Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 PM EST Mon Jan 08 2024

Valid 00Z Tue Jan 09 2024 - 00Z Fri Jan 12 2024

...Southern High Plains through the northern Great Lakes...
Days 1-2...

**Powerful winter storm to bring a myriad of hazards to the
eastern half of the Lower 48 through mid-week**

A winter storm in the central Plains is in its strengthening stage
in the TX/OK Panhandles and will only continue to intensify as the
storm system finds itself ideally placed beneath two divergent jet
streak zones overnight. The deformation zone will position itself
on the northern and western flanks of the 700mb low, which in this
case includes the northern TX Panhandle and OK Panhandle on north
into southern Nebraska this evening. NAEFS on the western flank of
the 850mb low shows 99.5 climatological percentile to even outside
of the CFSR observed wind gusts 00Z-06Z Tuesday in the TX/OK
Panhandles and western Kansas. Blizzard conditions are expected in
these regions as a result and will make for nearly impossible
travel conditions tonight. Wind gusts are forecast to range
between 45-65 mph in these areas. The latest WSSI shows a large
swath of Moderate Impacts from southeast Colorado and northeast
New Mexico through the TX/OK Panhandles and western Kansas. Expect
treacherous travel conditions due to blowing snow to linger in the
southern and central High Plains into Tuesday morning.

Meanwhile, farther north and east, strong 850-700mb warm air
advection and 300K isentropic ascent will foster a blossoming
precipitation field to fall in the form of snow north of the warm
front from Iowa to northwest Illinois and southern Wisconsin.
There will be no shortage of moisture for this winter storm to
work with thanks to an IVT over the Middle Mississippi River
Valley that will range between the 90-97.5 climatological
percentile according to NAEFS Monday night. As the 700mb low
continues its track north Tuesday morning, the deformation zone of
heavy snowfall will accompany it with up to 2"/hr snowfall rates
expected (according to WPC's Snowband Probability Tracker
utilizing the HREF). In fact, intense vertical velocities beneath
the TROWAL could support thundersnow and snowfall rates up to
3"/hr Monday night into Tuesday morning in parts of southern Iowa.
WPC PWPF sports high chances (>70%) for >6" of snowfall from
northern Missouri and southern Iowa to southern Wisconsin.
Southeast Iowa has the best odds for seeing >12" of snowfall, as
evident by moderate-to-high (50-80%) chances in the latest WPC
PWPF. To exacerbate the situation further, winds on the backside
of the storm (while not as intense compared to the central Plains)
will produce blowing and drifting of snow. The latest WSSI shows
Major Impacts from northern Missouri to southern and eastern Iowa.
Portions of southern Wisconsin also sport Major Impacts. Snow
Amount is the primary driver in the WSSI algorithm but there are
also some Minor Impacts related to Blowing Snow expected as well.
Overall, from eastern Nebraska on northeast to eastern Wisconsin,
this winter storm will be responsible for disruptions to daily
life, which includes (but are not limited to) dangerous to even
impossible travel, closures, and potential disruptions to
infrastructure.

By Tuesday afternoon, the 700mb low will traverse northern
Illinois while the same conveyor belt of 850-700mb moisture and
enhanced 300K isentropic ascent occurs over the northern Great
Lakes. By Tuesday night, the near 980mb surface low will track
through southern Michigan with heavy snow bands unfolding over
northern Michigan and the eastern half of the Michigan U.P.. SLRs
will be on the lower side due to the surge in southerly 850mb
winds warming the low levels, but the available QPF would still
support heavy/wet snow in the northern half of Michigan's Mitten.
WPC PWPF shows high chances (>70%) for snowfall totals >6" from
eastern Wisconsin to the eastern Michigan U.P. and northern
Michigan. There are also low chances (10-30%) for >12" of snow in
portions of the eastern Michigan U.P. and east-central Wisconsin.

...Southern Appalachians through New England...
Days 1-3...

