Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
459 AM EST Sun Feb 11 2024

Valid 12Z Sun Feb 11 2024 - 12Z Wed Feb 14 2024

...Southern Rockies to Texas Panhandle into Oklahoma...
Day 1...

A complex and dynamic synoptic and mesoscale setup will unfold
over the western Texas and into central Oklahoma today as a
vigorous 500mb low makes its way from southeast New Mexico this
morning to over the Red River late afternoon. As 850-700mb
moisture flux is advected northward, 850-700mb frontogenesis will
become more defined from the Red River on west to the lower Texas
Panhandle. 850-700mb moisture will wrap around the northern flank
of the 700mb low exiting eastern New Mexico and advancing east to
the south of Lubbock during the day. The region is also favorably
positioned at the left-exit region of a 90kt 500mb jet streak over
the Big Bend, whose wind speeds are topping the 99th
climatological percentile according to NAEFS at both 12Z and 18Z
Sunday. This is a classic setup for a warm conveyor and excellent
mesoscale forcing to result in an impressive deformation axis of
heavy snow that will stretch from east-central New Mexico on east
into the lower Texas Panhandle Sunday morning. WPC's Snowband
Probability Tracker (SPT) page shows a narrow corridor where
1-2"/hr snowfall rates are expected, and some sampled soundings
suggest enough of a warm nose aloft (particularly closer to the
TX/OK border) to where some elevated instability could manifest
itself within the deformation zone, thus leading to potential
cases of thundersnow (as high as 3"/hr snowfall rates could occur
in these cases).

As the upper low heads for the Red River, so will the deformation
axis as it makes its way into southwestern and central Oklahoma
late morning into the afternoon hours. The region lacks the colder
air within the boundary layer and the elevation that their
neighbors in the Texas Panhandle have. However, the same synoptic
and mesoscale drivers supporting the heavy snow threat in Texas
appear to sustain themselves long enough for dynamic cooling to
force a switch over from rain to snow. SLRs will remain <10:1 but
in areas where thundersnow could ensue, they could come awfully
close and force surface temperatures to drop <32F. Snowfall rates
could top out between 1-2"/hr in some parts of southwest Oklahoma
and possibly as far north and east as the OKC/Norman metro area.
Snowfall rates of this intensity can still lead to rapid
accumulations despite the snow coming during the daytime hours and
the initially meager surface temps. By Sunday night, the 700mb low
will make its way into eastern Oklahoma and bring the deformation
axis of snow along for the ride. Some snowfall accumulations
around I-40 in eastern Oklahoma are possible Sunday night and, in
turn, make for a treacherous evening commute for those on the
road. Periods of snow may linger across the eastern half of the
state before the shield of snow tracks into the Ozarks on Monday.

WPC PWPF is most confident in >6" snowfall amounts between
Amarillo and Lubbock along I-27 where there are now
moderate-to-high chances (50-80%) in place. In fact, WPC PWPF even
shows low chances (10-30%) for >12" where the band of heaviest
snowfall sets up. The WSSI shows Major Impacts along and just east
of I-27, suggesting these areas can expect considerable
disruptions to daily life. The WSSI depicts the Amarillo and
Lubbock metro areas as having Moderate Impacts, implying that
hazardous travel conditions are likely and to use extra caution
while driving. From the Red River and TX/OK border on east near
the OKC/Norman metro area and into eastern Oklahoma, the
expectation is for a narrow swath of 3-6" of snow to occur but
exactly where that band sets up will be heavily determined based
on the 700mb low track. The WSSI shows Moderate Impacts in
southwest Oklahoma to the west of Lawton, while areas along I-40
into the Norman/OKC metro areas are currently forecast to see
Minor Impacts.

...Oklahoma/Ozarks to Ohio Valley...
Day 2...

