SPC AC 120717

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0217 AM CDT Wed Aug 12 2020

   Valid 141200Z - 151200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE UPPER MISSISSIPPI VALLEY...

   ...SUMMARY...
   Severe thunderstorms with isolated large hail and wind damage will
   be possible in parts of the Upper Mississippi Valley on Friday.
   Marginally severe thunderstorms will be possible in parts of the mid
   Missouri Valley.

   ...Upper Mississippi Valley/Mid Missouri Valley...
   An upper-level trough will move across the northern Plains on Friday
   as an associated cold front advances southeastward into the upper
   Mississippi Valley. The front is forecast to be located from
   northwest Minnesota southwestward into eastern South Dakota by
   mid-afternoon. Ahead of the front, surface dewpoints in the upper
   60s to near 70 F will likely result in moderate to strong
   destabilization. As large-scale ascent moves in from the west and 
   low-level convergence increases along the front, thunderstorm
   development is expected to take place. Storms are forecast to first
   form in northwest Minnesota and then expand south-southwestward
   across far western Minnesota into eastern South Dakota. This
   activity should move eastward across much of Minnesota during the
   late afternoon and evening. Isolated thunderstorms may also develop
   across parts of the mid Missouri Valley.

   Model forecasts are in good agreement concerning the location of the
   instability axis. The NAM suggests MLCAPE could peak in the 3000 to
   4000 J/kg range across parts of southwest and central Minnesota,
   with less instability further north. The models also show moderate
   deep-layer shear along the front with 0-6 km shear ranging from 45
   to 55 kt. This environment would support severe thunderstorm
   development. At this time, it appears that a nearly continuous line
   segment with wind damage potential will organize along the front
   during the late afternoon. This wind damage potential is forecast to
   move east-southeastward reaching eastern and south-central Minnesota
   by mid-evening. In addition to the wind damage threat, isolated
   large hail will be possible with rotating cells embedded in the
   line.

   Further south-southwest into the mid Missouri Valley, convective
   development should remain isolated. This will likely be due to
   limited large-scale ascent and stronger capping. If any cell can
   develop in spite of the less favorable environment, a marginal
   severe threat would be possible.

   ..Broyles.. 08/12/2020

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