FOUS11 KWBC 070947
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
447 AM EST Sun Jan 07 2024

Valid 12Z Sun Jan 07 2024 - 12Z Wed Jan 10 2024

...New England
Days 1 & 3...

The strong coastal low will be positioned just south of the New
England coast this morning, and will likely experience some rapid intensification through the afternoon as a stripe of vorticity
embedded within a negatively tilting shortwave lifts northeast
downstream of the Great Lakes trough to enhance ascent over the
low. At the same time, the subtropical jet streak arcing out of
the TN VLY will pivot poleward and intensify to 190 kts (nearly +4
sigma according to NAEFS), placing intense upper diffluence within
the LFQ atop the best PVA, further enhancing the surface low
deepening. This evolution will result in a deepening low but also
lead to an occlusion as the low begins to stack vertically, which
will allow the cold air to collapse back to the southeast across
New England. This will result in a continuation of heavy snow
across Southern New England (SNE), especially within a deformation
band which is still progged to pivot eastward across the region.
Some lengthening of the heavy snow is possible during the first
half of Sunday as well as interaction with the primary trough
occurs, driving additional ascent from the west even as the low
begins to pull away. This overlap of ascent will likely produce
snowfall rates in excess of 1"/hr as reflected by the WPC
prototype snow band tool, and although some drier air will begin
to impinge on the area from the west, additional snowfall across
SNE will likely exceed 6 inches as reflected by WPC probabilities
for more than 6 inches reaching 40-80% from the southern Green
Mountains of VT eastward through far southwest ME.

Another strong system will develop over the Southern Plains on
Tuesday and then lift northeast into the Great Lakes by Wednesday.
This track is not favorable for cold/snow across New England,
especially as the surface high retreats quickly to the north.
Moisture will likely be excessive however, with NAEFS IVT
anomalies surging to +5 to +6 sigma into SNE by the end of the
forecast period. This will likely be heavy rain across all but the
highest terrain, but above 2000 ft, heavy wet snow is likely,
which will accumulate rapidly. Uncertainty still exists by D3, but
current WPC probabilities for more than 6 inches of snow are
30-50% in the southern Adirondacks, and above 80% in the White
Mountains of NH/ME where more than 12 inches is likely in the
highest terrain.

..Pacific Coast, Great Basin, Northern Rockies...
Days 1-3...

A series of shortwaves will continue an active pattern into the
West through mid-week. The first of these shortwaves will be
moving onshore the WA/OR coast to start the period as an
amplifying trough digs into the Great Basin. A sheared out
vorticity lobe extending along the coast will provide modest
ascent, with confluent mid-level flow in its wake driving some
enhanced moisture into the Cascades. The overlap of forcing and
moisture should wane by the end of D1, but in the OR Cascades, and
generally above 2000ft, WPC probabilities for more than 6 inches
are 30-40%.

Shortwave ridging follows in the wake of this lead shortwave
bringing a reduction in forcing and moisture into D2, but this
will quickly be overwhelmed by much more significant ascent and
moisture ahead of a more intense shortwave advecting to near the
WA/B.C. border Tuesday evening. Downstream of this feature,
mid-level flow will become increasingly confluent in conjunction
with a 130kt Pacific jet streak approaching zonally to the coast.
The overlap of these will result in increasing IVT shifting
onshore, driving an atmospheric river (AR) towards the coast with
moderate probabilities for IVT exceeding 500 kg/ms. This will
initially push a warm front onshore late Monday with snow levels
rising to 4000-6000 ft. However, a subsequent cold front following
rapidly behind the lead WAA will quickly plummet snow levels back
to 1500-2500 ft, with heavy precipitation occurring this
evolution. The intense and long-lasting ascent, aided by
pronounced upslope into the N-S ranges, combined with impressive
moisture, will result in very heavy snowfall beginning late D2 and
expanding across much of the region during D3. WPC probabilities
for more than 6 inches on D2 are above 70% in the WA/OR
Cascades,and 30-50% as far east as the Northern Rockies. However,
by D3, this becomes much more impressive with >80% probabilities
for 6+ inches encompassing all of the Cascades, Olympics, Shasta/Trinity/Siskiyou region, and eastward into the Salmon
Rivers/Sawtooth, Northern Rockies, and towards the Tetons. Just
during D3 alone, many of these ranges, especially above 2000 ft,
will receive more than 2 feet of snow. As snow levels crash,
considerable snow will also impact most of the area passes across
the Cascades and Northern Rockies.

