Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Mon Feb 12 2024

Valid 12Z Mon Feb 12 2024 - 12Z Thu Feb 15 2024

...Ozarks & Ohio Valley...
Day 1...

This morning, the 500mb low tracking through the ArkLaTex will
maintain a healthy and southerly conveyor belt of 850-700mb
moisture flux along the eastern flank of the upper low. This will
sustain the warm conveyor belt rotating around the 500mb low and
leading to heavy snow on the 500mb low's northern and western
flanks this morning. Snow will fall from northeast Oklahoma to the
Ozarks this morning, but as the upper low opens up into an open
trough by late afternoon, the deformation zone will become
increasingly narrow. Still, the Ohio and Tennessee Valleys will
remain at the nose of a 110+kt 500mb jet streak, keeping strong
vertical ascent atop the atmosphere in place. Latest CAMs are
identifying a SW-NE oriented 850mb frontogenesis area from
northern Tennessee to the Ohio River Valley, which would favor the
next potential banding setup to the north of the 500mb trough.
However, compared to those in the Ozarks where elevation is higher
and boundary layer temperatures were colder, the Ohio and
Tennessee Valley are lacking on both of those fronts. The Ozarks
(given their proximity to the upper low and higher elevation)
remain most favored for measurable snowfall. WPC PWPF shows
low-to-moderate chances (30-50%) for >4" of snowfall in northwest
Arkansas and southwest Missouri through Monday afternoon. WPC PWPF
currently shows parts of northern Kentucky and southern Ohio with
higher chances for >4" of snowfall, however, those totals are
sporting low chances (10-30%) at this time.

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

The Northwest will be caught in the middle of a series of
shortwave troughs that will produce multiple rounds of heavy
snowfall through the middle of the week. The first round of
snowfall occurs today as a weak shortwave trough over Washington
heads east over the Northern Rockies. A stream of 700-500mb
moisture aloft within westerly flow within the 850-300mb layer
allows for some enhanced snowfall rates via upslope flow in
orographically-favored mountain ranges. Newest WPC PWPF shows
moderate-to-high chances (60-90%) for >8" of snowfall in the
tallest peaks of the Bitterroots in northern Idaho and along the
Lewis Range (the latter of which is on the higher side of those
probabilities). As that disturbance races east Tuesday morning,
another one diving south from British Columbia will be accompanied
by a frontal boundary inching south along the Lewis Range. Snow
totals will not be as heavy along the Lewis Range, but WPC PWPF
still shows low-to-moderate chances (20-50%) for additional
snowfall amounts >6". It is the next approaching Pacific storm
system, courtesy of a negatively tilted 500mb trough that will
direct the next surge of 850-500mb moisture into the Pacific
Northwest on Wednesday and spilling into the Northern Rockies bu
Wednesday night. This time around, the surge in moisture will
bring about heavy snow into the Cascade Range, the Olympics, and
as far south as the Trinity/Shasta and northern Sierra Nevada of
northern California. WPC PWPF shows high chances (>70%) for
snowfall totals >6" in the Cascades and the northern California
ranges mentioned, but moderate chances (40-60%) in the Olympics.
As mentioned, the surge in Pacific moisture will advance well
inland and lead to heavy snow in the Blue Mountains of northern
Oregon, the Boise and Sawtooth of Idaho, and the Tetons in western
Wyoming. WPC PWPF shows high chances (>70%) for snowfall totals
>6" in these mountain ranges on Wednesday with periods of heavy
snow likely to stick around into Thursday.

...Northeast...
Day 1-2...

The main players involved in the evolving synoptic scale pattern
generally remain unchanged, as the 500mb low in the Tennessee
Valley will direct an anomalous plume of rich Gulf of Mexico
moisture northward into the Mid-Atlantic Monday night and into the
Northeast on Tuesday. The "kissing jets" setup (divergent left
exit region of a jet streak in the Southeast aligning with the
divergent right-entrance region of a jet streak over Nova Scotia)
will foster fantastic upper level divergence off the Mid-Atlantic
coast Monday night into Tuesday. This is also well depicted in
700-300mb layer averaged Q-vector convergence plots showing the
strongest convergence along the Jersey Shore 12Z Tuesday, then
near the "benchmark" (40N/70W) by 18Z Tuesday. Meanwhile, the same
rich stream of 850-500mb moisture aloft will wrap around the the
850mb low tracking over the northern DelMarVa Peninsula early
Tuesday morning and deepen rapidly throughout the day. As 300K
isentropic lift increases over the Mid-Atlantic and southern New
England early Tuesday morning, 850-700mb frontogenesis will become
increasingly robust along the Mid-Atlantic coast and over southern
New England. It is beneath this area of intense frontogenetical
forcing where a vigorous deformation axis of heavy snow is
expected to develop, initially from as far west as central
Pennsylvania, but really coming into its own over the Poconos,
Lower Hudson Valley, and southern New England. The storm will
rapidly intensify to <980mb levels by 18Z Tuesday but track
quickly east to the south of Nova Scotia by Tuesday evening,
effectively ending the snow event all except maybe the
Massachusetts Capes by 7PM EST Tuesday.

