SPC AC 091323

   Day 1 Convective Outlook CORR 1
   NWS Storm Prediction Center Norman OK
   0823 AM CDT Thu May 09 2024

   Valid 091300Z - 101200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM CENTRAL
   TEXAS TO PARTS OF THE GEORGIA/SOUTH CAROLINA COAST...

   CORRECTED FOR INCORRECT SLGT LABEL LOCATION

   ...SUMMARY...
   Very large hail with multiple supercells appears probable across
   parts of north and central Texas into the ArkLaTex vicinity this
   afternoon and evening -- including parts of the Dallas/Ft. Worth
   Metroplex.  A broader corridor of severe hail and damaging-wind
   potential will extend from east Texas into the lower Mississippi
   Valley and Southeast.

   ...Synopsis...
   In mid/upper levels, an extraordinarily long, more than continent-
   spanning, positively tilted trough was apparent in moisture-channel
   imagery.  The trough extended from an Atlantic cyclone south-
   southeast of Greenland, across Newfoundland/southern Labrador and
   southern parts of QC/ON, the Upper Great Lakes, upper Mississippi
   Valley, NE, the central Rockies, southern Great Basin, central CA,
   and west-southwestward over the Pacific.  On the western segment of
   the trough, a remnant cyclone near FSD/YKN will dissipate shortly,
   while a closed vortex develops over UT and slowly retrogrades
   through the period.  Meanwhile, weak shortwave troughs now over
   parts of MO/IA/IL will move eastward across the Ohio Valley region,
   in advance of a larger, amplifying shortwave trough now over Lake
   Superior and northeastern MN.  A weak, but still potentially
   influential southern stream shortwave will move east-northeastward
   from southwest to central TX today.  Picking up some convective
   vorticity and related amplification, this perturbation should
   accelerate eastward and reach parts of AL by 12Z tomorrow.

   At the surface, 11Z analysis showed a diffuse frontal-wave low over
   central IL, with quasistationary to locally warm front eastward
   across central/southern OH to northern WV.  A cold front extended
   from the low across southern IL to eastern AR, becoming wavy/
   quasistationary through a weak low south of ABI, then cold again
   into the Big Bend region.  This front, overall, will sag southward
   across TX through the period, but hang up for much of the day ahead
   of the low and around the DFW/SEP region, and across the Edwards
   Plateau.  The Ohio Valley low is expected to move eastward to near
   the southwestern corner of PA by 00Z, then move/redevelop eastward
   across VA to near the Delmarva Atlantic Coast by the end of the
   period.

   ...North, central and east TX...
   At least a few supercells are expected to erupt in a high-
   instability environment this afternoon, along and southeast of the
   front.  This activity will be capable of large hail -- some of it
   potentially 3+ inches in diameter -- as well as locally severe gusts
   and perhaps a tornado or two.  The threat area includes the DFW
   Metroplex -- unfortunately, no stranger to swaths of destructive
   hail -- and areas southward/eastward into the Hill Country, I-35/45
   corridors and Piney Woods.

   The airmass across the region begins the day quite moist, with
   surface dewpoints commonly in the low/mid 70s just southeast of the
   front, and mid/upper 70s over the Gulf Coastal Plain.  Steep
   midlevel lapse rates, related to an EML, were sampled by 12Z RAOBs
   at MAF and FWD.  This airmass aloft will remain over the region,
   with some subtle/reinforcing large-scale ascent possible ahead of
   the southern-stream perturbation.  Meanwhile, the boundary layer
   should destabilize amidst strong diurnal heating, causing the cap to
   weaken considerably through early/mid afternoon.  Development is
   possible as soon as late morning to midday near the front, over the
   Edwards Plateau, and midday to mid afternoon farther north near the
   low.  By then, 3500-4500 J/kg MLCAPE should be common, locally near
   5000 J/kg.  Low-level flow should remain weak, but with enough
   easterly component southeast of the front to foster strong veering
   with height and long (sometimes nearly straight) hodographs.  This
   will support splitting storms, with both left- and right-moving
   supercells offering the threat for significant (2+ inch) hail. 
   Sustained storms could produce 3-4-inch hailstones, based on
   historic analogs and a 2D hail model applied to forecast soundings.

