SPC AC 251257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0757 AM CDT Sat May 25 2024

   Valid 251300Z - 261200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   KANSAS...OKLAHOMA AND SOUTHWESTERN MISSOURI...

   ...SUMMARY...
   An outbreak of severe thunderstorms is possible from late afternoon
   into tonight over parts of the central and southern Plains.  A few
   long-lived supercells capable of intense tornadoes may occur.  Giant
   hail and destructive wind gusts are also expected.

   ...Synopsis...
   In mid/upper levels, a synoptic-scale trough over the western CONUS
   -- with several associated shortwaves and vorticity lobes -- will
   shift eastward to the Great Plains States by the end of the period. 
   A positively tilted, basal shortwave trough -- evident in moisture-
   channel imagery over parts of the lower Colorado River Valley and
   southern CA -- will eject east-northeastward slightly faster than
   the progress of the synoptic trough.  This shortwave trough should
   reach southwestern KS, the TX Panhandle, and western/southern NM by
   00Z, then extend near an MKC-CSM-HOB line by 12Z tomorrow. 
   Meanwhile, farther poleward, a shortwave trough now apparent over
   the northwestern NV/eastern OR area should amplify and pivot across
   the central Rockies, reaching eastern CO by 12Z.  

   A northern-stream shortwave trough now over parts of Lake Superior,
   Upper MI and WI should move east-northeastward to parts of Lake
   Huron and southern ON by 00Z, then break through a synoptic ridge to
   its east.  By 12Z, this feature should reach parts of the St.
   Lawrence Valley and northern New England.  Meanwhile, a broad fetch
   of difluent mid/upper-level flow will persist over the southeastern
   CONUS, with several embedded, convectively generated/augmented
   vorticity lobes. 

   The 11Z surface analysis showed a cold front from lower MI
   southwestward across southern IL and southeastern MO, becoming
   quasistationary over western AR, southeastern OK, and north-central/
   west-central TX.  Amid intense low-level mass response to the
   approaching mid/upper trough, the boundary is forecast to move
   northward across northwest TX and OK today as a warm front, becoming
   more diffuse with time into evening as the airmass now to its south
   approaches (and eventually moves into some of) southern KS.  A
   dryline will develop and sharpen with time across the area from
   west-central TX northward to western KS, as moist advection
   continues to its east.  An elongated area of low pressure --
   attached to a lee trough over eastern CO -- is expected to
   consolidate today, with the resultant low reaching the MCK vicinity
   by 00Z.  The low should move/redevelop southward into western KS
   overnight then shift eastward to near SLN by 12Z.  To its southwest,
   frontogenesis will occur overnight, leading to a 12Z cold-frontal
   position from the low across northwestern OK and parts of the TX
   Panhandle. 

   ...Central/southern Plains...
   Isolated to scattered thunderstorms should develop by midafternoon
   near the low and dryline, from parts of western NE to KS and perhaps
   over northwest TX and western OK.  The airmass ahead of this
   activity will destabilize, moisten and get more strongly sheared
   with time, supporting all severe hazards to significant levels
   (tornadoes, hail, gusts).  The activity in NE/KS will occur in less
   moisture than farther south, with deeper, strongly mixed subcloud
   layers promoting the possibility of earlier cold-pool development
   after initial evolution to supercell structures.  Relatively dense
   concentrations of large to very large hail, then severe wind, are
   possible -- along with potential for a band of thunderstorms to
   aggregate and sweep eastward toward the lower Missouri Valley
   tonight.  

   The low-level wind profile for much of the afternoon will support
   both right- and left-moving supercells (each capable of very
   large/damaging hail).  The tornado threat generally should increase
   with time from midafternoon into early evening before peaking,
   especially from about southern KS southward.  As the LLJ strengthens
   from late afternoon into evening, greatly enlarging hodographs, and
   the richest moisture spreads northward through southern/central OK,
   the parameter space will become much more suitable for long-lived,
   significant tornadoes.  Meanwhile the threat for large to very large
   hail and severe downdrafts will continue with any relatively
   discrete cells.  In parts of north TX to central OK, forecast
   soundings from multiple different models reasonably superimpose
   MLCAPE of 4000-5000 J/kg, upper 60s to mid 70s F dewpoints, 50-70 kt
   effective-shear magnitudes, and effective SRH as high as 400-700
   J/kg by early evening.  That is an uncommon (even for this time of
   year) environment supporting potentially dangerous supercells, and
   may spread north of I-40 as far north as southern KS.  

   However, some important caveats remain that preclude any greater
   unconditional severe probabilities for now.  The strongest
   deep-layer forcing across the central Plains will remaining
   displaced north of the remnant/diffuse frontal zone, and thus north
   of the richer moisture over TX and into southern OK, for much of the
   day and evening.  The development of only isolated/few storms in TX
   may greatly increase the threat in OK.  By contrast, dense coverage
   of afternoon convection over west-central/northwest TX may affect
   potential farther north -- whether directly through left-split
   interactions or indirectly via outflow-related theta-e deficits
   advected northward.  Also, a plume of high clouds is forecast to
   develop and stream east-northeastward for hundreds of miles off the
   mountains of south-central/southeastern NM as the shortwave trough
   approaches, potentially limiting diurnal heating/destabilization
   between about I-40 and southern KS.  North and south of that,
   heating will be stronger, for longer.  Still, any supercell(s) that
   can move through the high-SRH, large-buoyancy environment of north
   TX to southern KS from late afternoon onward will pose a threat for
   substantial tornado production. 

   ...Lower Great Lakes/upper Ohio Valley...
   Widely scattered to scattered thunderstorms are expected to develop
   near the cold front, and along a nearly parallel, prefrontal
   trough/convergence zone this afternoon, moving eastward to
   southeastward across the outlook area and persisting into early
   evening.  

   Aside from lift along those boundaries, activity will be supported
   by diurnally weakened MLCINH, with surface dewpoints in the upper
   50s to mid 60s F contributing to around 1500 J/kg MLCAPE.  Weak
   low-level flow and nearly unidirectional low/middle-level wind
   profiles will limit shear, except through the entire cloud layer. 
   Although the shortwave trough will affect very little of this area
   directly, its flattening of the ridge and related enhancement to
   mid/upper-level gradient winds will enable strong flow in higher
   portions of the buoyant layer to aid somewhat in multicell
   organization.  Damaging gusts and isolated hail will be possible
   from the best-organized convection.

   ...Southeastern CONUS...
   Multiple mesoscale boundaries -- both from ongoing convection over
   the region and from later-developing outflow and differential-
   heating zones -- will support isolated potential for severe gusts
   and/or hail today across a broad swath of the Southeast, from the
   Mid-South to GA and parts of the Carolinas.  Separately, a merger of
   sea breezes over southeastern FL should prompt scattered to widely
   scattered thunderstorms this afternoon.  

   Both areas will be characterized by rich moisture but generally weak
   flow in low/middle levels, though locally backed winds on the east
   and north sides of boundaries may augment vertical shear enough to
   support occasional supercell structures.  MLCAPE should range from
   around 3000 J/kg across the Mid-South to 1500-2000 J/kg over
   outlooked parts of AL/GA/FL/Carolinas.  Damaging to isolated severe
   gusts, as well as isolated large hail, will be possible, mainly from
   afternoon into early evening.  Any aggregated thunderstorm clusters
   in the Carolinas to AL corridor may move southeastward with
   damaging-gust potential continuing into the evening.

   ..Edwards/Goss.. 05/25/2024

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