SPC AC 260728

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0228 AM CDT Mon Jul 26 2021

   Valid 281200Z - 291200Z

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

   ...SUMMARY...
   Thunderstorms associated with severe wind gusts and isolated large
   hail will be possible on Wednesday across parts of the upper
   Mississippi Valley and Great Lakes region. Marginally severe storms
   could also occur in the central Appalachians.

   ...Upper Mississippi Valley/Western Great Lakes...
   A large area of high pressure will remain over the western and
   central United states on Wednesday. Northwest mid-level flow will be
   located over the north-central states as a subtle shortwave trough
   moves southeastward across the upper Mississippi Valley. At the
   surface, a cold front is forecast to advance southward across the
   northern Plains and upper Mississippi Valley. A very moist airmass
   will likely be in place south of the front with surface dewpoints in
   the lower to mid 70s F. This should allow moderate to strong
   instability to develop across much of the moist sector during the
   day, with convective initiation taking place along the front during
   the mid to late afternoon. NAM forecast soundings near Minneapolis
   by 00Z/Thursday have MLCAPE peaking in the 3500 to 4500 J/kg range.
   0-6 km shear is forecast to be about 35 kt with steep low to
   mid-level lapse rates. This environment should support supercell
   development with a potential for large hail and wind damage. As cell
   coverage increases during the early evening, MCS formation could
   also occur as the low-level jet strengthens. If this happens, a
   wind-damage threat could affect areas further downstream into
   central and southern Wisconsin.

   ...Central Appalachians/Lower Great Lakes...
   Northwest mid-level flow will be in place on Wednesday from the
   Great Lakes into the Northeast. At the surface, a moist airmass
   should be in place from the Ohio Valley into the central
   Appalachians, with moderate instability developing across much of
   the region by afternoon. Although large-scale ascent will not be
   that strong, low-level convergence and terrain influences will
   likely aid convective initiation. In response, isolated to scattered
   thunderstorms appear likely to develop in the central Appalachians
   during the afternoon. Model forecasts across the central
   Appalachians Wednesday afternoon suggest that 0-6 km shear will be
   strong enough for a marginal wind-damage threat with the stronger
   multicells. The wind-damage threat should be greatest during the
   late afternoon as low-level lapse rates become steepest.

   ..Broyles.. 07/26/2021

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