SPC Dec 9, 2023 1300 UTC Day 1 Convective Outlook

[NorthGeorgiaWX]

SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0644 AM CST Sat Dec 09 2023

Valid 091300Z - 101200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
THE MID-SOUTH AND DELTA REGIONS TO THE TENNESSEE VALLEY...

...SUMMARY...
A few tornadoes, damaging to severe thunderstorm gusts, and isolated
severe hail are possible today and tonight from portions of the Ohio
Valley southward to the central Gulf Coast.

...Synopsis...
In mid/upper levels, a positively tilted synoptic trough will
amplify and proceed eastward across the central CONUS, predominantly
in response to two accompanying shortwave troughs:
1.  A northern-stream perturbation -- initially manifest as a closed
cyclone over MN with trough southwestward over NE.  This feature
will devolve to an open wave across WI tonight, before reaching
Lower MI and Lake Huron around 12Z tomorrow.
2.  An elongated vorticity banner and speed max over the Four
Corners States, which should reorient into a positively tilted
trough by 00Z from southeastern KS across southwestern OK to
southeastern NM.  In doing so, it may absorb the southern portion of
the northern-stream trough's vorticity field.  By 12Z tomorrow, the
reconfigured perturbation should extend from southern IL across
central AR to south-central TX.

The 11Z surface analysis showed a weakening, occluded low near HIB,
with occluded front southeastward to a triple-point low near MKE.  A
cold front was drawn from there southwestward over the Ozarks to
north-central TX and the Trans-Pecos region of west TX. By 00Z, the
triple-point low will deepen into the dominant surface cyclone and
eject northeastward to that portion of ON northeast of Lake
Superior.  The cold front should extend across central portions of
OH/KY, southwestward over northern parts of MS/LA, to the middle TX
Coast and deep south TX.  The front should reach northern NY,
central PA, WV, the TRI area, northern GA, southern AL, and the
west-central Gulf by 12Z. 

...Gulf Coast to Ohio Valley...
Bands of scattered to locally numerous thunderstorms are expected to
develop along and ahead of the cold front, perhaps as early as
midday in northwestern areas of the outlook, but mainly from this
afternoon through overnight hours.  As activity sweeps eastward to
northeastward over the region, damaging to severe gusts, sporadic
hail near severe limits, and a few tornadoes are all possible.  

The tornado and wind threats should be relatively maximized from the
Mid-South to Tennessee Valley region, given that area's probable
overlap between the most favorable parameter space and thunderstorm
probability.  However, concerns over storm morphology --
particularly potential for messy/embedded convective modes and short
duration of favorably mature supercell/mesocyclone production --
remain large.  This notion is supported by the presence of short,
low- to medium-magnitude UH tracks in the preponderance of CAM
guidance.  This does not preclude the possibility of a significant
or longer-lived tornado threat locally, but makes it too conditional
and uncertainly focused for a specific, greater-potential outlook
area at this point.  Deep shear and hodograph size will strengthen
northward, with around 200-300 J/kg effective SRH being fairly
common over the warm sector prior to arrival of the main frontal/
prefrontal band.  With CINH being weak, additional convection may
develop atop weak convergence zones in the nearby warm sector as
well, especially over parts of northern AL and mid TN from late
afternoon into evening, before either merging with or taking over as
the main convective band.

Forecast soundings show modest low/middle-level lapse rates beneath
abundant cloud cover, with warm/theta-e advection contributing at
least as much to surface destabilization as diurnal/diabatic
warming.  Surface dewpoints ranging from the upper 60s F near the
coast to the 50s in the Ohio Valley (with temperatures not much
warmer) will lead to a gently northward-diminishing buoyancy
gradient.  MLCAPE should range from around 1500-2000 J/kg over
central/southern parts of the Delta region (where lift and shear
will be weaker) to 500-800 J/kg over the Ohio Valley, in a
prefrontal plume that narrows and has shorter duration with
northward/inland extent.  The eastern rim of the favorable
surface-based buoyancy will shift across AL and into parts of GA,
the FL Panhandle, eastern TN and perhaps western Carolinas
overnight, supporting an eastward extension of marginal severe
potential and more overlap with the start of the day-2 outlook area.

..Edwards/Gleason.. 12/09/2023

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