Subj : HVYSNOW: Us Winter Storms
To   : All
From : Dumas Walker
Date : Tue Jan 09 2024 10:26 am

FOUS11 KWBC 090944
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
443 AM EST Tue Jan 09 2024

Valid 12Z Tue Jan 09 2024 - 12Z Fri Jan 12 2024

....Midwest through the northern Great Lakes...
Days 1-2...

An intense closed mid-level low with height anomalies around -4
sigma will bowl its way from the Southern Plains early Tuesday
into the Great Lakes by Wednesday morning and then continue
northeast into Quebec Wednesday night. This extremely anomalous
low will be accompanied by intensifying jet streaks both upstream
and downstream, leaving an increasingly coupled jet structure
across the Upper Midwest. This setup is conducive to a rapidly
deepening surface low pressure area which will lift northeast from
near St. Louis, MO to the L.P. of Michigan by Wednesday morning.
Downstream of this low, intense moisture advection will continue
to surge out of the Gulf of Mexico on 290-295K moist isentropic
ascent, driving PWs up towards +2 sigma according to NAEFS. This
moisture laden air is then progged to wrap cyclonically within the
WCB into a modest TROWAL which will pivot back into the Great
Lakes and Upper Midwest, resulting in some enhanced elevated
instability to increase ascent and drive more impressive snowfall
rates.

There are likely to be two primary areas of snow as this low
translates northeast through Wednesday. The first will be along
and just ahead of the 850mb low where the most intense warm/moist
advection will drive enhanced 850-600mb fgen for ascent, while
also yielding an expanding area of precipitation. The accompanying
WAA is quite robust, and areas east of the low track will likely
start as snow but then change over to rain as far north as
Detroit, MI. However, this leading WAA will also result in a
front-end "thump" of heavy snowfall with rates of 1-2"/hr at times
as reflected by the WPC prototype snow band tool. Farther to the
NW but still within the best theta-e ridge, a longer duration of
these rates is likely, especially where they pivot beneath the
TROWAL.

Farther to the W/NW and along/west of the 700mb low track, there
is likely to be a rejuvenated deformation axis which will pivot
around the backside of the low and then translate northeast with
time. This is expected from far eastern KS early, through southern
IA, and into southern WI Tuesday night. The setup looks extremely
favorable for a strong deformation band, and both CSI/CI
parameters are noted in cross-sections indicating at least the
potential for thundersnow. With SREF DGZ depth probabilities for
100mb exceeding 50% from IA into WI coincident with the
deformation axis, if thundersnow does occur, rates could reach
3"/hr at times, which is now reflected by the latest WPC prototype
snow band tool, and could rapidly enhance snowfall totals. The
combination of these intense rates and gusty winds will result in
considerable impacts through D1, and the WSSI-P probabilities for
major impacts reach as high as 20-40% from southern IA through
southern WI. WPC probabilities on D1 for more than 6 inches are
above 70% from far northeast MO through the Door Peninsula of WI,
with the maximum snowfall potential exceeding 12" where the
WAA/deformation band both occur, possibly focused near Davenport,
IA. It is important to note that the track of this low, which had
been trending NW the last few nights, has corrected a bit back to
the southeast tonight. This has resulted in a modest SE shift in
the heaviest snow, and if this trend continues that potent
deformation axis could shift back into the Chicago metro area late
tonight.

As the low pulls into Canada late D1 and into D2, the best
synoptic ascent and deepest moisture will shift out of the region,
but increasing N/NW flow may result in some area of heavy lake
effect snow (LES), especially across the eastern U.P. and
northwest L.P. of MI. Inversion depths in regional soundings are
modest and winds are quite strong which will reduce parcel
residence time, but multi bands with heavy snow rates are likely,
and WPC probabilities for more than 6 inches are above 70% in the
Huron Mountains, and 30-50% across much of the eastern U.P. and
near the Tip of the Mitt.

....Southern Appalachians through the Northeast...
Days 1-3...

Extreme upper low with NAEFS standardized anomalies reaching -4
sigma at 500mb will progress northeast into the Great Lakes and
then Quebec through Wednesday, while spokes of vorticity shed from
the primary gyre and lift northward within pronounced southerly
flow downstream. Impressive moist advection embedded within this
southerly flow will drive IVT as high as +8 sigma over the
Mid-Atlantic, indicative of how anomalous the moisture is lifting
across the region. Ascent through PVA, mid-level divergence, and
increasing upper-level diffluence will act directly upon this
saturated column, resulting in a shield of heavy precipitation
lifting from the Southern Appalachians Tuesday morning, to New
England by Wednesday morning. Initially, this precipitation will
be either frozen or freezing, but will eventually transition to
rain in most areas due to the intense WAA. The exception will be
in the higher terrain of New England where significant heavy wet
snow accumulations are likely.

