Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Mon Jan 15 2024

Valid 00Z Tue Jan 16 2024 - 00Z Fri Jan 19 2024

...Deep South, TN Valley, Appalachians through Northeast...
Days 1-2...

The coldest air-mass so far this winter season is responsible for
the plethora of winter weather hazards along and east of the
Rockies today. The Arctic front is positioned from southern
Louisiana stretching northeast into the southern Appalachians.
Bitterly cold temperatures are in place north of the front, while
at the same time, a steady stream of 850-700mb moisture passes
over the Deep South and up the East Coast. Along and just north of
the Arctic front, surface temperatures will remain <32F while the
aforementioned 850mb moisture stream also cause a burgeoning warm
nose at low levels. This is a classic overrunning setup that
favors an icy wintry mix from as far south as the Upper Texas
coast to the Mississippi Delta region and as far north as the
southern Appalachians. Most areas can expect <0.10" of ice
accumulation the remainder of the afternoon and evening, but for
some portions of the Deep South where any ice accumulation is
dangerous on roads, the greatest concern is in southern Louisiana,
southern Mississippi, and north-central Alabama where WPC PWPF
shows moderate-to-high chances (50-80%) for additional ice
accumulations >0.01". There is even a chance, albeit low (5-20%)
for >0.1" of ice accumulation in southern Mississippi and
west-central Alabama. These kind of light ice accumulations,
especially at night, can make identifying icy roads very
difficult. Treacherous travel conditions are likely to persist in
these areas tonight and into Tuesday morning.

Farther north, the boundary layer will remain sufficiently cold
enough for snow to be the primary precipitation type from the
Tennessee Valley on east through the western Tennessee, into the
central Appalachians, and up the I-95 megalopolis of the
Northeast. Snow will fall heavily this afternoon and into the
early evening hours in western Tennessee and the central
Appalachians initially where the strongest 850mb frontogenetical
forcing will ensue and a persistent upslope component into the
southern and central Appalachians will allow for additional
snowfall totals of 2-4".

By this evening, the same 850-700mb moisture transport will become
accentuated by the approach of a 500mb shortwave trough racing
through the Midwest. Starting around 06Z tonight, a 100+ knot
500mb jet streak will develop over the OH-MS River Confluence with
the nose of said jet streak approaching the northern Mid-Atlantic
and Northeast. With the strengthening 850mb front oriented from
northern Virginia on northeast along I-95 to southern New England,
and ample upper level divergence present thanks to the upper level
trough to the west, strong vertical ascent within the column will
lead to a band of moderate-to-heavy snow from central and northern
Virginia this evening to the Delaware Valley and NYC metro area
overnight. This band will quickly move north through the Lower
Hudson Valley and into central New England Tuesday morning.
Eventually, the upper level vorticity maximum tracking through the
Great Lakes will spawn a strengthening area of pressure south of
Long Island. This will direct southerly 850mb flow into New
England and add additional moisture into an air-mass ripe with
high SLRs.

WPC PWPF show moderate chances (40-60%) for snowfall totals >4"
from northeast Maryland and eastern Pennsylvania and into the
Catskills. Chances grow to moderate-to-high chances (60-80%) in
the Green and White ranges of the interior Northeast. The region
with the highest chances for >6" of snowfall are present in
northern Maine where probabilities are moderate-to-high, or 50-80%
for >6". Far northern Maine (closer to Caribou) also sports
moderate chances (50-60%) for >8" of snow in eastern Maine.
Overall, snowfall totals will yield primarily Minor Impacts, as
evident by the Probabilistic WSSI showing >60% chances for Minor
Impacts from the DC/Baltimore metro areas on north and east along
I-95 to as far north as Portland, Maine. Some freezing rain is
also possible Tuesday morning along the I-95 corridor as the storm
system moves up the coast. This kind of event typically leads to
some inconveniences to daily life with commuting by road seeing
the most detrimental impacts. Especially when considering
temperatures will remain well below freezing for the duration of
the event along and both north and west of I-95, any untreated
surfaces are likely to be icy with bridges the most prone to
icing. Motorists from the Tennessee Valley to the Northeast are
urged to use caution while driving and prepare for hazardous
travel conditions through Tuesday.

...Prolonged Great Lake Effect Snow...
Days 1-3...

An enormous upper low over southeast Canada will prolong what
feels like a seemingly endless period of heavy lake effect snow
(LES) downwind of Lakes Superior, Michigan, Erie, and Ontario
through mid-week. Prolonged cyclonic flow across the Great Lakes,
which is simultaneously directing a series of 500mb disturbances
over the region, will sustain the multi-day lake effect snow. NW
flow will prompt the formation of multi-bands over the western
Lakes while W to WSW flow favors single bands for the eastern
Lakes. The single bands off Lakes Erie/Ontario will remain quite
intense at times for nearly all typical lake belts, with minor
variations in strength and orientation as well as the vort max
passes through Tuesday morning. The expectation is for another
12+" to fall around the Buffalo metro area and along the windward
side of the Tug Hill through mid-week. Farther west, the Keweenaw
Peninsula sports high chances (>70%) for >12" over the next few
days. The western and northwest coast of Michigan's Mitten show
low-moderate chances (20-50%) for >8" with the coast north of the
Grand Traverse Bay on the higher end of those probabilities.

