Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Mon Jan 23 2023

Valid 12Z Mon Jan 23 2023 - 12Z Thu Jan 26 2023

...Northeast...
Day 1...

A strong upper level trough responsible in part for the formation
of a coastal low along the New England coast this morning will
impose impressive vertical ascent within the atmospheric column
over the Northeast. Latest guidance shows impeccable positive
potential vorticity advection from the northern Mid-Atlantic to
southern England, allowing for a band of snowfall to ensue via
dynamic cooling. The locations with the coldest supply of air will
be most favored for heavy snowfall rates >1"/hr, but with such
impressive dynamics and banding, these rates have a good chance to
make their way to communities along and west of I-95 in southern
New England. The latest 00Z HREF shows a swath of 30-50%
probabilities for >1"/hr snowfall rates over the Catskills,
portions of the Lower Hudson Valley, and into the Green and White
mountains early-mid morning. Eventually, the strong corridor of
lift aloft will shift east, putting more of southern Maine,
southern New Hampshire, and all the way to northern CT/RI with
similar snowfall rates. This is due to the 850mb low tracking into
the Gulf of Maine directing a conveyor belt of 850mb moisture back
beneath the strong vertical ascent aloft. Should the 850mb low lag
a little longer than expected, it could cause heavy snowfall rates
>1"/hr in parts of southern New England to stick around a couple
more hours into the afternoon rush hour. The experimental PWSSI
does show a large portion of central MA with 60-70% chances for
Minor impacts, with 40% odds along the I-95 corridor in Boston and
on southwest to northern RI and northern CT. Minor impacts suggest
some hazardous travel conditions are possible in these areas and
motorists should use caution while driving. However, there are
also 40-50% probabilities for Moderate impacts, which suggests the
potential for increasingly hazardous travel conditions and
potentially some road closures. In total, WPC PWPF does show
probabilities for >6" of snowfall Residents in southern New
England should keep close watch on the situation as snowfall rates
are the primary input going into the PWSSI projected hazards.

...Southern Rockies, Southern Plains, & Ozarks...
Day 1-2...

The large and anomalous upper low in the Southwest is set to eject
east into the Southern Plains by Tuesday morning. The NAEFS showed
500mb heights by 12Z Tuesday that were <2.5 climatological
percentile with anomalous moisture to work with out of ahead of
the trough. A deep stream of 850-700mb moisture advected north
within a robust low level jet will wrap around the 850mb low
tracking through southeast New Mexico and just south of Lubbock,
TX. Periods of heavy snow will ensue from the Sacramentos of
southern NM to the TX Panhandle late Monday night into Tuesday.
This band then tracks across south-central OK and could result in
heavy snow in North TX. 00Z HREF does depict probabilities as high
as 60-80% for hourly snowfall rates >1"/hr across the TX Panhandle
and through the Red River Valley. It is worth noting soil/road
temperatures are elevated across the region, however in cases
where >1"/hr snowfall rates occur, accumulations on all surfaces
can occur and can result in treacherous travel conditions. In
addition, these snowfall rates can cause near zero-visibility for
motorists. WPC's experimental PWSSI shows >60% probabilities for
Moderate impacts across much of the TX Panhandle between Amarillo
and Lubbock, but given the banded nature of this event, snowfall
rates may be lighter compared to being located directly beneath
the TROWAL.

Throughout the day Tuesday, a jet streak over the Middle MS Valley
will place its diffluent right-entrance region over much of
eastern OK and into the Middle MS Valley as the 250-500mb mean
trough begins to take on a negative tilt Tuesday afternoon. As the
850mb low makes is way through the Red River, copious amounts of
850-700mb moisture emanating out of the Gulf of Mexico races north
and provides ample moisture for snow across the Ozarks and into
the Middle MS Valley. Boundary layer temperatures will be coldest
over the Ozarks but precipitation will fall in the form of snow
beneath the deformation axis as it tracks from eastern OK into
southern MO and southern IL. Hourly snowfall rates ranging between
1-2"/hr are likely here, and is one of the primary contributing
factors in the experimental PWSSI placing a large 60-80%
probability area for Moderate impacts in portions of northern AR
and southern MO. Latest WPC PWPF does contain 40-60% probabilities
for >6" of snowfall in these areas between Tuesday night and
Wednesday night with localized totals surpassing 10" within the
realm of possibility.

