FOUS11 KWBC 040943
QPFHSD

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
$$
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FOUS11 KWBC 050956
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
456 AM EST Fri Jan 05 2024

Valid 12Z Fri Jan 05 2024 - 12Z Mon Jan 08 2024

...Southern Appalachians through New England...
Days 1-3...

Major winter storm to bring combination of freezing rain and heavy
snow from the Southern Appalachians through New England.

A large trough across the center of the CONUS will expand to the
east Friday night in response to a subtle vorticity impulse and
accompanying weak shortwave trough rotating cyclonically through
the flow and into the Mid-Atlantic. This will be followed quickly
by a more substantial vorticity impulse streaking out of the Gulf
Coast, and it is this secondary trough that will drive the more
intense ascent and support cyclogenesis as a Miller-A type low
develops in the Southeast and then moves northeast into the
Mid-Atlantic and into New England. This shortwave will remain
progressive at least into Sunday morning, but guidance has
continued to become more aggressive with it tilting negatively
near the New England coast, which could slow/stall the surface
low, while yet a third piece of energy approaches from the Ohio
Valley to re-invigorate ascent and lengthen the period of
precipitation. At the same time, an intense subtropical jet streak
will arc out of the TN VLY, with wind speeds approaching +4 sigma
according to NAEFS, and the intense diffluent LFQ of this jet will
overlap the best height falls sufficiently to help rapidly
strengthen the low south of New England. While there is still some
uncertainty into the exact track which will wreak havoc with the
rain/snow line, there is increasing confidence that a stripe of
heavy and wet snow with significant accumulations will occur just
inland from the coast from WV through ME.

Late D1 /Friday night into Saturday morning/ the expanding
precipitation shield will begin to overrun cold Canadian air
across the Appalachians. Moist isentropic upglide at 285K will
feature impressive mixing ratios of 5-6 g/kg, suggesting a rapidly
moistening column and potentially heavy precipitation. This
overrunning will likely result in periods of snow changing to
freezing rain in the foothills of the Appalachians where the
surface wedge will at least briefly become reinforced by falling
precipitation before slowly eroding to the northeast. Guidance
tends to over-do the rate of this erosion, and with some weak
dry-drainage on NE winds, it is possible some areas could receive
around 0.25" of freezing rain. However, this system remains
progressive into Saturday so precip will either wane or turn to
rain quickly Saturday across VA/NC/SC, with the best freezing rain
potential shifting northward along the Blue Ridge where WPC
probabilities for more than 0.25" peak at 20-40%.

North of the freezing rain/mixed zone, an area of heavy snow will
develop early Saturday as the surface low tracks from near GA to
off the NJ coast by Sunday morning. This low will be progressive
during this time, but should be accompanied by strong WAA at onset
which when combined with the impressive synoptic lift will result
in some front end heavy snow most areas, before quickly changing
over to rain along and east of I-95, with some sleet-to-rain
farther inland. The WAA is impressive, 850mb flow is out of the
S/SE, and the Canadian high is not in a favorable place to lock in
cold air, so it is likely only elevated areas and regions well
inland that will receive significant accumulations across the
southern Mid-Atlantic states, but a burst of heavier snowfall
rates within the WAA/fgen band could still result in hazardous
travel for a time, and moderate accumulations as far south as the
Laurel Highlands and Blue Ridge. However, the more significant
snowfall is likely the latter half of D2 and into D3 as the low
stalls south of New England and rapidly deepens in response to a
strong baroclinic gradient offshore and better synoptic lift.
During this time, onshore flow will maximize noted by U-wind
anomalies approaching -3 sigma according to NAEFS, pushing
impressive theta-e advection into New England and lifting into a
TROWAL. While this will likely push warm air into the coast,
locations just inland and in higher elevations could experience a
longer duration of heavy snow as the setup seems to at least
marginally support a pivoting band of heavy snow N/NW of the
maturing and then occluding cyclone. During this time, likely late
Saturday night into the first half of Sunday, the occlusion and
isallobaric response to the strengthening low could cause the cold
air to collapse back to the southeast, bringing heavy snow back
towards the coast before the system winds down Sunday evening.
Despite marginal low-level thermals, the strong ascent should
result in heavy snow rates exceeding 1"/hr, especially in elevated
inland areas. There remains some uncertainty as to where this
impressive band will pivot, but WPC probabilities, in general,
have increased and shifted a bit NW with this update, and now are
above 50% for 6+ inches on D2 from the Poconos through the
Catskills and into much of southern New England away from the
immediate coast, shifting into northeast MA, southeast NH, and far
southwest ME on D3. Locally more than 12 inches of snow is likely
in some areas, especially in the higher terrain of the Catskills,
Berkshires, and Worcester Hills.