...Four Corners States across the Plains and through the western
Great Lakes...
Days 1-3...

A potent shortwave and accompanying vorticity maxima will rotate
across the Great Basin this morning and then amplify rapidly into
a closed low near the Four Corners, with 500mb height anomalies
reaching -4 sigma according to NAEFS ensemble tables in eastern
NM, with even further deepening to as low as -5 sigma progged over
the Southern Plains by Tuesday morning. This extreme mid-level low
will be accompanied by increasing upper diffluence as coupled jet
streaks intensify downstream of the amplifying longwave trough
axis, which will result in an impressive surface cyclone
developing in the lee of the Southern Rockies Sunday night and
then intensifying as it lifts northeast through Tuesday. This will
likely become an intense cyclone with heavy precipitation and
strong winds in many areas.

During D1, as the low begins to consolidate, most of the forcing
for ascent will be driven by deep layer forcing within the LFQ of
a jet streak pivoting around the amplifying trough, and within a
region of robust height falls/mid-level divergence downstream of
the deepening closed low. Moisture on D1 will be somewhat modest
across the Four Corners states as reflected by near-normal PW
anomalies, but the intense forcing will wring out what is
available, falling as moderate to heavy snow above 3000-4000 ft.
Increasing SW flow between 700-500mb will help advect at least
modest Pacific moisture into the region, and this will also
upslope into the higher terrain, especially around the San Juans,
Wasatch, and Mogollon Rim. It is in these ranges where WPC
probabilities for more than 6 inches maximize, reaching above 80%,
with locally higher amounts reaching 1-2 feet likely. With snow
levels crashing rapidly behind the cold front accompanying the
surface low, light snow accumulations are likely even into many of
the valley floors before 12Z Monday.

The system really begins to ramp up on Monday and Tuesday in
response to the most intense deep layer lift resulting from the
overlap of coupled jet streaks and impressive mid-level height falls/divergence. At the same time, the intense confluent and
southerly flow downstream of the closed low will drive strong
moist advection northward into the Southern and Central Plains.
This is noted in model progs via extreme mixing ratios of 8g/kg
surging northward on 295K moist isentropic upglide, with the
resultant theta-e ridge axis lifting N/NW into a TROWAL around the
surface low, and PWs forecast by NAEFS reaching +1 to +2 sigma.
This will fully saturate the column, and as deep layer ascent
maximizes both through synoptic forcing and the WAA, an expanding
shield of heavy precipitation will result. While the column in the
Southern Plains will be too warm for snow, areas into KS/MO and
points north will likely experience a long duration of moderate to
heavy WAA snow, with snowfall rates of 0.5-1"/hr likely as fgen
maximizes beneath a deepening DGZ. Cross sections at this time do
not suggest convective rates within the WAA snowfall, but a long
duration of snow as the storm lifts northeast will still produce
significant accumulations, and locally enhanced banding cannot be
ruled out. On the back side of this system, the setup does appear
to support a pivoting or laterally translating deformation band
surging eastward behind the low from the High Plains of CO/NM,
through the OK/TX panhandles Monday, and then lifting into lower
Missouri Valley Tuesday morning, and eventually into the Great
Lakes by the end of the forecast period. These bands are extremely
sensitive to initial position errors of the models, so confidence
in the exact placement is still uncertain, but it is possible that
the WAA snow will transition immediately to more intense
deformation snow across parts of MO/IA which may explain why
current PWPF data is most aggressive in that region. Heavy snow is
likely to spread from the Southern High Plains into the western
Great Lakes Monday and Tuesday, and WPC probabilities for more
than 4 inches exceed 70% on D2 from the Raton Mesa northeast
through the Corn Belt, and on D3 extend as far northeast as the
Door Peninsula and northern L.P. of MI. Locally up to 12 inches of
snow is possible, as reflected by WPC probabilities of 20-40% near
the MO/IA border. Note that ensemble trends have been shifting NW
the past few runs, so it is likely that additional adjustments to
the heaviest snow axes will continue over the next few model
cycles, and a subsequent additional shift to the NW is possible.

