SPC AC 130605

   Day 2 Convective Outlook CORR 1
   NWS Storm Prediction Center Norman OK
   0105 AM CDT Thu Aug 13 2020

   Valid 141200Z - 151200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE UPPER MISSISSIPPI AND MID MISSOURI VALLEYS...

   CORRECTED FOR WORDING

   ...SUMMARY...
   Severe thunderstorms with wind damage, large hail and perhaps a
   couple tornadoes will be possible in parts of the upper Mississippi
   and mid Missouri Valleys on Friday afternoon and evening. Marginally
   severe thunderstorms will be possible in parts of the central
   Plains.

   ...Upper Mississippi and Mid Missouri Valleys...
   An upper-level trough will move across the northern Plains on Friday
   as a 50 to 70 kt mid-level jet moves eastward into the upper
   Mississippi Valley. At the surface, a low will deepen and move
   northeastward across northwestern Minnesota as a cold front advances
   eastward through the northern Plains. Scattered thunderstorms are
   expected to develop to the north of the surface low during the day
   in parts of northern North Dakota and northwest Minnesota. As
   surface heating takes place, thunderstorms are forecast to form
   along the front during the late afternoon from north-central
   Minnesota south-southwestward into southwestern Minnesota. A line of
   organized storms may organize and move eastward across eastern and
   southern Minnesota during the evening. The tail end of the line may
   also affect parts of eastern Nebraska and northern Iowa.

   Model forecasts are in relatively good agreement concerning the
   position of the instability axis at 21Z on Friday. MLCAPE is
   forecast to be in the 3000 to 4000 J/kg range from far southeastern
   South Dakota into western Minnesota just prior to the rapid
   development of a convective line. NAM and GFS Forecast sounding
   along this corridor of instability during the late evening show 0-6
   km shear ranging from 30 to 40 kt. Lapse rates are forecast to be
   very steep in two layers within the soundings, from the surface to
   850 mb and from 650 mb to 500 mb. The latter would aid large-hail
   production in supercells. Hailstones of greater than 2 inches in
   diameter would be possible with the more dominant supercells. The
   steep lapse rates at low-levels would aid the potential for damaging
   wind gusts. In addition, the mid-level jet is forecast to become
   coupled to a 40-50 kt low-level jet, increasing the tornado
   potential. For that reason, a tornado threat will be possible with
   supercells embedded in the line or with bowing segments.

   At this time, it appears that strong low-level convergence along the
   front and a focused band of large-scale ascent will help a linear
   MCS to organize across Minnesota during the late afternoon and early
   evening. This MCS may have potential to produce widespread damaging
   wind gusts, which could necessitate an upgrade to enhanced in later
   outlooks if forecasts conditions warrant. But uncertainty is still
   too great for issuance of an enhanced risk.

   ...Central Plains...
   An upper-level ridge is forecast across the central Plains on
   Friday. Beneath the ridge, an axis of strong instability is expected
   to be in place by afternoon. Although temperatures will be very warm
   aloft, isolated thunderstorms will be possible along a cold front
   during the late afternoon as instability maximizes and persisting
   into the evening. Moderate deep-layer shear and steep lapse rates
   would make hail and marginally severe wind gusts possible.

   ..Broyles.. 08/13/2020

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