Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EST Sat Feb 10 2024

Valid 12Z Sat Feb 10 2024 - 12Z Tue Feb 13 2024

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

A 500mb shortwave trough approaching the Pacific Northwest
Saturday night 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 Cascades could also see some measurable snowfall,
indicated by WPC PWPF sporting low-to-moderate (20-50%) 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. WPC PWPF shows
moderate-to-high chances (40-70%) for snowfall totals >6" through
Monday. 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 as high as moderate chances (40-60%) for Minor
Impacts in the highest elevations of the Washington Cascades,
while low-to-moderate chances (20-50%) are in place over the
Bitterroots of northern Idaho.

...Southern Plains to the Ozarks...
Days 1-3...

A complex and dynamic setup, from the synoptic scale on down to
the mesoscale, will setup a potent winter storm whose snowfall
will be heavily dependent upon boundary layer temperatures and
where the deformation axis sets up. Big picture-- an upper low
over northern Arizona will foster strong vertical ascent aloft
over New Mexico and southern Colorado today. Farther north, high
pressure over the Northern High Plains will support S-SE 850-700mb
flow into a surface trough positioned along the Front Range of the
Colorado Rockies and on north into southeast Wyoming and western
Nebraska. In fact, latest hi-res guidance has remain steadfast in
a narrow 850-700mb frontogenesis area of western Nebraska that
will support a band of heavy snow early this morning and lingering
into the day on Saturday. The 00Z HREF does show show 20-40%
probabilities for >1"/hr snowfall rates in western Nebraska this
morning, then as the FGEN becomes more pronounced during the day
over central Colorado, probabilities spike to moderate-to-high
levels (40-70%) from the Palmer Divide to the CO/NE border
Saturday afternoon. WPC PWPF shows moderate-to-high chances
(50-70%) for >6" of snow over western Nebraska and far northeast
Colorado with low chances (20-40%) for Moderate Impacts according
to the WSSI-P.

Farther south, the 500mb low will emerge over eastern New Mexico
Saturday night and direct healthy PVA over the southern High
Plains. As the 700mb low tracks towards Texas, 700-850mb moisture
flux will begin to wrap around the 700mb low early Sunday morning
and develop a warm conveyor belt around the northern and western
flank of the low, giving birth to a healthy deformation axis of
heavy snow from northeast New Mexico to the lower Texas Panhandle.
This also coincides favorably with the strongest 700-300mb
averaged Q-vector convergence early Sunday morning. The 00Z HREF
depicts high chances (>70%) for snowfall rates of at least 1"/hr
just south of Amarillo and just north and east of Lubbock. How
much snow falls will be dependent upon the duration of the snow,
which could lead to some areas surpassing 8". Latest WPC PWPF
shows the greatest confidence for >12" totals along the Sangre De
Cristo in southern Colorado and northern New Mexico where there is
the best combination of upslope enhancement via healthy easterly
low level easterly flow, but synoptic scale ascent is also
maximized as they are initially located at the left front exit
region of the 500mb jet streak. Snow will fall in parts of the
lower Texas Panhandle until Sunday evening. Latest WPC PWPF shows
moderate chances (40-60%) for snowfall totals >6" between Amarillo
and Lubbock through Sunday evening. Farther east, boundary layer
temperatures will be marginal and snow accumulations will be more
heavily dependent upon the dynamic cooling within the atmospheric
column. There remains a good deal of uncertainty in snowfall
amounts along the Red River and as far east as the OKC metro area.
Latest WPC PWPF shows a sharp gradient from low chances (10-30%)
for >2" totals west of the OKC metro to moderate chances (40-60%)
along the TX/OK border and along I-40. Regardless, the WSSI shows
Moderate Impacts from I-27 between Amarillo and Lubbock on east to
southwest Oklahoma. Expect hazardous travel conditions in these
impacted areas from Saturday night in northeast New Mexico to
throughout the day in affected areas of the lower Texas Panhandle.

