Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
154 PM EST Tue Dec 12 2023

Valid 00Z Wed Dec 13 2023 - 00Z Sat Dec 16 2023


...Michigan U.P. into the Tug Hill...
Day 1...

A cold front moving across the region tonight into Wednesday
morning will leave pronounced CAA in its wake, with 850mb
temperatures forecast to fall to -10C to -15C, this will support
increasing instability over the Lakes, and with W/NW flow in
place, this should lead to bands of heavy lake effect snow (LES)
downstream of Lakes Superior and Ontario, with the most intense
LES east of Ontario due to a longer effective fetch as moisture
gets transported from Lake Huron and Lake Superior with
long/straight hodographs suggesting a single band or two. Steep
lapse rates and deepening inversion depths will likely support
1"/hr snowfall rates at times, which will likely accumulate to
around 4 inches in the eastern U.P., with a greater potential
reflected by WPC probabilities of 50-70% across the Tug Hill
Plateau.


...Interior Northeast and New England...
Day 1...

A strong cold front will cross from the Great Lakes through New
England, moving offshore into the Atlantic late Wednesday morning.
This feature will be driven eastward by strong vorticity lobe
embedded within a shortwave advecting eastward. Behind this
feature, low-level moisture in the 0-2km layer will increase
rapidly noted by RH surging above 80%, aided in some places by
flow off the Great Lakes, and as 850mb temps crash to around -10C,
low-level lapse rates will steepen to near dry-adiabatic levels.
This could support scattered convective snow showers and snow
squalls as reflected by an axis of 925mb fgen overlapping
efficiently with SBCAPE of 50-200 J/kg, more than enough for snow
squall development. The high-res simulated reflectivity also
supports convective snow showers, and with regional forecast
soundings indicating more up to 30kts of wind at the top of the
deepening PBL, there could be some scattered snow squalls on
Wednesday. The greatest risk for squalls appears to be where the
CIPS snow squall parameter maximizes above +4 in northern Upstate
NY and into Northern New England, but convective snow showers or
isolated squalls are possible across much of the Northeast.


...Southern Rockies into the High Plains...
Days 1-3...

A complex evolution of the synoptic pattern and associated
thermals result in a challenging winter forecast through Friday.

A deepening trough across the Great Basin tonight will amplify
into a closed low as it dives into the Four Corners region late
Wednesday, and is then forced almost due east into the Southern
Plains by Friday in response to an amplifying ridge to its north.
This closed low will weaken with time as it moves east, but is
still progged to be an impressive feature into Friday over Texas.
Aloft, a jet streak pivoting around the amplifying trough will
result in impressive downstream upper diffluence, which will work
in tandem with mid-level divergence and the accompanying height
falls to produce intense synoptic ascent across parts of the
Southern/Central Rockies and into the adjacent high plains. While
this is occurring, column moisture will become pronounced as
warm/moist advection intensifies downstream of this low, resulting
in PW anomalies progged to approach +4 sigma according to the
NAEFS ensemble tables, resulting in a prolonged period of heavy
precipitation.

Beginning Wednesday morning, precipitation will likely expand
considerably as deep southerly flow advects copious moisture and
interacts with the intensifying ascent. This will drive widespread
precipitation, with heavy snow likely in the terrain above 7000ft.
The impressive WAA will likely lead to periods of heavy snow rates
as both theta-e and theta-es lapse rates are progged to fold
beneath the most impressive ascent suggesting CI and possible
thundersnow at times as laterally translating bands shift
northward within the best WAA. Where these setup will be crucial
to snowfall amounts as the thermal structure will generally
support a cold rain outside of terrain, but this could dynamically
cool to produce snow in the intense lift. As the entire system
begins to shed to the east D2 into D3, there may be a break as the
WAA shunts east and the synoptic ascent lags a bit to the west.
However, as the core of this upper low moves east into the High
Plains late Thursday into Friday, a second round of intense ascent
is likely through the most impressive PVA overlapping still
intense upper diffluence. This will likely lead to an additional
area of heavy precipitation, generally farther east than the
first, with precip changing to snow from west to east both due to
CAA but also the more intense lift and strong dynamic cooling with
another round of CI possible from eastern NM into the TX/OK
Panhandles. Where this CI occurs, snowfall rates could reach
1-2"/hr as reflected by the WPC snow band prototype tool, with
locally higher snowfall rates possible in the higher terrain due
to upslope enhancement and higher SLRs.

WPC probabilities for more than 6 inches of snow on D1 are
generally 10-20% and confined to the higher terrain above 7000 ft
in the Sangre de Cristos and San Juans. During D2, the focus for
heavy snow continues in the San Juans and Sangre de Cristos where
WPC probabilities for more than 6 inches rise above 80%, with
snowfall exceeding 1 foot likely in the higher elevations of the
Sangres and Jemez Mountains. Farther east into the High Plains of
NM/CO and into western portions of KS/OK/TX, WPC probabilities or
2" reach 20-50%, but locally much heavier snow is possible where
any banding sets up, most likely in NM/CO, and along higher
terrain features like the Raton Mesa and even up into the Palmer
Divide. By D3, the focus shifts into the TX/OK Panhandles before
ending Friday morning. WPC probabilities are modest for 2", but
similar to D2, where any convective snowfall can more intensely
cool the column, locally higher snowfall is possible. At the same
time, if the column cools more slowly or precip rates are less
intense, snowfall could be less than forecast across this area
late D2 into D3.

Additionally, as precipitation begins to overrun the cold air mass
present at the onset D1, an area of freezing rain is likely across
the High Plains of KS and into NE. The cold high appears to erode
quickly to the northeast which will limit the freezing rain
potential, but at least some minor icing is likely. WPC
probabilities for 0.01" of ice both D1 and D2 are as high as 30%
along the KS/CO border and into southwest NE, but probabilities
for 0.1" are less than 5%.


Weiss