Over the last 24 hours, latest guidance has shown a general push
south with the expected snowfall axis that now sets up from the
central Appalachians and central Pennsylvania to the Poconos,
northern New Jersey, Lower Hudson Valley, and across southern New
England. These changes are due to a couple key factors. The first
is the amplitude and tilt of the 500mb trough in the Mid-South on
Monday. The trough is not generating as much convection and
leading to lower 500mb heights over the Ohio Valley and
Mid-Atlantic. The 500mb low is also tracking farther south due to
the 500mb trough maintaining a positive tilt longer and being more
progressive, reducing the strength of the PVA and WAA to the
north. The other big change is the upper trough tracking through
the Great Lakes. Guidance across the board shows this trough have
becoming stronger and allowing for less of a path for the upper
low to turn north farther up the coast thanks to strengthening
confluent flow. The upper trough is not only helping to help push
the storm track farther south and east, but it is imposing a
tighter QPF gradient on the northern and western flanks of the
storm track, making for drastic snowfall footprint changes over a
matter of just 30-50 miles in parts of northern Pennsylvania and
the Southern Tier of New York. Given the sensitivity in the
storm's track based on the track of the 500mb low and the upper
trough over the Great Lakes, it is still possible to see
additional shifts in the storm track, and that can occur from
something as minute as increased convection in the Southeast later
today.

So where we stand now with the forecast is still one that shows a
significant winter storm, thanks to the synoptic and mesoscale
features referenced in the first paragraph. However, the forecast
now places the axis of heaviest snowfall over of south-central
Pennsylvania, the Lehigh Valley, the Lower Hudson Valley, and
coastal New England. The 00Z HREF showed an impressive footprint
of CAMs members showing >2"/hr snowfall rates from the central
Appalachians and most of central Pennsylvania early Tuesday
morning, heading into the Lehigh Valley, Poconos, Lower Hudson
Valley, and NYC metro areas by mid-Tuesday morning, then across
southern New England for the midday and afternoon hours. Latest
WPC PWPF now shows high chances (>70%) for >8" of snowfall from an
area along I-80 in central Pennsylvania through the Poconos,
northern New Jersey, the Lower Hudson Valley, and across much of
southern New England. In fact from the Poconos to southern New
England, WPC PWPF shows moderate-to-high chances (50-70%) for
>12". Along the Laurel Highlands and as far south as Garrett
County, MD, WPC PWPF reaches moderate chances (40-60%) for >6" of
snowfall. The WPC Probabilistic WSSI (WSSI-P) shows a solid swath
of >50% odds of Moderate Impacts from the Laurel Highlands and
central Pennsylvania on east into southern New England. All
impacted areas within this corridor are likely to contend with
hazardous travel conditions, as well as some closures and
disruptions to daily life. From the Lower Hudson Valley to eastern
Massachusetts, there are moderate-to-high chances (50-70%) for
Major Impacts, implying these areas can anticipate considerable
disruptions to daily life which include dangerous to even
impossible travel. The combination of strengthening winds and in
some areas a heavy/wet snow consistency will likely lead to some
power outages and tree damage.

Mullinax


***Key Messages for Feb. 12-13 Winter Storm***

--Periods of Snow from The Ozarks to the Ohio Valley
The winter storm will track into the Mid-South today, and the Ohio
Valley tonight, producing a swath of disruptive snowfall
accumulations from central Oklahoma through southern Missouri.
Localized snowfall totals up to 6" are possible in the Ozarks,
with a few inches possible across the Ohio Valley tonight.

--Nor'easter Monday Night and Tuesday
A rapidly deepening low is expected to track off the Mid-Atlantic
coast early Tuesday morning. This will produce widespread heavy
snowfall with rates exceeding 2" per hour across central
Pennsylvania and southern New York Tuesday morning and southern
New England Tuesday afternoon.

--Significant Travel Impacts in the Northeast Tuesday
The fast-moving Norâ€easter is forecast to produce a swath of
6-12" of snow from the central Appalachians to the New England
coast. Powerful winds and heavy/wet snow could damage trees, power
lines, and disrupt travel.

--Moderate Coastal Flooding
Intensification of the coastal low south of Long Island Tuesday is
forecast to cause moderate coastal flooding at high tide Tuesday
along the Jersey Shore and portions of both the New York and New
England coasts.