SPC AC 081255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0755 AM CDT Wed May 08 2024

   Valid 081300Z - 091200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVE PORTIONS OF
   SOUTHERN/SOUTHEASTERN MISSOURI...EXTREME NORTHEASTERN AR...EXTREME
   SOUTHERN ILLINOIS...WESTERN/SOUTH-CENTRAL KENTUCKY...AND
   NORTHWESTERN TO MIDDLE TENNESSEE...

   ...SUMMARY...
   Severe thunderstorms appear likely from parts of the mid
   Mississippi, Ohio, and Tennessee Valleys into the southern Plains. 
   All severe hazards, including tornadoes, very large to giant hail,
   and potentially significant damaging winds are possible.  Some
   tornadoes may be strong.

   ...Synopsis...
   In mid/upper levels, a temporarily blocky pattern has evolved around
   a cutoff cyclone over the northern Great Plains.  The associated
   500-mb low is expected to drift erratically southeastward and fill
   slowly through the remainder of the period, while a series of
   shortwaves amplify associated synoptic troughing extending
   southwestward across the Intermountain West and Great Basin. 
   Meanwhile, a northern-stream perturbation over the Upper Great
   Lakes, that peeled eastward off the cyclone's influence, has a
   compact cyclone aloft centered near ANJ.  This feature should
   devolve into an open shortwave trough and accelerate eastward as it
   enters the northern stream today, reaching eastern NY and western
   New England around 00Z.

   In the southern stream, broadly cyclonic flow will prevail over the
   western CONUS.  A weak perturbation -- apparent in moisture-channel
   imagery over the central Gulf of California -- should move
   east-northeastward through the period, reaching northern Coahuila
   and adjoining southwest TX around 00Z, then parts of LA/northern MS
   by the end of the period.  Downstream perturbations over the
   Southeast and central/southern Appalachians should be predominantly
   convectively derived.

   At 11Z, a surface lows were evident over southwestern ND and between
   OFK-YKN, along an occluded front.  An occluded/cold front arched
   from the NE low across northeastern KS to another weak low near CNU,
   then southwestward over southwestern OK and the TX South Plains. 
   Today, the cold front will overtake the northern part of a dryline
   now over northwest TX to the Big Bend region.  By 00Z, the front and
   dryline together will act as a western limit of substantial
   convective/severe potential, from eastern OK to
   north-central/central TX.  A Gulf warm front over northeastern OK
   and central/northeastern AR was moving northward and should cross
   the Ozarks this morning, moving into southern MO.  A progressive
   outflow boundary was apparent from northern VA across southern WV,
   southeastern KY, to northern parts of middle/western TN and
   southeastern MO.  The western part of this boundary is stalling, and
   soon should retreat northward through the rest of the morning across
   parts of western KY and southeastern MO, essentially linking with
   the warm front to form a baroclinic zone focusing the greatest of
   today's severe risks.

   ...Ozarks to Mid Mississippi, lower Ohio and Tennessee Valleys...
   A complicated, potentially dangerous severe-weather event is
   possible today into this evening.  Multiple rounds of strong-severe
   thunderstorms are expected today across this region, additively
   contributing enough severe hazard for the "moderate" (level 4 of 5)
   risk area.  A swath of damaging to severe gusts and at least a few
   embedded tornadoes are likely to accompany a complex of
   thunderstorms from southern MO across parts of the multi-state area
   near the Ohio/Mississippi River junction, to at least northern parts
   of the Tennessee Valley region.  The main MCS should move
   east-southeastward to southeastward along and near the instability
   gradient associated with the aforementioned outflow boundary. 
   Before and during that process, several supercells are expected,
   offering tornadoes (a few potentially significant/EF2+), large to
   very large hail, and localized severe gusts.

   An ongoing area of strong-severe thunderstorms across portions of MO
   should pose mainly a severe-hail threat for another couple hours,
   though the gust potential will increase as it grows upscale.  See
   severe thunderstorm watch 202 and related mesoscale discussions for
   the near-term threat.  As the morning progresses, the wind threat
   will ramp up further, with some tornado potential starting as well,
   as this activity and/or additional convection developing later over
   western MO interacts with 1) a northward-moving plume of
   surface-based inflow air and 2) rich low-level moisture now found
   south of the outflow boundary.  Meanwhile, diurnal destabilization
   of an already richly moist airmass south of the boundary will grow
   MLCAPE commonly into the 3000-4000 J/kg range, beneath favorable 
   (50-60 kt) deep shear.  Low-level shear/vorticity, SRH and hodograph
   size will peak near the boundary, supporting a corridor of
   relatively maximized tornado potential with any sustained/discrete
   or semi-discrete supercells in the area, as well as a focus for MCS
   forward propagation from the northwest.  MCS activity should merge
   upscale further this evening and tonight, maintaining severe-wind
   potential into parts of the Tennessee Valley region before moving
   into gradually less instability late tonight.