The 500mb low will make its way into Arkansas Monday morning and
strong 700-300mb layer-averaged Q-vector convergence will pivot
over northern Arkansas and southern Missouri. The same plume of
850-700mb moisture flux will wrap around the 850mb low and
continue to sustain the same deformation axis that produced heavy
snow in parts of western Texas and Oklahoma. Latest CAMs show the
850-700mb frontogenesis are trending stronger in recent runs, and
given the benefit of elevation (southern Missouri and northern
Arkansas most notably), chances for a plowable snow have gone up
compared to 12-24 hours ago. Latest WPC PWPF now shows
low-to-moderate chances (20-40%) for snowfall totals >4" in
southwest Missouri along the higher terrain of the Ozarks through
Monday. The WSSI does sport Minor Impacts in parts of the Ozarks
and even into parts of southern Illinois where minor snow
accumulations are possible. As the upper low tracks into the
Tennessee Valley Monday evening, the deformation axis will follow
along the northern flank of the upper low, still resulting in low
SLR (heavy/wet) snow in the Upper Ohio Valley. The strongest
synoptic scale forcing will shift towards the Mid-Atlantic by this
point, but enough dynamic cooling aloft should support moderate
snowfall rates Monday evening. Latest WPC PWPF shows low chances
(10-30%) for >4" of snowfall from the southwest OH/IN border on
north and east to the PA/OH border. Minor travel impacts are
depicted on the WSSI in parts of these areas Monday night.

...Northwest...
Days 1-3...

A 500mb shortwave trough approaching the Pacific Northwest this
morning will direct an influx of 850-500mb moisture into the
region that leads to mountain snow in the Cascades on Sunday. Snow
levels in the Cascades will be as low as 3,000ft, but the heaviest
totals will generally be confined to elevations above 5,000ft.
Snow will continue along the Cascades into Monday as a trailing
500mb disturbance sustains orthogonally-oritented westerly
850-300mb winds into the region, keeping a prolonged period of
upslope flow into the mountain range. WPC PWPF shows high
probabilities (>70%) for snowfall ttoals >6" at elevations above
5,000ft in the northern Washington Cascades. The tallest peaks of
the Olympics could also see some measurable snowfall, indicated by
WPC PWPF sporting moderate-to-high (50-80%) chances for >4" of
snowfall at elevations above 5,000ft. Farther inland, the initial
shortwave trough will advance inland into the Northern Rockies and
delivering the same surge in 850-500mb moisture flux into northern
Idaho and western Montana starting Sunday afternoon and lingering
into the day on Monday. By Tuesday, a frontal boundary will bank
up against the eastern slopes of the Northern Rockies in western
Montana and keep snow in the forecast through Tuesday night. WPC
48-hr PWPF shows high chances (>70%) for snowfall totals >12"
through  Tuesday along the Lewis Range, including ridges
surrounding Glacier NP. Elevations above 5,000ft in northern Idaho
and above 6,000ft along the Lewis Range are on the higher end of
the probabilistic range for snowfall totals >6". The Probabilistic
WSSI (WSSI-P) shows high chances (>70%) for Minor Impacts in the
highest elevations of the Washington Cascades, the Bitterroots of
northern Idaho, and the Lewis Range of western Montana.

...Northeast...
Days 2-3...

By Monday morning, the upper low over Arkansas will have been
working in tandem with an upper ridge located over the Caribbean
to foster a robust subtropical jet (STJ) over the Deep South.
ECMWF ensembles centered on Tuesday 00Z show winds at the 850mb
and 700mb levels to be above the 90th climatological percentile
and tapping into a deep reservoir of Gulf of Mexico moisture. This
will be the same moisture source that the 500mb low tracking into
the Tennessee Valley Monday evening will direct northward into the
Mid-Atlantic and along the Northeast coast. In fact, PWs over the
Mid-Atlantic 06Z Tuesday 90th climatological percentile, so there
is no shortage of moisture available. Farther north, confluent
flow over southeast Canada and northern New England will support a
150kt 250mb jet streak and place its diffluent right-entrance
region off the Mid-Atlantic coast Monday night. As a second 150kt
250mb jet streak over the Southeast approaches Monday night, a
"kissing jets" setup will foster excellent upper level divergence
along the Mid-Atlantic coast, allowing for a surface low to
rapidly organize by early Tuesday morning while the initial low
over West Virginia gradually weakens.

