SPC AC 111622

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1122 AM CDT Sun Oct 11 2020

   Valid 111630Z - 121200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON INTO EARLY TONIGHT FROM SOUTHWEST MN TO EXTREME NORTHEAST
   KS...

   ...SUMMARY...
   Severe thunderstorms are possible late this afternoon and evening
   across the Upper Midwest and Middle Missouri Valley. A brief/weak
   tornado or two and isolated damaging gusts will be possible this
   afternoon across parts of the Carolinas.

   ...MN/IA/eastern NE this evening through early tonight...
   In association with a midlevel shortwave trough moving eastward from
   WY/MT, a surface cold front will progress east-southeastward across
   the northern/central Plains today and reach the upper MS Valley
   overnight.  A narrow corridor of low-level moistening is underway
   ahead of the cold front (boundary-layer dewpoints of 56-60 F),
   though the breadth and quality of the moisture return has been
   limited by the influence of tropical cyclone Delta across the
   Southeast.  The modest low-level moistening is occurring beneath a
   plume of steep midlevel lapse rates near 9 C/km, which is also
   contributing to a strong cap.  It appears that surface
   heating/mixing, continued moisture advection, and focused ascent
   along the front will all be necessary for thunderstorm initiation
   this evening, starting around 23z near the MN/ND border and
   developing southward toward eastern NE/western IA by late evening.

   Strong/linear forcing for ascent along the front and storm motions
   undercut by the front support rather rapid upscale growth into a
   squall line, while the narrow corridor of buoyancy and cap will
   limit the potential for pre-frontal storm development.  The steep
   midlevel lapse rates and MLCAPE of 1000-1500 J/kg will support some
   potential for large hail, especially with any embedded supercells
   given effective bulk shear in excess of 40 kt.  Otherwise,
   convection evolving quickly into a line suggests that damaging
   outflow gusts will become the main threat this evening.  The severe
   threat will diminish gradually overnight as buoyancy diminishes and
   convective inhibition increases with eastward extent.

   ...Carolinas this afternoon...
   The remnants of tropical cyclone Delta will continue to move
   east-northeastward from the southern Appalachians and weaken. 
   Vertical shear will remain at least marginally favorable for
   supercells given lingering 30-40 kt southwesterly midlevel flow and
   backed low-level winds/stronger low-level shear along a warm front
   approaching the SC/NC border.  The threat for a brief tornado or two
   will be focused where the convection in central SC (along the
   primary confluence band) moves northeastward and interacts with the
   zone of somewhat stronger low-level shear along the warm front this
   afternoon.

   ..Thompson/Squitieri.. 10/11/2020

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