Across the Southern Appalachians, primarily for the Blue Ridge of
VA into the Alleghenies and Laurel Highlands, some snow at onset
will transition to freezing rain as warm air overruns the cold
surface layer. The intensity of the WAA will eventually cause a
transition to all rain, and heavy rainfall rates should limit ice
accretion efficiency, there is likely to be at least light icing
Tuesday morning across this area, but WPC probabilities for more
than 0.1" of ice are less than 10%.

The more significant threat is heavy snowfall from the Catskills
and Adirondacks northeast through the Greens and into much of the
higher terrain of NH and ME. Here, extreme moisture flux will
impinge into the higher terrain to drive more pronounced ascent
through upslope enhancement, which should produce intense snowfall
rates of 1-2"/hr. This is reflected by the WPC prototype snow band
tool as well as NBMv4.1 1-hr snowfall probabilities. What is most
concerning about this snowfall is that SLRs are likely to be well
below climo, so in places above 4000 ft where the p-type may stay
all snow for the event, the heavy wet snow could result in major
impacts as reflected by the snow load parameter within the WSSI-P.
WPC probabilities for more than 6 inches of snow on D1 are above
80% in the White Mountains and higher terrain of Maine, and 40-70%
in the Adirondacks and Greens, generally above 2000 ft. The
heaviest snow is almost certainly going to occur in the White
Mountains, where D1 into D2 snowfall could exceed 18" in the
highest terrain.

Then, as the low over the Great Lakes occludes to a secondary
center lifting across ME on Wednesday, increasing NW flow in its
wake will result in strengthening CAA over the anomalously warm
Great Lakes. This could result in efficient lake effect snow (LES)
with inversion heights climbing to around 8000 ft coincident with
lake-induced instability rising towards 250 J/kg. There is some
uncertainty as to how much of the DGZ will be saturated, so
extremely intense and efficient LES is not currently progged, but
with the lake temperatures still well above normal for early
January, this could result in heavy accumulations exceeding 6
inches in the Tug Hill both D2 and D3 as reflected by WPC
probabilities exceeding 50% both days.

....The West...
Days 1-3...

An extremely active pattern continues across the Pacific Northwest
and into the Northern Rockies/Great Basin through late week as a
series of shortwaves drop onshore from the Pacific

The first of these will will move onshore this morning and drive a
surface low/associated cold front into the Pacific Northwest and
as far east as the Northern Rockies tonight. Although the synoptic
forcing responsible for driving this feature eastward will weaken
with time into D2, the modest IVT (less than +1 sigma according to
NAEFS) will still push elevated mid-level RH above 90% across much
of the Northwest, which will manifest as snow above snow levels
which will fall gradually through the period from 2500 ft early,
to as low as 500 ft east of the Cascades late, 1500 ft in the
Cascades. With impressive and confluent mid-level flow advecting
onshore and impinging orthogonally into the terrain, the heaviest
snow is likely in the Cascades.

Following immediately in the wake of this first shortwave, a more
intense shortwave and accompanying vorticity maxima will drop
along the PacNW coast and come onshore near the OR/CA border
before amplifying into a more potent longwave trough near the Four
Corners Thursday night. This feature will again be accompanied by
impressively confluent mid-level flow to surge moisture inland,
and although PW anomalies are modest, intense ascent in the
vicinity of a translating low and strong surface cold front, and
within the LFQ of a more potent jet streak will drive strong
ascent to wring out available moisture. Snow levels late D1 into
D2 may briefly rise on the warm/moist advection, but will
generally continue to fall, and will likely reach the surface
across much of the interior Northwest, and fall to around just
1000 ft near the coast of WA/OR, higher into the Sierra. The
heaviest snow with this second wave will likely shift a bit
southeast from the greatest accumulations on D1.

Maybe more importantly, the guidance has become more intense with
this low pressure and cold front moving towards Portland, OR late
tonight into Wednesday morning. Steep lapse rates beneath the cold
pool aloft suggest some elevated instability which could support
convective precipitation rates aided by the impressive synoptic
deep layer lift overlapping concurrently the most intense
850-600mb fgen surges onshore. This suggests that snowfall rates
in excess of 1"/hr are possible, which is reflected by the latest
WPC prototype snow band tool. During the period of greatest
snowfall rate potential, RAP regional soundings indicate that WBZ
heights are below the critical 750ft level, which indicates that
snow accumulations could reach down into the lowlands around
Portland, OR and the Columbia River Gorge/Basin. The high-res
guidance is most intense with this potential, but latest WPC WSE
plumes have also shown an increase in snowfall, and after
coordination with SEW/PDT/PQR, some enhancements to snow has been
included in the lowlands, and WPC probabilities for 2+" reach
30-40% in the Portland, OR metro area.