The remarkable surge of warm air and moisture advection out ahead
of this robust winter storm will overrun a marginally cold
air-mass over the Mid-Atlantic and Northeast. Anchored by high
pressure over Quebec, a weak cold air damming signature will ensue
from the southern Appalachians on north along the spine of the
Appalachians into northern New England. The exceptional isentropic
glide into the Mid-Atlantic and added topographic enhancement as
far south as the southern Appalachians will initially lead to an
icy wintry mix from the Smokey Mountains on north to the Blue
Ridge Mountains and Potomac Highlands tonight and Tuesday morning.
WPC PWPF shows low chances (10-30%) for ice accumulations >0.1"
most notably in the central Appalachians. The Probabilistic WSSI
(WSSI-P) shows moderate chances (40-60%) for Minor Impacts from
Ice Accumulation in west-central VA, which includes the I-81/I-64
merger near Staunton.

As the precipitation shield makes its way north into the northern
Mid-Atlantic midday Tuesday, temperatures will be cold enough to
initially fall as snow in central Pennsylvania and into both the
Poconos and Catskills, but the the roaring 850mb jet aloft (NAEFS
shows >70kt winds, which would be above the observed max 850mb
wind speeds in the CFSR climatology in central NC 18Z Tuesday)
will escort copious amounts of moisture and a burgeoning >0C warm
nose into the region Tuesday afternoon. This will lead to a
changeover to a wintry mix initially, with surface temperatures
managing to stay below 32F in portions of the northern
Mid-Atlantic and interior Northeast thanks to lingering snow pack
from this past weekend's winter storm. Given the strength of the
warm air advection and lack of a deeply Arctic air-mass, the
changeover to all rain should occur quick enough over Pennsylvania
to limit significant snow and ice accumulations. Farther north
into New England, however, a greater source of snow pack and a
more sufficiently cold air-mass will support a longer duration of
heavy snow Tuesday evening and into Tuesday night. The WPC
Snowband Probability Tracker shows the potential for 2-3"/hr
snowfall rates in New Hampshire and into Maine Tuesday night. The
White Mountains in particular have the highest odds (>70% chance)
for >12" of snow, which given the exceptional moisture content
will also support a very heavy/wet snowfall. The Adirondacks sport
high chances (>70%) for snowfall >6", while the Catskills have
moderate odds (50-60%) for >4" of snowfall.

The WSSI-P depicts moderate-to-high chances (60-80%) for Major
Impacts due to Snow Load alone, and these impacts could extend as
far south as the Berkshires of western Massachusetts. The
combination of dense snow pack on trees/power lines, combined with
the growing threat for high winds, will likely result in tree
damage and power outages in parts of northern New England.
Elsewhere, the WSSI-P shows at least moderate odds (>60%) for
Moderate Impacts from the Catskills and Adirondacks through much
of interior New England with Snow Load being the primary driver in
these impacts.

...The West...
Days 1-3...

A rapidly strengthening storm approaching British Columbia this
afternoon will direct a strong cold front and IVT (>600 kg/m/s,
topping out near 97.5 climatological percentile according to NAEFS
this evening) at the Pacific Northwest. Snow levels will plummet
in wake of the cold frontal passage early Tuesday morning to as
low as 1,000ft while 50 knots worth of 850-700mb westerly flow
enhances the upslope flow into the Olympics and Cascade Range. The
region will see yet another potent storm system be directed at the
Pacific Northwest Tuesday evening, this time favoring the Oregon
Cascades for very heavy snowfall. Still, the diffluent left-exit
region of a 140 knot 250mb jet streak Tuesday evening will be
ideally placed over the Washington Cascades to sustain the
onslaught of heavy snow. Cumulatively through mid-week, the
Cascades and Olympics will be bombarded with rounds of heavy snow.
Snowfall totals through mid-week will be measured in feet with
totals topping 5 feet in the tallest peaks. Major to Extreme
Impacts are depicted on the WSSI with Snow Amount being the
primary driver, although strong winds at 700mb would also
exacerbate the impacts by causing Blowing Snow. The Probabilistic
WSSI, which takes into account Snow Load, shows moderate-to-high
chances (60-80%) for Major Impacts due to Snow Load. Whether it be
due to snowfall totals, snow load, or near blizzard conditions,
the Olympics and most notably the Cascades can expect dangerous
travel conditions even at most pass levels that may be impossible
to travel through over the next couple days.