Recent trends in guidance have led to a more pronounced surge in
southerly flow into southeast Pennsylvania, the Delaware Valley,
and into the NYC metro area Monday night into early Tuesday
morning. This has led to a reduction in probabilities for seeing
>4" of snow in these areas. What this has done, however, is flexed
the robust 850-700mb frontogenesis signal to align better over
northern Pennsylvania, the Lower Hudson Valley, and through
southern New England. Impressive vertical velocities coinciding
with a highly saturated DGZ will foster ripping snowfall rates
that could top 2"/hr, most notably from the Poconos and Catskills
to southern New England Tuesday morning. Latest Probabilistic WSSI
(WSSI-P) shows moderate-to-high chances (50-70%) for Moderate
impacts from north-central Pennsylvania and the Lower Hudson
Valley to eastern Massachusetts, implying these areas are likely
to contend with hazardous travel conditions and disruptions to
daily life. The Snow Amount and Snow Rate algorithms are the
primary drivers in producing these probabilities. The WSSI-P even
shows moderate chances (40-60%) for Major Impacts in parts of
northern Connecticut, northern Rhode Island, and eastern
Massachusetts on Tuesday, implying considerable impacts to daily
life in these areas. WPC PWPF shows high chances (>70%) for
snowfall totals >8" along the northern PA/southern NY border, in
the Catskills, the Berkshires, and eastern Massachusetts. In fact,
WPC PWPF sports moderate chances (40-60%) for snowfall >12 in the
Catskills and Berkshires.

While these areas sport the highest confidence in expected
impacts, there are still forecast details that need ironing out
over the next 24 hours. On the northern flank of where the
deformation zone sets up, a trailing shortwave over the Great
Lakes will race in from the west, attempting to advect drier
tongue of 700-300mb moisture into the eastern Great Lakes. In
addition, such a robust area of frontogentical forcing over
northern PA, southern NY, and southern New England would
inevitably lead to some modest subsidence to the north. This is
depicted quite well on the 07Z NBM 50th percentile snowfall
totals, where amounts in Rochester could end up between 1-2", but
southeast towards Ithaca, totals were around 9". This is a common
occurrence from northeast Pennsylvania and western New York across
southern VT/NH. Any additional shifts in track can still mean
increases (decreases) in snow totals. Meanwhile, to the south, the
recent trend of a northerly track would in fact make it tough for
accumulating snowfall from Philly on north up I-95 to the NYC
metro and the immediate coast of New England. However, ensemble
guidance continues to show solutions where the deformation axis
pivots over northern NJ, the NYC metro, and Long Island. By 00-18Z
Tuesday, the ECMWF ensemble mean shows the low deepening from
~995mb to ~976mb, which would lead to a dramatic increase in winds
as the low tracks off the coast Tuesday morning. There is still a
scenario where as the low rapidly deepens, a burst of snow beneath
the deformation axis and strong wind gusts could cause
significantly reduced visibilities. While totals could still be
minor, the combination of just moderate snow and wind gusts >40
mph may lead to notable impacts from blowing snow. The latest
WSSI-P show 40-50% chances for Minor Impacts due to the Blowing
Snow algorithm from the NYC metro to Long Island. Probabilities
for Minor Impacts due to Blowing Snow jump to as high as 60-70%
along the coast of CT, RI, and southeast MA.

Mullinax


Key Messages For the Southern Rockies to Northeast Winter Storm...

--Heavy Snow Southern Rockies/Plains
A winter storm tracking across the southern High Plains will
produce a swath of heavy and disruptive snowfall accumulations
from the Southern Rockies to central Oklahoma today and into
Sunday night. Hazardous travel conditions are expected in affected
areas.

--Narrow Heavy Snow From Ozarks to Ohio Valley
The storm will make its way east into the Mid-South where a band
of heavy snow will set up over southern Missouri. Localized
snowfall totals up to 6" are possible in the Ozarks.

--Norâ€easter Forms Monday Night Into Tuesday
Low pressure in the Ohio Valley Monday evening will give way to a
rapidly deepening storm off the Mid-Atlantic coast. Heavy snowfall
will begin across northern PA, southern NY, and southern New
England early Tuesday morning.

--Significant Travel Impacts in the Northeast Tuesday
This fast-moving storm currently has the highest potential for
over 8" across northern Pennsylvania through southern New England.
Powerful winds and heavy/wet snow could damage trees, power lines,
and disrupt travel. Coastal Flooding is also expected.