By D3 the pattern begins to shift a little as the primary trough
amplifies over Alberta and then digs southward, driving a surface
cold front into a more NW to SE orientation as it drops into
WA/ID/MT by the end of the forecast period. South of this trough,
mid-level flow becomes increasingly pinched with W/NW flow
impinging again into the Cascades. Although the highest IVT/PWs
will remain just offshore by the end of the period, increasing
moisture being acted upon by intensifying fgen along the cold
front and amplifying upper diffluence ahead of an approaching jet
streak should result in widespread snowfall once again spreading
across the Northwest, with snow levels low enough to support light
accumulations even into the lowlands around Seattle, WA by the end
of the forecast period.

WPC probabilities D1, D2, and D3 all feature high potential (>80%)
for more than 6 inches across much of the western terrain from the
Olympics eastward through the Northern Rockies and southward along
the Cascades, into the Sierra, across much of the Great Basin, and
even into the Wasatch. The most significant snowfall D1 is likely
in the WA/OR Cascades where several feet of snow is likely above
2500 ft, and major impacts to mountain passes is expected. Also on
D1 as moisture spills eastward, 1-2 feet, with locally higher
amounts, is likely across parts of the Northern Rockies. During D2
the heaviest accumulations shift to focus primarily in the OR
Cascades and down into the Sierra, where 1-2 feet of additional
snowfall is likely, which will again cause major impacts to the
passes. By D3 the focus returns to the WA/OR Cascades and Northern
Rockies where multiple feet of additional snow is possible, and it
is also during this time when snow levels crash to support more
widespread, albeit light, snowfall accumulations in the valleys
and lowlands.

Finally, the strong cold front moving across Oregon Tuesday night
into Wednesday /late D1 into D2/ will continue to shift rapidly
southeast across the Great Basin and into the Central Rockies by
Thursday morning. This could spread convective snow showers and
snow squalls across the area, with the CIPS snow squall parameter
showing values above +1 spreading as far south as the Four Corners
states. Evaluation of the fields which drive the snow squall
parameter suggest coverage of snow squalls may be modest as there
exists only pockets of overlapping sufficient 0-2km RH/925mb
fgen/SBCAPE>100 J/kg to support intense squalls, but scattered to
widespread convective snow showers are possible, which could cause
significant travel impacts even beyond the areas of heavier
synoptic snow.

Weiss


***Key Messages for Jan 8-10 Major Winter Storm Central Plains
through the Midwest and Eastern U.S.***
--Dangerous Travel in Heavy Snow
Heavy snow bands with rates of 1-2"/hr will shift from the central
Plains to the Upper Midwest today, and then into the Great Lakes
tonight. These snow rates will be accompanied by gusty winds
reaching 35-45 mph, making for dangerous travel due to low
visibility and snow covered roads. More than 12 inches of
storm-total snowfall is likely (70-90%) in some areas.

--Impactful Snow in the Northeast
Heavy/wet snow will move across interior Northeast terrain tonight
into Wednesday. This snow will cling to trees and power lines,
which when combined with gusty winds reaching 40 mph, could result
in sporadic power outages and scattered tree damage.

--Significant River, Flash, and Coastal Flooding
Widespread and potentially significant river and flash flooding is
likely for much of the Eastern U.S. today. Powerful onshore winds
will lead to widespread moderate to locally major coastal flooding
through the Mid-Atlantic and southern New England tidal rivers and
bays through Wednesday.

--Damaging Winds May Lead to Power Outages
Widespread wind gusts in excess of 55 MPH are likely across the
central Appalachians, much of the Eastern Seaboard, and New
England today into tonight. Prepare for power outages. Severe
thunderstorms are also possible across the Southeast today.

***Key Messages for Jan 8-11 Northwest Winter Storm***
--Blizzard for the Cascades & parts of the Rockies
A series of two strong cold fronts will cross the Pacific
Northwest through Wednesday morning, bringing several feet of
heavy snow, and strong winds gusting to 60 mph, to the Cascades
and portions of the Northern Rockies. Snow levels will fall to
between 1000-2000 ft by Wednesday, leading to considerable travel
impacts for many mountain passes.

--Heavy Snow elsewhere in the West
Heavy snow exceeding 12 inches is likely (>70%) for much of the
Northern Rockies of Montana, Idaho, and eastern Oregon. Heavy
snowfall will also spread into the Sierra and Wasatch by Thursday.

--Lowland Snow Accumulations
Heavy snow rates exceeding 1"/hr combined with lowering snow
levels may allow for light to moderate accumulations even into the
lowlands of northern OR/southern WA tonight into Wednesday.
Significant travel disruptions are possible.

--Storm will move through Central U.S.
This storm system will dig across the Western U.S. through
Thursday before restrengthening over the central U.S. Friday.
Those in the Plains and Midwest should prepare for another
powerful winter storm and stay tuned for updates.
$$
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