Absolute Vorticity

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.NEAR TERM /THROUGH SATURDAY/...
We continue to see an influx of low clouds N/NE into the ILN FA
this afternoon, a manifestation of the tremendous LL moisture
transport that has already begun across the region as a whole.
And as expected, despite the LL WAA, sfc temps have struggled a
bit to warm without much in the way of sunshine to help the
process as temps are ranging from the upper 50s in N KY to lower
50s in w-cntrl OH.

SW flow has become established both at the sfc and aloft as the
main trof axis digs and ejects into the central plains,
promoting strong deep-layer WAA across the entire region. Have
already seen the primary sfc low set up across the s-cntrl
plains, with the attendant surging theta-e boundary arcing to
the NE across the mid MS/OH Rvr Vlys. With the deepening of the
system as it ejects east into the mid MS Rvr Vly by this
evening, will see robust theta-e advection begin to evolve late
this evening and early overnight near/E of the MS Rvr and even
into the ILN FA. The entire setup will have the local area well
within the broadening open warm sector of the system, with
tremendous moisture and mass transport north into the region
through early tonight. Moisture depth thus far has been rather
shallow in nature, but will gradually deepen through the evening
hours. Have kept PoPs largely in the chance category until late
afternoon before the depth of moisture availability becomes such
that the isentropic lift/WAA-wing is able to generate a NW to
SE band of SHRA, which will pivot NE, mainly for areas NW of
I-71, during the late afternoon and early evening. Many spots
may remain dry until late evening.

At this juncture, we shift our focus to tonight into the day on
Saturday -- a setup that continues to be rather concerning for
the region. There are several items to break down as we discuss
the expected evolution as we continue to hone in on particular
impacts locally.

As has been mentioned for several days now, the overall track of
this system is a climatologically-favorable track that is
supportive of a severe threat across the OH Vly. A slowly-
deepening sfc low will track from N MO to Lake Michigan by tonight.
Deep lows on this track have a rather long history of being
supportive for severe weather events across the OH Vly. And
although the low will be pulling away/N through MI by the time
the FROPA actually occurs locally mid/late Saturday morning, it
will continue to deepen as it does so. Both deterministic and
ensemble solutions seem to have locked in a deeper system, with
the prospect of a sub-985mb low in MI Saturday morning seeming
very probable. This track and deepening of the sfc low as it
progresses from MO to MI puts the region squarely in a favorable
zone for robust isallobaric and LL mass field response, aiding
in enhanced convergence and large-scale ascent for tonight. And
with this track and a very favorable dynamic and kinematic
environment, traditional thermodynamic evolution, such as
instby, tends to become slightly less important as the mass
transport/response can often be sufficient to produce strongly-
forced vertical ascent amidst an increasingly-frontogenetic
response.

Looking more closely at some of the ingredients... the effective
bulk shear of 50+ kts will be /more/ than supportive of storm
organization likely initially to the W/SW of the ILN FA (more
toward SW IN, W KY) in the form of one or more clusters with
supercellular structure. Ahead and to the N/E of this activity
will be LLJ-induced warm-sector storms, which will develop
quickly and overspread the Tri-State/EC IN around/shortly after
02z. This initial activity looks to be primarily elevated in
nature, with the better LL thermodynamic environment still
hanging back to the west quite a bit. So even though we will be
dealing with some heavy rain and some thunder (and perhaps even
some small hail) between 02z-07z, we do not expect much of a
severe threat during this time frame.