...Lower Great Lakes, northern Mid-Atlantic, & Northeast...
Days 2-3...

This textbook (from a synoptic and mesoscale standpoint) winter
storm makes its way into the Mid-South Tuesday night with a large
shield of precipitation running out ahead of it along the warm
front, as well as on its north and west flank where the
deformation axis sets up. At 250mb, diffluent regions of two jet
streaks (one over West Texas, the other over the northern Great
Lakes) will provide excellent vertical ascent aloft to strengthen
a surface low tracking north and east. In addition, a robust 850mb
jet is supplying the necessary moisture, while coinciding with
strong 290K isentropic glide and strong frontogenetical forcing at
850-700mb along the warm front. Dynamic cooling from the heavy
precipitation rates and strogn omega aloft will result in periods
of heavy snow throughout the Lower Great Lakes Tuesday night and
into Wednesday. Areas on the northwest track of the 850mb low are
likely to witness the best snowfall rates with a sufficiently cold
thermal layer and very saturated DGZs. NAEFS shows >90th
climatological percentile values of mean specific humidity in the
500mb layer throughout the region by 06Z Wednesday and within the
deformation axis, and this only continues to grow in size into the
day on Wednesday as heavy snowfall moves into eastern IL, central
IN, and into southern MI and northwest OH. Latest WPC PWPF shows
50-70% probabilities for >6" of snowfall in these areas between
Wednesday 00Z - Thursday 00Z. Farther east, the initial front end
"thump" of heavy snow via intense WAA will give way to a 700-300mb
dry slot racing through the Upper Ohio Valley on Wednesday. This
area is among the lowest in confidence regarding snowfall amounts
due to what should be a heavy initial band of snow, followed by
warmer/drier mid-level air rushing in and shutting off the ability
to produce heavy snow.

This applies to the central Appalachians and northern Mid-Atlantic
as well. High pressure to the north, intense omega within the DGZ
via strong isentropic ascent, and heavy precipitation rates
dynamically cooling the column allows for several hours of heavy
snowfall.The initial arrival of snowfall Wednesday morning is
likely to come down quite hard, perhaps 1-2"/hr in some cases
across central PA and into the Poconos. The moisture supply in
particular is significant with the IVT in the Southeast topping
out >1,000 kg/m/s. However, the intense southerly LLJ delivering
the moisture will lead to a protruding warm nose within the
850-750mb layer over the northern Mid-Atlantic. Meanwhile, the
same dry slot in the Ohio Valley will arrive by late Wednesday
afternoon. This will eventually result in a changeover to an icy
wintry mix for the Poconos on north into the Catskills. That being
said, latest WPC PWPF does show 50-70% probabilities of snowfall
>6" in central PA on northeast into the Catskills and Poconos. The
experimental PWSSI also shows 40-50% probabilities for Moderate
impacts, almost solely driven by the Snow Rate algorithm. This
speaks to how heavy the snowfall rates can be, and how disruptive
to travel and daily life snowfall across the northern Mid-Atlantic
could be Wednesday morning.

The robust syonptic-scale ascent via dueling jet streaks at 250mb
makes its way into the Northeast Wednesday afternoon and into the
overnight hours. Similar to the northern Mid-Atlantic, strong
850-700mb WAA will result in periods of heavy snow throughout the
northern Appalachians. There is a little more uncertainty beyond
Wednesday night, as there is the expectation that a coastal low
will form along the New England coast sometime Thursday morning.
If it forms earlier, heavier snowfall totals are expected over
northern New England with snowfall totals having a good shot of
eclipsing 12". However, if it forms later, the dry slot will have
a better shot of shutting off snowfall and allow for lesser
amounts and more wintry mix. The ECMWF EFI had its best heavy
snowfall signature from the Adirondacks to the White mountains and
throughout much of northern Maine. Residents in New England will
want to follow this storm system closely the next couple days as
it has the potential to be yet another significant winter storm,
highlighted by a large 60-80% probability area for Moderate
impacts throughout northern New England according to the
experimental PWSSI.

Mullinax