Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
442 AM EST Thu Jan 04 2024

Valid 12Z Thu Jan 04 2024 - 12Z Sun Jan 07 2024

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

A potent and closed mid-level low centered over the southern Great
Basin to start the period will advect eastward while continuing to
deepen, reaching -2 to -3 sigma with respect to 500mb heights over
the Desert Southwest Thursday night. This will result in
impressive downstream height falls into the Four Corners and
Southern High Plains Thursday through Friday, with ascent
maximizing beneath the LFQ of a robust subtropical jet streak
overlapping the greatest height falls. This overlap will also
drive surface cyclogenesis, with a wave of low pressure moving
eastward from near the AZ/NM border through the Red River Valley
of the South. Downstream of this low, moisture advection will
steadily intensify on nearly unidirectional southerly flow noted
in regional forecast soundings, with 290-295K moist isentropic
upglide driving PW anomalies to +1 to +2 sigma as far east as the
High Plains. Across the Southern Rockies Thursday, this will also
result in increasing upslope enhancement to the ascent, driving
locally higher precipitation amounts and rates, especially in
southern and eastern facing slopes, and this is where the greatest
WPC probabilities exist for more than 4 inches, reaching 80% in
the San Juans, Jemez, and Sangre de Cristos, with lower
probabilities extending into the northeast High Plains, as well as
south into the Sacramento Mountains. Late D1 into D2, the surface
low briefly intensifies more robustly, resulting in an area of
more intense WAA and associated fgen downstream and to the north
of the surface low, potentially focusing near the OK/KS border
Friday morning where theta-e lapse rates fall to near 0C/km
accompanying the more intense fgen. This could result in a band of
heavier snowfall which is reflected by the WPC prototype snow band
tool indicating at least a low potential for 1"/hr rates, aided by
dynamic cooling to overcome marginal surface temperatures. For D2,
WPC probabilities for 4+" peak at just 5-10%, and will be reliant
on this band setting up robustly enough and long enough to
accumulate significantly.

This first system ejects later D2, but is followed almost
immediately but yet another shortwave digging through the
exceptionally active flow. This shortwave is progged to dig out of
the Great Basin Friday night and then deepen as it approaches the
Four Corners and into the Southern Rockies once again Saturday
morning. Height falls and PVA should again drive ascent supporting
a wave of snowfall. Available column moisture for this second
impulse will be less, but the column will also be colder and the
best ascent may efficiently overlap the DGZ to support periods of
moderate snowfall, especially in the terrain. WPC probabilities
late D2 into D3 are modest, generally 10-30%, for an additional 4+
inches from the Sangre de Cristos through the southern Front Range.


...Pacific Northwest into the Great Basin and the Sierra...
Days 1-3...

An active pattern will bring multiple rounds of heavy snow to much
of the West into the weekend, with snow levels lowering with time
as well. During D1, broad zonal flow will surge modest moisture
onshore ahead of a shortwave which is progged to lift into the
Pacific Northwest Friday morning. This overlap of moisture and
modest ascent will bring a quick wave of precipitation to the
region, with snow levels generally 4000-5000 ft within the axis of
greatest PW and heaviest precip. Above these levels, briefly
moderate to heavy snow is likely before some subsidence develops
behind the first shortwave, and WPC probabilities for more than 6
inches of snow are above 30% only in speckles of the highest
terrain of the Cascades and into the Northern Rockies.

After brief shortwave ridging early D2, a more pronounced trough
will dig across the Pacific Ocean and then dive southward into the
Pacific Northwest, eventually advecting into portions of central
CA by the end of D3 while the primary longwave trough axis pivots
into the Great Basin. Pronounced confluent mid-level flow
downstream of the primary vorticity streamer will advect anomalous
moisture eastward, and the GEFS IVT probabilities are forecast to
exceed 60% for 250 kg/ms. At the same time, a potent Pacific jet
streak will dive southward along the Pacific coast, placing
impressive upper diffluence atop the best PVA/height falls, and
merging with the onshore flow to produce widespread precipitation
across the West. Although IVT is relatively modest, the prolonged
overlap of moisture due to onshore flow and synoptic ascent will
result in heavy precipitation, especially in the Cascades and
Sierra where upslope flow will contribute. Snow levels within this
deepening trough will fall to as low as 1500 ft in the Cascades,
and 3000 ft across the Sierra, so impactful snowfall is likely
even into lower elevations around the foothills of these ranges,
with snowfall also impacting the northern CA ranges, some of the
coastal ranges, and even spilling as far east as the
Northern/Central Rockies and higher terrain of the Great Basin. By
D3, WPC probabilities for more than 6 inches of snow are above 80%
in the Cascades and Sierra, with locally more than 18 inches
possible in the highest terrain.


...Southern Appalachians through New England...
Days 2-3...

A shortwave digging across the Four Corners Friday night into
Saturday will advect rapidly eastward across the Southern Plains
and then get sheared into pinched westerlies across the MS VLY
before arcing northward across the Mid-Atlantic and into the
Northeast late Saturday night /end of D3/. This will lead to a
modest intensity, fast moving area of low pressure which is
progged to develop near the Gulf Coast Saturday morning and then
lift northeast as a Miller-A type cyclone, reaching the southern
New England coast by Sunday morning. While this system continues
to look progressive, and has shifted a bit NW with recent runs, at
least through the Mid-Atlantic, it will likely result in a
significant winter event from the Southern Appalachians, the
interior Mid-Atlantic states, and into much of New England.
Uncertainty remains high due to the variations in speed and track
of this low, however, significant icing for the Southern/Central
Appalachians, and at least plowable snow, is becoming more likely
for much of the area.