...Southern Rockies and Four Corners...
Days 1-3...

A series of shortwave rotating through broad cyclonic flow
encompassing much of the CONUS will bring waves of precipitation,
primarily as snow, to the Four Corners and Southern Rockies each
day. One impulse moving into the region late tonight into Saturday
morning will bring a round of snow, generally to the higher
terrain of UT/CO and the Sangre de Cristos, with NW flow providing
favorable upslope into these regions. The shortwave is rather
transient and moisture is modest, but above 3000 ft WPC
probabilities for more than 4 inches reach 30-50% in these ranges.

As this first shortwave ejects to the east and into the Plains
Saturday, brief shortwave ridging will follow in its immediate
wake, before another, stronger, impulse digs in from the Pacific
Northwest. This next feature will amplify into a potent closed low
centered over the eastern Great Basin Sunday evening and continue
to deepen as it pivots into the Southern High Plains by the end of
the forecast period. The result of this evolution will be a
pronounced longwave trough which sets up over the Intermountain
West, leaving both upstream and downstream jet streaks to amplify
and support enhanced coupled divergence aloft. The overlap of this
synoptic ascent will likely lead to surface cyclogenesis around
the Four Corners Sunday evening, and this low is progged to then
strengthen as it shifts to the east on Monday. Downstream of this
wave, at least modest moisture advection will emerge from the
south and pivot into the Four Corners, with mid-level RH
increasing dramatically late Sunday. Snow levels ahead of the wave
may rise to as high as 4000-5000 ft within the WAA, but will fall
quickly to as low as 1500 ft behind the accompanying cold front.
The duration of this impressive ascent into a moistening column
will result in areas of heavy snow beginning late D2 but
especially on D3. WPC probabilities for more than 4 inches during
this second wave reach above 80% from the Wasatch Front and Uintas
along the Wasatch, southeast into the San Juans, and then much of
the higher terrain from the Mogollon Rim into the Sangre de
Cristos of northern AZ and NM. Locally more than 12 inches of snow
is possible on D3, with the highest probability of that occurring
in the White Mountains of AZ and the southern San Juan Range.

..Pacific Northwest/Great Basin/Sierra...
Days 1-3...

An active period of weather will be accompanied by lowering snow
levels as an expansive trough amplifies across the West. The
period begins with one shortwave impulse racing eastward across
the inland Pacific Northwest and diving into the Great
Basin/Northern Rockies. This will have limited impact to the
sensible weather/precipitation, but will result in more zonal flow
in its wake to surge additional moisture onshore. Within westerly
flow, a shortwave will approach from British Columbia, pinching
the flow to become more intense, while also producing downstream
divergence ahead of the approaching impulse. This feature will
then intensify as it drops southeast into the Pacific Northwest
and northern CA Saturday evening, amplifying into a closed low
over the Great Basin by Sunday morning before slowly pulling off
to the east by the end of the forecast period. Broad but
impressive synoptic lift will accompany this evolution, with lift
being provided through substantial height falls, pockets of
impressive PVA, and robust upper diffluence as the LFQ of a 150kt
Pacific jet streak arcs down the coast and into the Intermountain
West.

