SPC AC 231250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0750 AM CDT Fri Apr 23 2021

   Valid 231300Z - 241200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE SOUTHERN PLAINS TO LOWER MISSISSIPPI VALLEY...

   ...SUMMARY...
   Severe wind, large to very large and destructive hail, and a few
   tornadoes are possible today and tonight across parts of the
   southern Plains to lower Mississippi Valley.

   ...Synopsis...
   An active, progressive mid/upper-level flow belt will exist across
   most of the CONUS through the period, south of a substantial cyclone
   covering most of mainland Nunavut.  The key feature for this
   forecast is a pronounced trough -- evident in moisture-channel
   imagery initially from the Four Corners area south-southwestward
   across AZ, Sonora, and central/southern Baja.  This trough should
   become somewhat less positively tilted today, as it progresses to
   western KS, the TX Panhandle/South Plains, and the Big Bend area by
   00Z.  The trough should reach western MO, eastern OK and the middle
   TX Coast by the end of the period. 

   At the surface, 11Z analysis showed an elongated area of low
   pressure in a frontal zone across parts of eastern CO, western KS,
   the OK and northern TX Panhandles, and northeastern NM.  The low
   should consolidate and deepen through today over the TX Panhandle
   and northeast TX, then move roughly eastward near the Red River
   toward TX overnight.  The trailing cold front will move
   southeastward across northwest and west-central TX, while the
   frontal segment northeast of the low also shifts southeastward over
   OK.  Meanwhile, a marine/warm front initially drawn over south TX
   and just offshore LA will shift northward over central/north TX
   today, into southern and perhaps central OK this evening, as well as
   parts of east TX and LA, modulated on the mesoscale by extensive
   convection forecast to develop along and north of the boundary.  A
   dryline will become more sharply defined across the Permian Basin,
   South Plains and Panhandle today, mixing eastward over the Edwards
   Plateau, Concho Valley and Big Country regions today, while hanging
   up some nearer the surface low over northwest TX.  

   ...Southern Plains to portions of MS...
   Multiple episodes of strong-severe thunderstorms are possible
   through the period, with some spatial overlap between each of them
   accounting for the aggregate shape of the outlook area.  Two regimes
   will account for the bulk of severe potential with these episodes:

   1.  Central TX to MS and central OK:  A large area of scattered to
   numerous thunderstorms should form and grow throughout the midday to
   late-afternoon period over central/east TX, with a lobe potentially
   extending across portions of north-central TX into at least central
   OK.  Thunderstorms should aggregate and shift eastward across the
   east TX Piney Woods, LA and the lower Mississippi Valley.  Expect
   mainly a damaging-wind threat, with a few tornadoes also possible
   from embedded heavy-precip supercells and QLCS mesocirculations. 
   Without the optimally steep lapse rates and buoyant profiles of
   areas farther west, and with messier storm modes, only sporadic
   large hail is likely from east TX eastward .  However, relatively
   early and more westward in the episode, severe to significant (2+
   inch diameter) hail will be possible as well from supercells.  

   This activity will develop gradually but expansively and be
   supported by: 
   a) Gently sloping, warm-frontal lift; 
   b) A broad area of low-level warm advection and moisture transport
   ahead of the mid/upper trough; 
   c) Favorable vertical shear/CAPE parameter spaces, with 500-1500
   J/kg MLCAPE spreading northeastward near the warm/marine frontal
   zone, enlarged hodographs in the frontal zone, and 40-55 kt
   effective-shear magnitudes in forecast soundings not convectively
   contaminated;
   d) Weakened MLCINH/MUCINH through warm-advection-related lift along
   and east of the eastern rim of the EML.

   This convection should shift eastward across the lower Mississippi
   Valley tonight, with considerable uncertainty remaining on the
   northern (instability) and eastern (timing) edges of the severe
   threat in particular. 

   2.  Northwest TX, southeastern Panhandle, southwest/west-central OK,
   to eastern OK/AR:  Initially more isolated to widely scattered
   thunderstorms should form late this afternoon near the cold front
   and northern part of the dryline, becoming supercellular and
   offering large to very large/damaging hail (some potentially around
   4 inches in diameter).  Convection should aggregate into a cluster
   or small MCS with the severe threat evolving toward wind as it moves
   eastward across the remainder of OK, possibly as far east as western
   AR tonight.  Sufficient agreement in available guidance exists in
   the evolution of this activity to assign a second enhanced-wind risk
   area. 

   This activity will be supported by intensifying large-scale
   lift/DCVA ahead of the mid/upper trough, and related cooling aloft
   (-16 to -20 deg C 500-mb temperatures amidst 8-8.5 deg C/km midlevel
   lapse rates).  When superimposed with increasing low-level moisture
   (dew points rising into the upper 50s to mid 60s F), this yields
   around 2000 J/kg MLCAPE in forecast soundings.  The most favorable
   buoyancy will be over a narrow (around 75-90-nm wide), diurnally
   destabilized corridor of moist advection/transport between the
   dryline and departed midday/afternoon activity discussed above.  A
   substantial fraction of the CAPE will be "top-heavy" on sounding
   diagrams and occupying ideal hail-growth layers.  Updrafts will be
   very intense, thanks to supercell mode supported by 40-50-kt
   effective-shear magnitudes in the CDS-LTS-FDR-SPS corridor
   (weakening with northward extent over western OK) and enhancements
   to low-level shear provided by modest but backed near-surface winds
   northeast through east of the surface low.  

   Although instability will diminish with eastward extent and time
   across OK and into AR this evening and tonight, low-level theta-e
   advection behind the earlier activity may support surface-based
   effective-inflow parcels with neutral to weakly unstable boundary
   layer lapse rates.  Given this, and the potential for an organized
   complex and cold pool to develop, 30% wind probabilities are being
   introduced for that stage.

   ..Edwards/Jewell.. 04/23/2021

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