...Southern/Central Appalachians...
Days 2-3...

An intense 500mb low lifting across the Upper Midwest will result
in downstream divergence across the Appalachians on Tuesday.
Downstream of this feature, extreme moist advection is likely on
impressively pinched flow driving IVT as high as +6 sigma
according to NAEFS, and extreme mixing ratios of 8g/kg surging
northward on the 295K-300K isentropic surfaces. This will result
in an expanding shield of heavy precipitation emerging from the
Gulf Coast, and although the associated WAA will be intense,
initially the surface wet-bulb temperatures will be below 0C.
These sub-freezing temperatures will likely erode rapidly from
south to north during Tuesday, but precipitation at onset could
feature a mix of sleet and freezing rain before changeover to
rain. Heavy rates and limited duration of freezing rain should
limit ice accretions overall, but WPC probabilities peak around
30-50% for 0.1" of ice accretion on D2.5 along the NC/VA Blue
Ridge and into the Laurel Highlands.

Weiss


***Key Messages for Jan 7-11 Major Winter Storm***

--Heavy Snow Will Create Hazardous Travel
The winter storm will deliver heavy snow in the Plains and Midwest
on Monday and Tuesday. A heavy, wet snow will develop in the
central Appalachians and interior Northeast Tuesday Night. The
heaviest snow totals are most likely in parts of the Midwest,
where local maxima up to 12 inches are likely.

--Extremely Dangerous Blizzard Possible
Blizzard conditions are most likely in the central Plains where
wind gusts in excess of 50 MPH will lead to near zero visibility
at times and extremely dangerous travel. Blizzard conditions are
possible in the Midwest as well.

--Significant River, Flash, and Coastal Flooding
Widespread and potentially significant river and flash flooding
are likely for sections of the central Gulf Coast and the Eastern
U.S. early this week. Powerful onshore winds will lead to
widespread coastal flooding along the eastern Gulf Coast and much
of the East Coast. Significant coastal flooding is likely in some
areas, especially in the Carolinas and Mid Atlantic.

--Damaging Winds May Lead to Power Outages
Widespread wind gusts in excess of 50 MPH are likely in the
eastern Gulf Coast, central Appalachians, much of the East Coast,
and New England. Prepare for power outages.


***Key Messages for Jan 8-11 Northwest Winter Storm***

--Major Winter Storm for the Cascades
A winter storm will peak on Tuesday and Wednesday in the Cascades
in Washington and Oregon, with several feet of heavy snow and
gusty winds. Snow levels will quickly fall to between 1500 and
2500 feet behind a cold front on Tuesday, leading to considerable
impacts for many mountain passes.

--Heavy Snow Expected Elsewhere in the West
Over a foot of snow is likely (70-90%) in the higher elevations of
the Northern Rockies, with the heaviest snow falling on Tuesday
and Wednesday. Snow levels falling to below 1000 feet later on
Wednesday and early Thursday may lead to some snow accumulations
in the valleys also.

--High Surf, Gusty Winds on the Pacific Northwest Coast
The active storm track into the Pacific Northwest is likely to
lead to renewed periods of high surf and gusty onshore winds early
this week on the coastlines of Washington and Oregon.

--Low Pressure System Likely to Move into Central U.S.
The storm system is likely to advance through the Western U.S. and
reach the Plains by late in the week. People in the Central U.S.
should be aware of the potential for another winter storm and stay
tuned for updates.

$$
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