SPC AC 181953

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0253 PM CDT Thu Apr 18 2024

   Valid 182000Z - 191200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
   EASTERN MISSOURI INTO SOUTHERN ILLINOIS...EXTREME SOUTHWEST
   INDIANA...FAR SOUTHWESTERN KENTUCKY...FAR NORTHWESTERN TENNESSEE...

   ...SUMMARY...
   Damaging gusts to 70 mph, large hail up to 2 inches in diameter, and
   a couple of tornadoes remain possible this afternoon into early
   tonight across the middle Mississippi and lower Ohio Valleys.
   Isolated large hail and damaging winds are also still possible
   farther south from northern Mississippi and Arkansas into central
   Texas.

   ...20Z Update...
   The only changes made to the ongoing outlook were to trim thunder
   and severe probabilities in areas that have been cleared by the
   passing surface cold front. Otherwise, the previous forecast (see
   below) remains generally on track, with no major changes made.

   ..Squitieri.. 04/18/2024

   .PREV DISCUSSION... /ISSUED 1124 AM CDT Thu Apr 18 2024/

   ...Mid MS/lower OH Valleys through tonight...
   Recent surface analysis places a low just south of MCI along the
   KS/MO border vicinity. A warm front extends eastward from this low
   across central MO, far southern IL and into KY. Airmass south of
   this warm front is moderately moist, with observations sampling mid
   60s dewpoints along much of the AR/MO border, with some upper 60s
   farther east over the MO Bootheel. Expectation is for this moist
   airmass to continue progressing northward throughout the day, likely
   reaching the I-70 corridor by the mid-afternoon. A cold front also
   extends southwestward from this low through north-central to
   southwest OK, continuing through northwest TX and southwest TX. An
   outflow boundary from overnight convection precedes this cold front,
   although by only about 30-50 miles or so.

   General expectation is for thunderstorm intensity and coverage to
   increase early this afternoon as these surface boundaries interact
   with the increasingly moist and unstable boundary layer downstream. 
   Overall convective evolution is a bit more uncertain, particularly
   regarding when the storms become more balanced with the cold front
   and/or outflow no longer undercutting updrafts. Current thinking is
   that this occurs during the early afternoon across central MO, with
   quick upscale growth promoting an organized convective line. Primary
   risk within this line will be damaging wind gusts from 45 to 65 mph.
   Large hail from 1 to 2" is also possible, particularly with any more
   cellular development ahead of the front and/or outflow.

   A similar convective evolution appears likely farther north (from
   central MO into central/southern IL and western IN) near the surface
   low, but with augmented mesoscale ascent near the low. This
   augmented ascent could contribute to more vigorous updrafts and a
   somewhat more organized convective structure, despite being
   displaced north of the better low-level moisture and buoyancy.
   Additionally, the eastward-progressing low will allow storms here to
   be more favorably aligned with the deep-layer vertical shear. This
   could contribute to a more forward-propagating structure and the
   potential for damaging gusts, despite modest thermodynamics. As a
   result, the outlook probabilities were shifted a bit northeastward
   across IL and IN. Primary threat here is wind gusts, although the
   backed low-level flow near the surface low could contribute some
   tornado threat as well.

   ...Much of AR through southeast OK and into North/Central TX this
   afternoon/evening...
   A cold front extends southwestward from a low near the KS/MO border
   southwestward through north-central to southwest OK, continuing
   through northwest TX and southwest TX. Surface heating ahead of this
   front is expected to result in strong buoyancy within the moist
   airmass that is already in place across the region. Widely scattered
   to 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...
   As mentioned in MCD #480, slightly elevated convection continues to
   evolve across the Arkansas/Louisiana border area this morning,
   within a zone of QG ascent ahead of a subtle shortwave trough moving
   across east Texas. This shortwave is expected to continue
   progressing northeastward, with the ongoing storms likely persisting
   as they move northeastward as well. General expectation is for this
   cluster to maintain its intensity for the next few hours, with some
   hail possible, before likely waning thereafter as the low-level jet
   weakens and veers. However, that is a fairly low confidence scenario
   given that this cluster developed about 4 hours ahead of when much
   of the guidance indicated.
    
   Additional development appears probable in the wake of this cluster
   over northern LA/southern AR this afternoon. In this area, weak
   ascent preceding the shortwave will interact with the unstable warm
   sector 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, with the
   threat expanding eastward into northern/central MS over time.

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