SPC AC 221252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Wed May 22 2019

   Valid 221300Z - 231200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM WESTERN
   MISSOURI TO SOUTHEASTERN KANSAS AND NORTHEASTERN OKLAHOMA...

   ...SUMMARY...
   The greatest potential for severe thunderstorms is this afternoon
   into evening, from western Missouri to southeastern Kansas and
   northeastern Oklahoma.  Tornadoes, large/damaging hail and severe
   gusts all are possible.

   ...Synopsis...
   The period begins with a large area of mid/upper-level cyclonic flow
   covering most of the central/western CONUS, anchored by embedded
   lows initially evident in moisture-channel imagery over the north-
   central Plains and the Great Basin.  The eastern of the two lows is
   occluded in low levels and nearly stacked, and is forecast to weaken
   while ejecting northeastward toward Lake Superior through 12Z. 
   Meanwhile the western low should pivot slowly around the southern/
   eastern Great Basin, while assuming less-positive tilt.  In between,
   shortwave ridging and height rises will build over the
   southern/central Plains. 

   At the surface, 11Z analysis showed the surface manifestation of the
   eastern mid/upper cyclone, with a low over southeastern SD and
   occluded front southeastward to the EVV area.  A wavy warm front was
   drawn from there across southern KY and eastern TN, to southern NC
   and northern SC.  This warm front will move northward across the
   Ohio Valley to the lower Great Lakes through the period.  A
   stationary front extended from the triple point southwestward over
   the Mid-South, becoming a warm front across north-central to central
   TX.  This boundary will become diffuse and spread northward over OK
   and AR.  A secondary cold front extended from the low across eastern
   KS and central/western OK. This boundary should stall then retreat
   northward over OK today, ultimately merging with the remains of the
   front to its south.  The combined front should become better-defined
   this afternoon and evening across southeastern KS and northern OK. 
   A forecast veering wind shift behind the front -- related to the
   departure of the deep-tropospheric low -- will make it more
   convergent this afternoon, while warm-sector theta-e advection
   contributes to strengthening the associated baroclinic gradient. 

   ...Southern Plains...
   Scattered to numerous thunderstorms are expected to develop
   gradually from mid afternoon through this evening along the
   strengthening frontal zone over the southeastern KS/southwestern MO
   and northeastern OK region.  A more-isolated/conditional threat is
   evident farther west and late afternoon into evening along the front
   over north-central OK, and southward into the warm sector this
   afternoon in central/eastern OK.  Any sustained, relatively discrete
   convection could become supercellular and offer the risk of
   tornadoes, large to very large hail, and severe gusts.  Isolated
   strong-severe convection may also develop late overnight near the
   dryline as it retreats across the south-central High Plains. 

   Even with background height rises and lack of substantial mid/upper
   forcing, intensifying thermodynamic support from below will
   contribute to a favorable warm-sector parameter space this
   afternoon.  As the boundary layer recovers northward through OK and
   around the western Ozarks vicinity, the air mass will destabilize
   from a combination of moist/warm advection and diabatic surface
   heating.  Given the reservoir of available moisture upstream, the
   return-flow regime should become characterized by surface dew points
   upper 60s to low 70s F, mean mixing ratios strengthening to the
   16-18 g/kg range, and PW 1.5-2 inches.  This process will reduce
   MLCINH to negligible levels by early/mid afternoon, in a northward-
   shifting way, with MLCAPE reaching the 2500-4000 J/kg range.  The
   LLJ will help to maintain large hodographs with effective SRH above
   200 J/kg, amidst 50-60-kt effective-shear magnitude.

   The more-certain area for convection, and ultimately greatest
   cumulative concentration, should be with the corridor of low-level
   frontogenetic forcing and convergence, where severe probabilities
   remain greatest.  Farther south across the warm sector, CAM progs of
   gradual convective/supercell genesis are uncertain but possible. 
   Based on character and motion of shallower/pre-supercellular
   elements in high-res guidance, any such development appears related
   to initial elevated convection in the warm-advection regime becoming
   surface-based, as the boundary layer moistens, deepens and
   destabilizes beneath, 0-3-km lapse rates steepen, and CINH erodes. 
   However, increasing boundary-layer theta-e may be offset somewhat,
   as other progs (e.g., NAM, RAP) also show warming near 700 mb
   related to poleward advection of northern portions of a residual
   Mexican EML.  Special afternoon sounding releases from SGF/OUN are
   planned to sample this airmass evolution diagnostically, given that
   the absence of substantial boundaries south of the front indicates
   crucial processes could occur mainly above the surface.  

   ...Mid Mississippi Valley to WI...
   Additional strong-severe convection should develop near the front
   northeastward across the mid Mississippi Valley, and perhaps as far
   as southeastern WI/northern IL.  This threat will peak late
   afternoon into early evening, as the warm sector destabilizes and
   moistens, also with vertical shear supporting supercell and
   organized multicell potential.  A nearly separate regime of
   destabilization, related less to low-level theta-e advection and
   more to cooling aloft ahead of the ejecting/occluded low,
   conditionally may support isolated/brief/marginal severe farther
   north across parts of eastern/northern WI this afternoon. 

   ...OH and vicinity...
   Isolated to widely scattered thunderstorms are possible near the
   warm front and southward into the warm sector this afternoon, near a
   diurnally destabilizing theta-e axis.  The strongest cells may
   produce marginally severe hail and gusts.  Low-level hodographs and
   deep shear will be favorable for organized multicells or even
   isolated supercells, especially near the warm front where near-
   surface flow is most backed.  However, the lack of substantial lapse
   rates aloft and of deep-layer forcing casts convective
   maintenance/coverage into doubt.

   ..Edwards/Leitman.. 05/22/2019

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