SPC AC 280537

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1237 AM CDT Fri Aug 28 2020

   Valid 281200Z - 291200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE UPPER MISSISSIPPI VALLEY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER GREAT LAKES AND TENNESSEE VALLEY...

   ...SUMMARY...
   Severe storms with large hail, damaging winds and a couple tornadoes
   are possible today over parts of the upper Mississippi Valley and
   southern Great Lakes. Other severe storms may occur across the
   Tennessee and lower Mississippi Valleys with a few tornadoes and
   damaging winds the primary threats.

   ...Upper Mississippi Valley/Southern Great Lakes...
   An upper-level trough will move southeastward into the upper
   Mississippi Valley today. At the surface, a cold front will advance
   southeastward across the upper Mississippi and mid Missouri Valleys.
   Ahead of the front, dewpoints in the lower to mid 70s F will
   contribute to moderate to strong instability by afternoon. By 21Z,
   the front is forecast to be located from near Omaha, Nebraska
   northeastward to Madison, Wisconsin. Thunderstorms are expected to
   form along the front during the mid afternoon with convection moving
   southeastward across the region. Multiple line segments appear
   likely to organize along the front during the late afternoon and
   early evening.

   Forecast soundings along the instability axis for 00Z/Saturday show
   MLCAPE in the 2000 to 3000 J/kg range. Wind profiles are forecast to
   have strong speed shear in the mid-levels, resulting in 0-6 km shear
   of 40 to 50 kt. In addition, 700-500 mb lapse rates are forecast to
   be near 7.0 C/km. This environment should support supercells with
   large hail. However, supercell development may remain confined to
   areas that destabilize the most. The favored storm mode could
   transition to linear as low-level convergence increases along the
   boundary in the early evening. Wind damage would be possible with
   the faster moving line segments. The severe threat could continue
   into the mid to late evening with the more persistent line segments.

   Further to the east across the southern Great Lakes region, the
   airmass will be very moist with surface dewpoints in the 70s F. In
   response, a corridor of moderate to strong instability is forecast
   to develop from northern Indiana into central Ohio. Thunderstorms
   that can form along this corridor of instability during the late
   afternoon will have potential to produce damaging wind gusts and
   hail.

   ...Tennessee and Lower Mississippi Valleys...
   Tropical Depression Laura will move east-northeastward across the
   mid Mississippi Valley today. Multiple bands of convection
   associated with Laura will move across northern sections of the
   central Gulf Coast states and the Tennessee Valley. Pockets of
   moderate instability are forecast to develop in some areas as
   surface temperatures warm. This combined with strong low-level shear
   profiles associated with Tropical Depression Laura, will create
   conditions favorable for rotating storms and possibly a couple
   tornadoes. The area with the greatest tornado potential should be
   from north-central Mississippi northeastward into western and middle
   Tennessee where low-level shear is forecast to be maximized this
   afternoon.

   ...Central Plains...
   West-northwest mid-level flow will be in place today across the
   central Plains as a cold front advances south-southeastward across
   the region. By 21Z, the front should be positioned from western
   Kansas northeastward into far southeast Nebraska. Along and south of
   the front, surface dewpoints in the lower to mid 60s F will allow
   moderate instability to develop by afternoon. Although large-scale
   ascent across the central Plains will be weak, increasing low-level
   convergence along the front should be enough for isolated
   thunderstorm development. The moderate instability, combined with
   steep low to mid-level lapse rates and 0-6 km shear in the 20 to 30
   kt range, will support an isolated severe threat. Hail and strong
   gusty winds will be the primary threats near and after peak heating.

   ..Broyles/Lyons.. 08/28/2020

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