SPC AC 261256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0756 AM CDT Sun May 26 2024

   Valid 261300Z - 271200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE
   EASTERN OZARKS TO PARTS OF THE OHIO VALLEY REGION...

   ...SUMMARY...
   Multiple rounds of severe storms are possible from the Ozarks and
   nearby Mississippi Valley into the Ohio Valley States today, with
   threats for tornadoes, severe gusts and occasional large hail.

   ...Synopsis...
   A progressive mid/upper-level pattern is apparent across the CONUS,
   perturbed mainly by a synoptic-scale trough initially over the
   central High Plains, and related cyclonic flow and embedded
   vorticity maxima/shortwaves over the Central States.  The trough
   should shift eastward to WI, eastern IA, northern MO and central OK
   by 00Z, then over the Upper Great Lakes, Lower MI, and the lower
   Ohio Valley by 12Z tomorrow.

   At the surface, 11Z analysis showed a primary synoptic low over
   southwestern IA, with warm front across western IL behind convective
   outflow, and ahead of the outflow from central IL across southern IN
   to northeastern KY.  The warm front should move slowly northward
   across the Ohio Valley through the afternoon.  The cold front was
   drawn from the IA low southwestward to another low between ICT-PNC,
   then across western OK and northwest TX.  This front will overtake a
   dryline today over northwest/west-central TX.  By 00Z, the cold
   front should extend from a triple-point low near MSN across central
   IL, southern MO, southeastern OK, and north-central/central TX.  By
   12Z, the cold front should reach eastern IN, central KY, and western
   TN, becoming quasistationary southwestward over the Arklatex to
   central TX.  The cold front will be preceded by an MCS outflow
   boundary over parts of the Ozarks and Ohio Valley region today.

   ...Ozarks/Arklatex to Ohio/Tennessee Valleys and south-central
   Appalachians...

   A complex, multi-episode severe scenario is apparent, still bearing
   some important mesoscale uncertainties.  However, probabilities
   supporting an "enhanced" category appear solidly warranted over most
   of the original area that has not been too convectively processed,
   and mesoscale foci may yield denser swaths of severe (wind in
   particular) within that remaining broad corridor.

   An ongoing severe MCS, with embedded supercells and bowing segments
   and a history of several overnight tornadoes in the Ozarks, will
   continue to pose a threat for all severe hazards (including
   tornadoes) for at least the next several hours past midday.  This
   threat should persist as activity exits the Ozarks, crosses the
   adjoining Mississippi Valley, and moves into parts of the lower Ohio
   Valley vicinity.  Some northward expansion toward/past the warm
   front is possible over the Ohio Valley.  This activity should only
   gradually outpace the most favorable buoyancy as it approaches the
   Appalachians, and may still have a severe threat across the higher
   terrain.  Confidence in its persistence is heightened by the
   presence of at least one MCV over the eastern Ozarks, which should
   enhance ascent and lower/middle-tropospheric vertical wind
   profiles/shear on the mesoscale.  Organization of these
   thunderstorms should continue to take advantage of forced lift with
   cold pools, as well as lift from internal rotational dynamics of
   supercells and bookend vortices, to overcome modest MLCINH this
   morning through midday.  Inflow air will contain rich low-level
   moisture (surface dewpoints commonly in the upper 60s to low 70s F,
   locally higher), and will be diurnally destabilizing from heating
   and warm advection near and south of the warm front.  For near-term
   guidance, see SPC Tornado Watch 314 and related mesoscale
   discussions.

   Details of subsequent round(s) of convection still remain to be
   resolved, based substantially on the character and extent of
   poleward return of the outflow boundary from the leading activity. 
   As such, further modifications (some substantial) may be needed to
   especially western parts of the outlook areas today.  One zone of
   relatively maximized deep ascent appears to be associated with
   passage of the convectively reinforced prefrontal boundary over
   parts of MO and southeastern KS, initially penetrating outflow air
   from the leading activity.  This regime should continue to benefit
   from large-scale DCVA/ascent aloft, related to the shortwave trough
   and closely associated convectively induced/enhanced vorticity
   fields.  Activity interacting with the outflow boundary's theta-e
   gradient, backed near-surface winds, and maximized mesoscale
   vorticity in low levels should pose the greatest potential for
   tornadoes.  However the afternoon/evening tornado risk is not yet
   focused tightly, due to uncertainties regarding the evolution of a
   boundary still being produced at this time.

   The general large-scale ascent, along with direct frontal lift and
   heating of a very moist airmass in the warm sector, also should
   support at least isolated to widely scattered, near-frontal
   convective development farther south across parts of AR and eastern
   OK, southwest of the outflow boundary.  This activity, including
   supercells, should occur in an environment of stronger EML-related
   MLCINH for much of the day, but also, steep midlevel lapse rates,
   optimal moisture (surface dewpoints commonly low-mid 70s F), and
   weakening capping with time this afternoon.  Despite some veering of
   near-surface winds, which may limit convergence, any activity that
   develops still will occur in an environment of 50-65-kt effective-
   shear magnitudes, and peak local MLCAPE commonly in the 3500-4500
   J/kg range.  Forecast hodographs and 2D hail models suggest very
   large hail of 3+ inches in diameter may occur with any sustained,
   relatively discrete storm this afternoon into early evening. 
   Additional development is possible farther east in the warm sector,
   near and east of the Mississippi River and south of the MCS outflow,
   off the eastern rim of the EML where MLCINH is weaker but moisture
   and heating are strong.

   ..Edwards/Goss.. 05/26/2024

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