   ...Eastern OK to AR, Mid-South and central TX...
   Thunderstorms should develop by late afternoon near the front and
   dryline, as a combination of lift along those boundaries and strong
   surface heating combine with very rich low-level moisture to erode
   the EML cap sampled well by the 12Z FWD RAOB.  Large to giant hail,
   occasional severe gusts, and a few tornadoes are possible as
   activity shifts eastward to northeastward across central/east TX,
   the Arklatex, and AR through the evening.  The northern part of this
   convection may merge with or be overtaken by MCS activity moving out
   of MO and into parts of the Mid-South.

   The afternoon/preconvective environment should be characterized by
   steep low/middle level lapse rates on either side of the weakening
   cap, along with surface dewpoints commonly in the 70s F.  This will
   contribute to strong buoyancy with the peak/preconvective MLCAPE
   reaching 4500-5500 J/kg, and SBCAPE values topping 6000 J/kg. 
   Although near-surface flow generally will be 10 kt or less, limiting
   lowest-km hodographs/shear, effective-shear magnitudes in the
   40-55-kt range indicate supercells will be possible.  These will be
   capable of very large/destructive hail exceeding 3 inches in
   diameter.  Clusters or upscale mergers of convection also may offer
   deep, precip-loaded, hail-cooled downdrafts with locally severe
   gusts.  Cell mergers and interactions with boundaries will factor
   into tornado potential on the storm scale, since the environmental
   low-level shear appears on the margins.

   ...Southern Appalachians/Cumberland Plateau into Carolinas...
   Thunderstorms are ongoing along and behind the outflow boundary near
   the TN/KY border area, and may pose a severe threat through the
   remainder of the morning while moving generally southeastward.  See
   SPC mesoscale discussion 693 for near-term details.  Further
   intensification and/or additional development is possible into the
   afternoon over and east of the mountains, moving into an environment
   of favorable moisture and diurnal destabilization.  MLCAPE in the
   1500-2500 J/kg range appears attainable, amidst roughly 35-45-kt
   effective-shear magnitudes.  A mix of supercell and multicell modes
   is expected, with small bows possible, supporting localized threats
   for damaging wind, large hail and at least marginal tornado
   potential.

   ...Northeastern CONUS...
   Widely scattered to scattered thunderstorms may develop as soon as
   mid/late morning over parts of eastern NY and New England.  Activity
   should form atop a diurnally destabilizing and weakly capped
   boundary layer, along/ahead of a prefrontal surface trough and
   behind a plume of non-severe thunderstorms and precip now crossing
   parts of the region.  Thunderstorms should move generally eastward,
   offering damaging to isolated severe gusts and isolated large hail. 
   A combination of low-level warm advection, spotty/irregular diurnal
   surface heating, and cooling aloft related to the approaching Great
   Lakes shortwave trough, will steepen low/middle-level lapse rates
   substantially.  With favorable moisture (surface dewpoints generally
   50s to low 60s F), a plume of 500-1000 J/kg MLCAPE should develop,
   amidst effective-shear magnitudes that reach 40-50 kt, and despite
   strongly veered/westerly surface wind components.  A mix of
   multicell and modest supercell characteristics should be observed
   before activity moves over more-stable air and/or offshore, and
   weakens by early evening.

   ...Eastern SD to eastern IA and vicinity...
   Widely scattered to scattered thunderstorms are possible from early
   afternoon in western parts to evening closer to the Mississippi
   River, with isolated damaging gusts, large hail and possibly a
   tornado or two.  Activity should form in an area of deep-layer lift
   (including low-level convergence/mass response) related to a
   vorticity lobe in the southeastern part of the mid/upper-level
   cyclone.  Associated cooling aloft will combine with residual
   low-level moisture (dewpoints mid 40s to mid 50s F) to yield pockets
   of around 500-1000 J/kg MLCAPE.  Backed low-level winds will aid
   convergence and storm-relative low-level flow, and may contribute to
   locally enlarged hodographs.

   ..Edwards.. 05/08/2024

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