   With a deep troposphere and abundant inflow-layer moisture, deeply
   precip-loaded downdrafts are possible, with some midlevel momentum
   augmentation and related severe-gust potential.  The wind threat
   could evolve upscale and start forward-propagating eastward into LA
   wherever early cells can aggregate into clusters.  The supercell
   tornado threat is conditional, mainly dependent on local storm-scale
   processes such as mergers and boundary interactions.  QLCS tornadoes
   are also possible with any MCS.

   ...Lower Mississippi Valley to southern Atlantic Coast...
   Ongoing, organized areas of thunderstorms over the Southeast,
   interacting with and focused astride baroclinic/instability
   gradients from outflow boundaries, will continue to offer damaging
   wind, a few tornadoes, and isolated severe hail through at least
   midday as they move across parts of northern MS, AL, territory
   eastward/southeastward toward the GA/SC coast, and perhaps northern
   FL.  For near-term concerns, refer to tornado watches 215-216 and
   related mesoscale discussions.

   The related outflow/baroclinic gradient should serve as a focus for
   forward propagation, and perhaps initiation, of thunderstorms this
   afternoon into tonight.  Surface dewpoints commonly in the 70s F and
   strong diurnal heating south and west of the outflow boundaries will
   contribute to peak afternoon MLCAPE of 3500-4500 J/kg, only slowly
   diminishing through the evening and overnight hours.  Given the very
   richly moist and favorable unstable environment in place, beneath
   parallel-oriented midlevel flow, one or two organized MCSs may
   result.  A conditional derecho potential exists, depending on timing
   and location of upscale organization within this corridor.  If a
   complex moves out of east TX, or develops far enough west in MS/LA
   and is sustained all the way across the remainder of the Gulf Coast
   States tonight, either readily could qualify.

   One factor that may limit overall wind potential is the gradual
   nocturnal stabilizing of the boundary layer.  However, sufficient
   cold-pool organization, rear-inflow-jet development and forced
   ascent of the still-favorably moist air may produce enough of a
   vertical circulation throughout an MCS to overcome the nocturnal
   influence.  At this time, too many mesoscale uncertainties remain to
   insert an unconditional 45%/MDT wind corridor, but that may need to
   be done in a succeeding outlook if convective guidance comes into
   better agreement and/or mesoscale trends warrant.  A few tornadoes
   also may occur, especially near the outflow boundaries where
   low-level vorticity/shear will be maximized.

   ...Mid/upper Ohio Valley to Tidewater...
   Scattered to widely scattered thunderstorms are possible today
   across parts of the Ohio Valley region, with damaging to isolated
   severe gusts and marginally severe hail possible.  Activity should
   occur in an environment characterized by steepening midlevel lapse
   rates ahead of the ejecting MO/IA/IL perturbations, modest
   convergence near and ahead of the surface low, adequate near-surface
   moisture with dewpoints in the 50s to mid 60s, and modest diurnal
   heating.  MLCAPE up to the 1000-1500 J/kg range is possible.  Though
   upper flow will be strong, nearly unidirectional profiles and lack
   of greater midlevel winds should keep effective-shear magnitudes in
   the 30-40 kt range over much of the area.

   Farther east/southeast, cool/stable air north of a damming/preceding
   frontal zone across VA and the Delmarva should limit the northern
   end of severe potential.  Destabilization and available moisture
   will be substantially limited today in the nominal warm sector,
   between the damming front and a great deal of MCS/outflow activity
   to the south over the Carolinas/GA.  Therefore, while thunderstorms
   are possible this afternoon near the max temperature hours, and deep
   shear may be adequate for a few organized cells, severe potential
   appears isolated and marginal at best.  As such, severe
   probabilities over much of the region have been reduced.

   ..Edwards.. 05/09/2024

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