warm conveyor belt of 850mb moisture will wrap around the
northern flank of the 700mb low. It is here, and along the front
range of southeast Colorado and northwest New Mexico, where
periods of snow will breakout and fall heavily at times beneath
the developing deformation axis. The steady conveyor belt of 850mb
moisture flux will give this storm system enough moisture with
SLRs in the 11-14:1 range that there can be a 6-9 hour window
where hourly snowfall rates could range between 1-2"/hr Friday
evening and into the overnight hours. The 12Z ECMWF EFI continues
to hint at southeast Colorado and southwest Kansas as seeing the
best odds for anomalously heavy snowfall in the 00Z Sat - 00Z Sun
time frame as well. Latest WPC PWPF gave southwest Colorado and
portions of far northeast New Mexico a 60-80% chance for snowfall
totals >6", and a 40-60% chance for snowfall totals >8". The
experimental PWSSI does depict a swath of 40-50% odds of Moderate
overall impacts in southeast Colorado, far northwest New Mexico,
and into southwest Kansas, with Snow Rate being the primary driver
in causing such impacts.

The upper low begins to merge with another upper level disturbance
diving south into the Central Plains Saturday afternoon, resulting
in a weakening in the forcing over central Kansas on east into
Missouri. This will result in periods of snow, but totals will be
more tame compared to their neighbors to the west.


Key Messages for the Central Plains, Great Lakes, and Northeast
Winter Storm:
--A winter storm will produce heavy snow across the Upper Midwest
and Great Lakes today and across the Northeast by this evening and
into Friday morning.

--Intense snow rates greater than 2"/hr are possible  at times
within the heaviest snow bands from northern Wisconsin through
northern Michigan. The forecast calls for an additional snowfall
of 6-12 inches, producing event total snowfall in excess of 16
inches in some areas.

--Heavy snow rates of 1-2â€/hr combined with winds gusting up to
35 mph will result in dangerous travel due to blowing snow with
near-zero visibility at times.

--Farther east, heavy snow rates of 1+â€/hr will create dangerous
travel Thursday night and into Friday morning across New England,
with lighter periods of snow continuing through much of Friday.

--Freezing rain may lead to ice accumulations of more than 0.1â€,
especially in the Catskills, Poconos, and Berkshires, creating
slippery travel and isolated power outages.

Mullinax