As the shortwave and associated vorticity max lift northeast D2
into D3, they will be accompanied by a rapidly intensifying
subtropical jet streak which may reach as high as 170 kts, or +4
sigma according to NAEFS, over the TN VLY late in the forecast
period. This will drive tremendous LFQ diffluence for ascent,
ideally overlapping the best height falls and PVA from the modest
vort/shortwave. At the same time, moist advection will gradually
intensify downstream of the surface low, and while much of the
best theta-e advection and isentropic upglide appear to lift
parallel to the track (no significant TROWAL), there should still
be plentiful moisture for heavy precipitation as noted by NAEFS PW
anomalies rising above +1 sigma, resulting in an expanding
precipitation shield. The impressive WAA and fast-moving low will
likely result in a p-type challenge from the Southern Appalachians
through the Mid-Atlantic states, with additional changeover likely
along the coast of southern New England. The greatest risk for
freezing rain is likely from NW GA through the Piedmont and into
far northern VA where where WPC probabilities for more than 0.1"
reach as high as 70%, and there is some potential for more than
0.25" in Upstate SC and into southwest VA. Some sleet and snow is
also possible in this area, but the predominant p-type outside of
the highest terrain is likely to be ZR as the wedge holds surface
wet bulb temps below freezing.

Farther north into the Mid-Atlantic, the guidance has continued to
trend a bit farther NW as the low lifts along the coast and begins
to deepen. This will likely produce at least a swath of moderate
snow at onset across most areas, but onshore/SE flow within the
WAA and from the still warm Atlantic waters will shift p-type
quickly over to rain generally along and east of I-95, and some
mixing with sleet/freezing rain is possible much farther NW from
there as well as the 850mb warm nose surges. WPC probabilities for
at least 0.01" of freezing rain reach above 10% as far west as
eastern KY and southwest PA, and as far north as the southern
Poconos during D2-D3. Significant snowfall exceeding 4" during
this event will likely be confined to D3, and from the higher
terrain of eastern WV through the interior Mid-Atlantic and
northward through Upstate NY and southern/central New England.
There is increasing confidence that as the low approaches southern
New England it will stop its northward progression and track
eastward as it deepens, leading to a band of heavy deformation
snow from near the Poconos through southern New England, with
heavy snow also collapsing back to the southeast during this time.
Some uncertainty continues near the immediate coast, including the
Boston area, due to onshore flow off the warm waters, but a period
of intense snow rates and rapid accumulation is likely during D3.
WPC probabilities D3 for more than 6 inches of snow have
increased, and are now above 50% from central PA through the
Catskills and into much of southern New England except the
immediate coast, and locally more than 12" of snow is possible in
some areas as reflected by the WSE mean and low, but increasing,
PWPF. Some of this snow could be heavy and wet, and the WSSI-P
suggests a high potential for at least moderate impacts from this
event D3 into D4, including the Boston metro area.


...Northern MN into the Western U.P. of MI
Days 2-3...

A closed low over the Corn Belt region Friday night will drive
downstream divergence and periods of PVA to produce ascent into
the northern portions of the Upper Midwest and into the western
Great Lakes. This synoptic ascent will combine with increasing WAA
ahead of a weak surface wave which may develop near the ND/MN
border, and then push eastward followed by a cold front Saturday
night driven by a secondary axis of vorticity strung out across a
northern stream. The accompanying WAA will surge PWs to +1 sigma
according to NAEFS in a thin ribbon focused in northern MN, which
will result in a corridor of moderate snow beginning Friday night,
and expanding with time through Saturday. The best frontogenetic
ascent appear to lie within the 925-700mb layer, which is well
below the elevated and shallow DGZ, and even then the total
forcing is modest. This should limit snowfall intensity across the
region, although there may be a brief uptick ahead of the cold
front as the DGZ deepens/lowers coincident with increasing ascent,
or along the Iron Ranges of the Arrowhead where additional
moisture from Lake Superior may upslope into the region. This will
produce generally light accumulations of snow, with locally higher
amounts across the Arrowhead. WPC probabilities for more than 4
inches reach 20-40% for much of northern MN, and above 50% along
the shore of Lake Superior into the Arrowhead.


***Key Messages for Jan 6-7 East Coast Winter Storm***

--Developing coastal low pressure
An area of low pressure is forecast to develop near the Gulf Coast
late Friday, and then track northeast to the DelMarVa coast
Saturday. This low will then likely deepen south of New England
before ejecting eastward into the Atlantic Ocean by Monday
morning.

--Significant Icing Possible
There is increasing potential for significant accumulations of ice
for portions of the southern and central Appalachians. Ice
accumulations exceeding 0.1" could produce hazardous travel due to
slippery roads Friday night and Saturday.

--Heavy snowfall amounts becoming likely
For the interior Mid-Atlantic, Northeast, and New England, there
is increasing confidence that an axis of heavy snow will develop
Saturday afternoon into Monday morning. Although the details are
still uncertain, significant accumulations of heavy and wet snow
may cause considerable travel impacts as well as possible impacts
to infrastructure.

--Coastal Flooding a concern
As the low intensifies on Sunday, gusty onshore winds may lead to
minor flooding along the Mid-Atlantic and southern New England
coasts.

 
Weiss