This moisture plume is strong enough to spill well over the
Cascades and into the Blue Mountains and Northern Rockies. A
prolonged plume of 700mb moisture flux will be directed into the
northern Rockies, northern Great Basin, the Tetons and Wasatch,
and as far south as the the Sierra Nevada through Wednesday. Snow
levels will steadily decrease in wake of the strong cold front
passing through these mountain ranges, supporting snow in many
valleys of the Intermountain West and rising SLRs throughout the
region. Latest WPC PWPF for Tuesday favors the Blue, Boise,
Sawtooth, Bitterroot, and Lewis Ranges with high chances (>70%)
for >8" of snowfall. Similar high-end probabilities exist in the
Salmon Mountains of northern California. By Wednesday, the heavy
snow threat shifts south into northern California and along the
Sierra Nevada with WPC PWPF showing moderate-to-high chances
(50-80%) for >8" of snow. The same can be said for the Wasatch
where high probabilities (>70%) for >8" of snowfall are present.
Finally by Thursday, periods of snow will blanket the southern
Sierra Nevada, the Mogollon Rim and Gila Mountains of Arizona, and
the Rockies of Colorado and northern New Mexico. WPC PWPF sports
high chances (>70%) for snowfall totals >4" in these ranges. This
shortwave trough, responsible for heavy snow into the higher
terrain of the Southwest, will march through the Four Corners
region Thursday afternoon with its sights set on the Nation's
Heartland, where it will become yet another significant winter
storm for parts of the Midwest late week.

Mullinax


***Key Messages for Jan 7-11 Major Winter Storm Southern Rockies
through Midwest and Eastern U.S.***
--Hazardous Travel Expected in Areas of Heavy Snow
Snow will develop in the Plains on Monday, and advance into the
Midwest on Monday Night and Tuesday. Bursts of heavy snow may
accumulate up to 2 inches in an hour, and result in hazardous
travel. A heavy, wet snow will also develop in the interior
Northeast Tuesday Afternoon and Night.

--Extremely Dangerous Plains Blizzard
A blizzard is expected in the plains of northeast New Mexico,
eastern Colorado, the Oklahoma and Texas Panhandles, western
Kansas, and southwest Nebraska. Wind gusts as high as 60-70 MPH
will create ferocious blizzard conditions with whiteouts. Travel
will become extremely dangerous to impossible. If you must travel,
pack a winter survival kit as wind chills will plummet below zero.

--Significant River, Flash, and Coastal Flooding
Widespread and potentially significant river and flash flooding
are likely for the central Gulf Coast and Eastern U.S. Monday
through early Wednesday. Powerful onshore winds will lead to
widespread coastal flooding along the eastern Gulf Coast and much
of the East Coast. Significant coastal flooding is likely,
especially on the Mid Atlantic coastline, including tidal rivers
and bays.

--Damaging Winds May Lead to Power Outages
Widespread wind gusts in excess of 50 MPH are likely in the
eastern Gulf Coast, central Appalachians, much of the East Coast,
and New England. Prepare for power outages.



***Key Messages for Jan 8-11 Northwest Winter Storm***
--Major Winter Storm for the Cascades
Back-to-back storms will cross the Pacific Northwest Monday night
and Tuesday night, with several feet of heavy snow and strong
winds across the Cascades. Blizzard conditions are likely. Snow
levels rise to around 5000 feet Monday night with an atmospheric
river but quickly fall to between 1500 and 2500 feet behind a cold
front on Tuesday, leading to considerable impacts for many
mountain passes with the second storm.

--Heavy Snow Elsewhere in the West
Over a foot of snow is likely (70-90%) in the higher elevations of
the Northern Rockies on Tuesday and Wednesday. Snow levels falling
to below 1000 feet later on Wednesday and early Thursday should
lead to some snow accumulations in valleys as well.

--Storm Will Move to the Central U.S.
The storm system is forecast to dig across the Western U.S.
through Thursday and reach the Plains late in the week. People in
the Plains and Midwest should prepare for another winter storm and
stay tuned for updates.