SPC AC 301956

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0256 PM CDT Tue Jun 30 2020

   Valid 302000Z - 011200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON INTO THIS EVENING ACROSS PARTS OF THE NORTHERN PLAINS...

   ...SUMMARY...
   A few severe thunderstorms are possible late this afternoon and
   evening, mainly across parts of central and eastern South Dakota and
   Nebraska, into eastern North Dakota, posing a risk for large hail
   and strong wind gusts.

   ...20Z Outlook Update...
   Categorical and probabilistic lines have been adjusted in an attempt
   to better account for gradual progression of synoptic and
   sub-synoptic features, and latest trends concerning destabilization
   which will impact convective potential late this afternoon through
   tonight.

   The surface cold front has advanced eastward/southeastward into
   central South Dakota and Nebraska, to the south of an area of low
   pressure migrating north-northwest of Pierre SD, toward the Mobridge
   SD area, within deep surface troughing.  The front is approaching
   Pierre, but has advanced east/southeast of the Valentine and North
   Platte NE vicinities.  Outflow from prior convection has impacted
   the boundary layer across much of central North Dakota, but its
   leading edge appears to have stalled east of the Devils Lake into
   the Jamestown vicinity and areas south of Bismarck.

   A narrow corridor of strong daytime heating, just ahead of the front
   across central Nebraska and South Dakota, seems to provide the focus
   for the most substantive convective potential late this afternoon
   and evening.  Beneath steep mid-level lapse rates associated with a
   lingering plume of very warm elevated mixed-layer air, mixed-layer
   CAPE appears in excess of 4000 J/kg.  This appears to be occurring
   just ahead of the stronger south-southeasterly mid-level flow in
   advance of the approaching negatively tilted troughing.  However,
   deep-layer shear may still be sufficiently strong where storms
   initiate to support isolated supercells initially, then organizing
   clusters as activity gradually grows upscale this evening.

   ..Kerr.. 06/30/2020

   .PREV DISCUSSION... /ISSUED 1103 AM CDT Tue Jun 30 2020/

   ...Northern Plains...
   A persistent upper low remains centered over ID today, with
   moderately strong and deep southerly flow across the
   central/northern Rockies and northern Plains.  A large cluster of
   thunderstorms has been affecting western ND this morning, providing
   abundant clouds/precipitation, and limiting heating/destabilization
   across the region.  There is limited confidence whether this area
   can recover from morning storms and pose an afternoon/evening threat
   of severe storms.  Will maintain the ongoing SLGT risk, but portions
   of western/central ND may be removed in later outlooks.

   Farther south, strong heating will lead to a very unstable air mass
   across portions of southeast ND/SD/NE, with forecast soundings
   showing MLCAPE values of 3500-5000 J/kg.  A shortwave trough
   currently rotating around the base of the low across WY/CO will
   approach the corridor of strong CAPE by late afternoon, resulting in
   rather widespread thunderstorm development.  Initial storms may be
   supercellular, but it appears activity will rapidly evolve into
   multiple linear structures capable of damaging winds and some hail. 
   These storms will likely persist through much of the night as they
   spread eastward into parts of western MN/IA.

   ...TX...
   Similar to yesterday, hot surface temperatures exceeding 100F are
   expected along the dryline that will extend from the San Angelo area
   into southwest OK.  East of the dryline, near dry-adiabatic low/mid
   level lapse rates and dewpoints near 70F will yield extreme CAPE
   values.  Forcing is weak.  But thermal circulations along the
   dryline may result in isolated thunderstorms for a few hours late
   this afternoon and early evening.  Those storms that form may
   produce locally damaging wind gusts and hail.

   ...Mid MS Valley...
   Water vapor imagery shows a remnant MCV north of St Louis.  There is
   a corridor to the south of the MCV from southeast MO into western
   KY/TN where strong heating and significant destabilization is
   occurring.  This area is likely to see scattered thunderstorm
   development this afternoon.  Vertical shear is weak, suggesting
   activity will be chaotic and weakly organized.  But large CAPE
   values and steep low-level lapse rates will support a risk of
   locally gusty/damaging winds.

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