SPC AC 211230

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0730 AM CDT Fri Aug 21 2020

   Valid 211300Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS
   AFTERNOON/EVENING FOR PARTS OF THE CENTRAL/EASTERN DAKOTAS...

   ...SUMMARY...
   Severe storms with large hail and damaging winds are possible this
   afternoon/evening across parts of the Dakotas.  Isolated strong
   thunderstorm gusts will also be possible this afternoon across parts
   of the Southeast and the Northeast.

   ...Northern Plains and Upper MS Valley through tonight...
   A midlevel shortwave trough over southern SK and northeastern MT
   will progress east-southeastward over ND today, and reach MN
   overnight.  A weak surface trough/low will likewise develop eastward
   across ND through early tonight.  East of the trough and the
   dissipating remains of overnight convection, boundary-layer
   dewpoints in the mid 60s and afternoon surface temperatures near 90
   F, beneath midlevel lapse rates of 8 C/km, will result in MLCAPE of
   2500-3000 J/kg and only small convective inhibition.  Thunderstorm
   development is expected along the surface trough by about 21z, and
   storms will subsequently build southward into SD and move eastward
   toward eastern ND/northwestern MN through late evening. 
   Thermodynamic profiles will favor a threat for large hail/damaging
   winds, while effective bulk shear of 30-35 kt and orthogonal shear
   vectors to the weak boundary suggest a broken band of cells, with a
   mix of low-end supercells and multicell clusters.

   ...Upstate NY eastward into New England today...
   A northern stream shortwave trough will move over the Saint Lawrence
   Valley and northern New England today, as an associated/weak surface
   cold front drifts southward into Upstate NY and northern New
   England.  Along the southern fringe of a band of clouds/rain with
   the cold front, surface heating and modest low-level moisture will
   contribute to weak surface-based buoyancy.  Storms that form in this
   corridor later today will have some potential to produce strong
   outflow winds given effective bulk shear up to 35 kt, but the
   overall severe threat will be tempered by poor lapse rates and weak
   buoyancy.

   ...Southeast today...
   A slow-moving midlevel trough persists over the lower MS Valley,
   with a band of somewhat enhanced (25-30 kt) southwesterly midlevel
   flow to its east, from the northeast Gulf coast into GA/SC.  Storm
   development will be most robust today where pockets of surface
   heating will drive moderate buoyancy (MLCAPE of 1500-2500 J/kg), and
   effective bulk shear of 25-30 kt will be sufficient for a few
   organized clusters capable of producing strong outflow winds and
   isolated tree damage.

   ...Central/southern AZ this afternoon/evening...
   Relatively widespread convection overnight has left a cooler/moister
   environment across southern AZ, compared to prior days.  The flow
   regime and stabilizing influence of the overnight convection/outflow
   are less favorable for storms to form over the Rim and move into the
   lower deserts, and the resultant threat for isolated damaging winds
   is also lower compared to recent days.

   ..Thompson/Leitman.. 08/21/2020

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z