SPC AC 211633

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1133 AM CDT Fri Aug 21 2020

   Valid 211630Z - 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 shortwave trough over southern Saskatchewan will continue to
   progress east-southward toward southern Manitoba/North Dakota today,
   reaching northern Minnesota late tonight. A weak surface trough/low
   will likewise develop eastward across North Dakota/northeast South
   Dakota through early tonight. 

   Boundary-layer dewpoints in the mid 60s and afternoon surface
   temperatures near 90 F, beneath mid-level lapse rates of 8 C/km,
   will result in MLCAPE of 2500-3000 J/kg and only small convective
   inhibition. Although questions exist regarding the coverage of more
   intense storm development, at least isolated thunderstorms are
   expected to develop by mid/late afternoon along the surface trough.
   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 multicellular clusters.

   ...Upstate NY to 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 States...
   A slow-moving mid-level trough persists over the Lower Mississippi
   Valley into the Tennessee Valley, with a band of modestly enhanced
   (25-30 kt) southwesterly mid-level winds ahead of it. 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. Somewhat more probable/focused areas of
   strong/severe thunderstorm development could occur across the
   north-central Florida Peninsula and/or in South Carolina/southern
   North Carolina in vicinity of a frontal boundary.

   ...Southeast Texas...
   Isolated to widely scattered thunderstorms should develop near a
   weak boundary this afternoon. With ample moisture/instability and
   upwards of 2500-3000 J/kg MLCAPE, thermodynamic profiles appear to
   be conducive for the possibility of a few wet microbursts with
   potential for localized wind damage.

   ...Central/southern Arizona 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.

   ..Guyer/Dial.. 08/21/2020

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