Although IVT will remain modest during the duration of this event,
generally AOB 300 kg/ms, the prolonged moisture fetch within the
large scale ascent will result in heavy precipitation across much
of the West, especially late D1 through early D3. The heaviest
precipitation will spread southeast with time, starting in the
Pacific Northwest (Cascades, Olympics) Saturday morning and then
expanding and extending across most of the region through Sunday.
For D1, the surface cold front will drop southeast through the
PacNW with upslope flow enhancing snowfall across the Cascades,
primarily above 2500 ft. WPC probabilities for more than 4 inches
are 70-90%, with 1-2 feet likely in the highest terrain. During
D2, WPC probabilities for more than 4 inches expand considerably
as the front and waves of low pressure move east beneath the
amplifying trough. The highest risk areas for more than 4 inches
on D2 are again across the Cascades, but also extending south into
the Northern CA ranges and along the Sierra, with additional high
probabilities above 80% reaching the higher terrain of the
Northern Rockies, Blue Mountains, and ranges in the Great Basin.
Although the heaviest snowfall, which could exceed 1 foot, should
be confined to the higher terrain of these areas, snow levels will
be collapsing to as low as 1000 ft in WA and the Northern Rockies,
and 2000-3000 ft elsewhere indicating an increasing potential for
impactful snow even into lower elevations and many of the area
passes to produce hazardous travel. By D3, the most intense ascent
shifts southeast again into the Four Corners, but renewed
precipitation is expected in the Cascades where WPC probabilities
for more than 4 inches reach 20-40% in advance of yet another
shortwave moving onshore.

...Northern Minnesota...
Days 1-2...

A cold front will waver across northern MN Friday night before
returning north as a warm front by Saturday morning in response to
a shortwave emerging from the Central Plains and lifting northward
into the Upper Midwest. At the same time, a weak secondary
shortwave will dig southeast from Saskatchewan, and the
interaction of these features will drive weak cyclogenesis which
will shift eastward, trailed by a cold front into the Great Lakes
by Sunday morning. Increasing moist isentropic upglide on the
285K-290K surfaces concurrent with the warm front will result in
an expanding shield of light to moderate snowfall, although modest
ascent and low probabilities for even 50mb of DGZ depth from the
SREF indicate rates should remain modest. The column will be cold,
so efficient and fluffy SLRs are likely, and a long duration of
this snow should result in moderate accumulations across the area.
As the low shifts eastward Saturday into the Great Lakes, the
trailing cold front may be accompanied by some enhanced 850-600mb
fgen which could drive a narrow band of heavier snowfall across
these same areas, adding light accumulations through D2. The
heaviest snow this period will likely be across the Iron Ranges in
the Arrowhead where some additional moisture from Lake Superior
will contribute, and low-level flow with the warm front will
upslope effectively to wring out additional moisture, but WPC
probabilities for more than 4 inches D1.5 peak around 50% along
the lake shore, with locally more than 6 inches possible.
Elsewhere across northern MN WPC probabilities for more than 4
inches reach 20-40%.

Weiss

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

--Heavy Snow in the Northeast
For the interior Mid-Atlantic and New England, heavy snow is
expected from Saturday Afternoon through Sunday. Although there is
uncertainty into where the rain-snow transition will occur, there
is a high chance (50-90%) of exceeding 8 inches of snow from the
Poconos eastward through much of interior southern New England.
Locally more than 12 inches is possible at higher elevations.

--Impacts from heavy, wet snow and wind
The combination of heavy, wet snow and gusty winds from northeast
Pennsylvania through much of southern and central New England will
result in difficult travel, and may lead to some power outages and
tree damage.

--Minor Coastal Flooding Possible in the Northeast
Gusty onshore winds may lead to minor flooding along the
Mid-Atlantic and southern New England coasts, particularly for the
Sunday morning high tide cycle.

--Ice Impacts to Travel in Southern Appalachians
Along and east of the Southern Appalachians, accumulations of ice
in excess of 0.1 inches, are likely (70-90%) with locally higher
accumulations possible. This icing, along with some areas of
sleet, may produce hazardous travel conditions on Friday Night and
Saturday.

$$
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