As the upper low tracks along the Red River Sunday night, dynamic
cooling aloft within the deformation axis will be able to sustain
periods of moderate-to-heavy snow, particularly in the Ozarks.
However, boundary temperatures remain marginal and SLRs are
noticeably lower compared to those observed in the lower Texas
Panhandle. Expect any snow accumulations Sunday night to be a
heavy/wet consistency that could lead to some slick conditions on
untreated surfaces early Monday morning. Latest WPC PWPF shows
low-to-moderate (20-50%) chances for >2" of snow through Monday
morning with portions of south-central Missouri closest to the
higher end of those listed probabilities.

...Northeast...
Day 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 at 18Z Monday 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. In fact, PWs
over the Southeast coast Monday morning are above the 99th
climatological percentile. 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. 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.

This setup features no shortage of atmospheric moisture (ECMWF ENS
shows >90th climatological percentile 500-700mb specific humidity
values from the Laurel Highlands on east to the Northeast coast
06Z Tuesday) and the aforementioned synoptic-scale setup is
classic for strong upper level divergence. What this setup lacks,
however, is a sufficiently cold air-mass anchored by cold high
pressure to the north (the high is closer to Hudson Bay than it is
Montreal). For this reason, mountainous or simply more elevated
terrain will have better odds for measuring heavy snowfall for
this event. One method to overcome marginal boundary layer temps
is via dynamic cooling within the atmospheric column. Both the
deterministic GFS and ECMWF show this in spades thanks to robust
850-700mb frontogenesis setting up from the upper Ohio Valley to
south-central PA Monday night, then tracking towards southern New
England Tuesday morning. These areas beneath this axis of strong
frontogenesis could witness snowfall rates between 1-2"/hr. Along
and east of I-95 Monday evening, the lack of cold air will be
difficult to over come in areas such as the DC/Baltimore metro
areas and even as far north as the Philly metro area. However,
when and where the coastal low develops will be critical in where
snowfall Tuesday morning sets up. By 12Z Tuesday, ECMWF ENS and
GEFS low positions vary from as far north as the coast of Long
Island to as far south as the VA/NC border. The farther north low
positions would favor placing heavy snow farther north into more
interior locations such as the Catskills, the Berkshires, and
central New England. The farther south tracks would support snow
closer to the I-95 corridor from the Lehigh Valley and northern NJ
to NYC and over southern New England. Intensity also matters as
the deeper solutions would favor stronger dynamic cooling within
the deformation axis, so even slightly farther north tracks could
still generate ripping snowfall rates along the I-95 corridor in
southern New England.

Latest WPC PWPF shows moderate chances (40-60%) for >4" of snow
from the the Allegheny Mountains and Laurel Highlands of western
Pennsylvania across the heart of Pennsylvania and into the
Poconos. The Probabilistic WSSI sports moderate chances (40-60%)
for Minor Impacts from the central Appalachians through Monday
night on east through eastern PA and into southern New England,
which would include metro areas such as New York City, Hartford,
Providence, and Boston during the day on Tuesday. The
Probabilistic WSSI shows the primary impacts (from a travel,
infrastructure, and disruption to daily life standpoint) are most
likely to be from a combination of Snow Amount, Snow Load, and
Snow Rates, with Blowing Snow an issue closer to the coast should
the coastal low be on the stronger side of guidance. This is a
fluid situation as minor adjustments in the depth/tilt of the
500mb trough approaching from the west, the strength of confluence
to the north, and a combination of the placement/track/intensity
of the coastal low can all lead to shifts in the heavy snow axis
over the next 48-72 hours. That all being said, ensemble guidance
across the board is keying in on a winter storm that is likely to
be loaded with moisture and have exceptional synoptic and
mesoscale forcing at its disposal. This storm is likely to produce
hazardous travel conditions across much of the northern
Mid-Atlantic and southern New England starting Monday night and
lasting into Tuesday.

Mullinax