By 07z, we will be likely be seeing the remnants of supercellular
structures, which had developed earlier in SW IN and W KY, finally
make their way closer to the ILN FA, riding a more favorable LL
thermodynamic environment that will be supportive of more sfc-
based activity. Expected to see one or more SW-NE additional linear
convective structures move into SE IN and N KY around/shortly
after 07z, with the /largely/ unidirectional profile and /very/
favorable vertical speed shear still in place (including a H9
LLJ of 45-50kts and H8 LLJ of 60-65+ kts). It continues to be
apparent that there will be at least /some/ cross component of
the LL bulk shear vector to the convective structure
orientation, helping maintain and enhance LL shear and near-sfc
SRH, supporting circulations within any convection that moves
through the ILN FA during the heart of the overnight. Anticipate
that the magnitude of the LL shear (45-50kts) may be sufficient
to overcome the larger angular overlap with the convection
itself, lending itself to the potential for a few tornadoes
within the region. And as for the sfc-based instby... despite
the impressive theta-e advection/LL moisture transport north
into the area ahead of the front (with sfc DPs in the upper 50s
and lower 60s and PWATS 250-300% of seasonal norms), will likely
not arrive until after 07z locally. But once this occurs, the
damaging straight-line wind and tornado threat will begin to
maximize, perhaps in a 6-hr period from 07-13z before the
activity quickly moves E toward S-cntrl OH and NE KY after
daybreak. Synoptic-based and deterministic CAM to depict a
narrow ribbon of several hundred J/kg SBCAPE nosing north into
IN and parts of N KY and W OH after 07z. This narrow ribbon of
slightly more favorable thermodynamics will overlap with an
impressive kinematic environment, suggesting the SLGHT/ENH risks
placed across the local area are /very/ reasonable and well-
warranted. The UH tracks in the various deterministic and
ensemble CAM guidance suggest we will be contending with several
storm clusters with rotating updrafts beyond 07z, especially
across the Tri-State/northern KY/SE IN and EC IN.

One trend that has been particularly concerning in today`s hi-
res guidance has been an increasingly-stronger signal for a
heavy rain and flooding threat. With a very moist and strongly-
forced environment, any training convective activity may yield
localized flooding, despite the progressive nature of the system
as a whole. With decent overlap between the steering-layer flow
and the orientation of the individual convective elements
themselves, some training of activity is likely, even with
individual elements moving quickly within the overall evolution.
Have seen a signal toward this training convection/heavy
rain/flood threat become the strongest across the Tri-State area
and especially northern KY, especially with the elevated
convection priming the soils in the 02z-07z time period before
the more significant storm activity arrives a bit later in the
overnight. While confidence is not quite there yet to issue a
Flood Watch, this trend will be monitored closely and we have
added mention in the HWO/official wx briefing for now. An uptick
in messaging may be required later this evening should trends
become more concerning.

The other item, and it is not an insignificant one, will
be the robust synoptic-based winds, especially during the day
on Saturday. With the trend of a stronger system, a Wind
Advisory has been issued for locations near/N of I-70, with
additional expansions possible as we get closer to the event.
In the WAA advection tonight ahead of the FROPA, could very
well see sustained winds of 20-25kts, especially near/N of I-70,
which in-and-of itself would be close to advisory-level
criteria. However, behind the front during the day/afternoon on
Saturday, within the increasingly- well mixed BL environment,
we will see gusts 40+ kts at times for locations further north
where the advisory will be in place. Will see gusts 30-35kts
further south (between I-70 and OH Rvr), with gusts to 30kts
expected south of OH Rvr.

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Severe Weather Statement
National Weather Service Memphis TN
656 PM CST Fri Dec 10 2021

ARC031-055-093-111-110130-
/O.CON.KMEG.TO.W.0060.000000T0000Z-211211T0130Z/
Greene AR-Craighead AR-Poinsett AR-Mississippi AR-
656 PM CST Fri Dec 10 2021

...A TORNADO WARNING REMAINS IN EFFECT UNTIL 730 PM CST FOR SOUTH
CENTRAL GREENE...CRAIGHEAD...NORTH CENTRAL POINSETT AND WESTERN
MISSISSIPPI COUNTIES...

At 655 PM CST, a confirmed large and extremely dangerous tornado was
located near Greenfield, or near Jonesboro, moving northeast at 60
mph.

This is a PARTICULARLY DANGEROUS SITUATION. TAKE COVER NOW!

HAZARD...Damaging tornado.

SOURCE...Weather spotters confirmed tornado.

IMPACT...You are in a life-threatening situation. Flying debris may
         be deadly to those caught without shelter. Mobile homes
         will be destroyed. Considerable damage to homes,
         businesses, and vehicles is likely and complete destruction
         is possible.

The tornado will be near...
  Trumann and Bay around 705 PM CST.
  Lake City and Brookland around 710 PM CST.
  Monette and Caraway around 720 PM CST.

Other locations impacted by this tornadic thunderstorm include
Hergett, Apt, Vail, Valley View, Macey, Schug, Gum Point, Farrville,
Lester and Mandalay.