SPC AC 181247

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0747 AM CDT Thu Apr 18 2024

   Valid 181300Z - 191200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS MAINLY THIS
   AFTERNOON INTO EARLY TONIGHT FOR PARTS OF THE MID MS AND LOWER OH
   VALLEYS...

   ...SUMMARY...
   Damaging gusts to 70 mph, large hail in excess of 2 inches in
   diameter, and a couple of tornadoes are expected mainly this
   afternoon into early tonight across the middle Mississippi and lower
   Ohio Valleys.  More isolated large hail and damaging winds will be
   possible farther south from northern Mississippi and Arkansas into
   central Texas.

   ...Mid MS/lower OH Valleys through tonight...
   An occluded cyclone will move eastward into western ON, as part of a
   deep mid-upper low over western ON/northern MN by the end of the
   period.  Farther to the south, a secondary cyclone will move across
   MO to southeast Lower MI in association with an embedded jet on the
   southern periphery of the deep midlevel low to the north, as a cold
   front progresses southeastward across the mid MS Valley and the
   southern Plains.  Ahead of the secondary low and cold front, a
   moistening warm sector will spread northeastward from MO to
   southern/central IL and southwest IN (boundary-layer dewpoints well
   into the 60s) beneath lingering midlevel lapse rates of 7-8 C/km. 
   The net result will be an unstable warm sector with MLCAPE near 2000
   J/kg and only weak convective inhibition.

   The remnants of morning convection across MO could be rejuvenated by
   midday-early afternoon, with an intensifying band of storms expected
   to spread east-southeastward through the afternoon across the mid MS
   Valley and into tonight across the lower OH Valley.  The combination
   of moderate-strong buoyancy and sufficient hodograph
   length/low-level curvature will support a mix of embedded supercells
   and bowing segments capable of producing damaging winds of 60-70
   mph, large hail of 1-2.5 inches in diameter, and a couple of
   tornadoes.

   ...AR/southeast OK/north and central TX this afternoon/evening...
   Surface heating within an already moist air mass (68-72 F
   boundary-layer dewpoints) will drive strong buoyancy (MLCAPE of
   2500-3500 J/kg) this afternoon ahead of a surface cold front. 
   Scattered thunderstorm development is expected by about 21z along
   the front from northwest AR across southeast OK into north and
   central TX.  Deep-layer shear will be strong enough for some
   supercell structures capable of producing isolated very large hail
   (up to 2.5 inches in diameter), along with damaging outflow gusts
   given the potential for strong downdrafts.  These storms will last a
   few hours into late evening and then weaken as convective inhibition
   increases.

   ...Southeast AR into northern MS this afternoon/evening...
   An embedded speed max noted over north TX in the subtropical stream
   will progress eastward toward the MS River by late afternoon.  Weak
   ascent preceding the wave within an unstable warm sector, along with
   surface heating in cloud breaks, could be sufficient for widely
   scattered thunderstorm development this afternoon.  If a few storms
   do form, the environment will conditionally favor supercells capable
   of producing large hail/damaging winds.

   ..Thompson/Gleason.. 04/18/2024

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