...Northwest, Northern Rockies, Northern Plains...
Days 2-3...

The Arctic air-mass that has firmly entrenched itself over the
Pacific Northwest, the northern Rockies, and northern High Plains
will stick around through Thursday while a pair of Pacific storm
systems direct heavy precipitation at the region. The first round
arrives Tuesday afternoon as an IVT of >500 kg/m/s (above the 90th
climatological percentile according to NAEFS) forces Pacific
moisture and warm air aloft to overrun the stubborn sub-freezing
air-mass within the boundary layer. Freezing rain will likely
continue into the early Wednesday morning hours, but more westerly
flow in wake of a Pacific cold frontal passage will eventually
help to scour out the remains sub-freezing temps west of the
Cascades by midday Wednesday. Sampled atmospheric soundings along
the leeward slopes of the northern Oregon Coastal Ranges, much of
the Willamette Valley, and along the foothills of the Oregon and
Washington Cascades will be most prone to significant ice
accumulations. Latest forecast call for 0.2-0.5" for most of these
areas, including the Salem and Portland metro areas.  In the
foothills of the OR/WA Cascades, there is the potential for >0.75"
of ice accumulations. It is in these areas where significant tree
damage and power outages are possible. The latest WSSI shows
Moderate Impacts in the Willamette Valley and Portland Metro area,
with some Major and even a couple Extreme Impact areas in the
upslope areas of the Cascades east of Portland.

Farther north and west, snow will be the primary precipitation
type with significant amounts in the Cascades, Blue, Boise,
Bitterroots, Lewis, and Teton mountain ranges. This comes to
fruition thanks to the onslaught of rich 700mb moisture flux
arriving Tuesday night and lasting well into the day on Wednesday.
Strong high pressure building in from Alberta will also support
upslope flow not only in the Northern Rockies, but also the
central Montana plains but the Big Snowy and both the Little and
Big Belt mountains of central Montana. The Cascades will rack up
the most snowfall through Thursday with WPC PWPF showing very high
chances (>80%) of measuring >18" through Thursday afternoon. The
Bitterroots, Lewis Range, and Tetons also sport high chances
(>70%) for >18" of snow through Thursday. The Big/Little Belt and
Big Snowy mountains of central Montana sport moderate-to-high
chances (50-80%) for >12" of snowfall through Thursday afternoon.
Travel will be treacherous within these mountains' passes.

By Wednesday night, a shortwave trough tracking southeast from
southern British Columbia will foster sufficient vertical ascent
aloft, along with the aid of upslope NE-Erly surface winds to
support periods of snow in southern Montana, eastern Wyoming, and
as far east as the South Dakota/Nebraska border. The latter of
these regions will be ideally placed beneath the left-exit regin
of a strengthening 250mb jet streak, resulting in potentially
banded heavy snow in north central Nebraska and south-central
South Dakota. Latest WPC PWPF shows low chances (10-20%) for >4"
of snowfall between Wednesday afternoon and Thursday afternoon.

...Mid-South...
Day 3...

On the heels of an icy and disruptive winter storm, the Mid-South
(most notably from the ArkLaTex on east through Arkansas, northern
Mississippi, and into western Tennessee) may have to contend with
another fast moving icy setup on Thursday. Latest WPC PWPF keeps
ice accumulations <0.1" at the moment, but within the region
listed above, WPC PWPF does show low-to-moderate chances (20-50%)
for ice accumulations >0.01". Minor ice accumulations on surfaces
that will remain well below freezing through mid-week could still
lead to hazardous travel impacts.

Mullinax


***Key Messages for Southern and Eastern Winter Storm***

--Wintry Mix Continues this Evening across the South
Sleet and freezing rain will continue to impact areas from the
lower Mississippi Valley to the southern Appalachians this
evening. 

--Snow Lifting North from the Southern Appalachians and
Mid-Atlantic to the Northeast
Areas of snow will continue from eastern Tennessee to the
Mid-Atlantic tonight.
Snow will reach southern New York and New England this evening
before shifting farther north overnight into Tuesday as an area of
low pressure develops and moves north along the coast.  

--Plan for Hazardous Travel
Plan on slippery roads and hazardous travel conditions along the
stormâ€s path.  Travel could become very difficult, especially in
areas impacted by heavy snow or significant ice.