SPC AC 241654

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1054 AM CST Tue Nov 24 2020

   Valid 251200Z - 261200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE LOWER AND MIDDLE MS...LOWER OH AND TN VALLEYS...

   ...SUMMARY...
   Isolated strong to severe thunderstorms may occur Wednesday across
   parts of the lower/mid Mississippi Valley into the Ohio/Tennessee
   Valleys and Southeast. Locally damaging winds should be the main
   threat, although a tornado or two also appears possible.

   ...Synopsis...

   A mid/upper level shortwave trough centered over the Ozark Plateau
   early Wednesday will lift northeast across the Midwest to the upper
   OH Valley/central Appalachians by 12z Thursday. Surface low pressure
   will move in tandem with the upper trough from MO toward the lower
   Great Lakes while a trailing cold front pushes eastward across the
   mid/lower MS valley and the lower OH/TN Valley. Ahead of the front,
   southerly low level flow will continue to transport Gulf moisture
   northward, with 60s F dewpoints reaching as far north a TN
   potentially. Surface dewpoints in the 50s will be more common
   farther north across the Ohio Valley vicinity. Surface heating will
   be limited by cloudiness and showers ahead of the front, but cooling
   aloft is expected to result in modest midlevel lapse rates.
   Furthermore, strong shear profiles are forecast as winds veer with
   height and 40-60 kt 850-700 mb southwesterly flow overspreads much
   of the southern/central U.S. This should support at least isolated
   strong-to-severe thunderstorms capable of strong/locally damaging
   wind gusts across the lower/mid MS Valley to the OH/TN Valley. 

   ...Lower/Mid MS Valley to the OH/TN Valley Vicinity...

   Higher-quality low level moisture will be confined to the lower MS
   Valley into MS/AL on Wednesday. While stronger heating will be
   limited by cloudiness and showers ahead of the cold front, modest
   midlevel lapse rates atop rich boundary layer moisture should result
   in at least weak surface-based instability (typically 750 J/kg or
   less). Strong shear will reside over the region to aid in organized
   convection. Deep layer flow will mainly be parallel to the front,
   favoring line segments, but a couple of semi-discrete cells also are
   possible. Strong forcing closer to the trough/surface low will be
   shifting away from the region through the day, but the front and
   weak buoyancy will be sufficient for strong-to-severe convection
   into parts of MS/AL and middle TN through early evening. Storm mode
   and quickly strengthening flow with height will mainly favor
   damaging wind potential. However, where backed low level flow is
   present, low level SRH will be enhanced and a tornado or two cannot
   be entirely ruled out. 

   Farther north, instability will be even weaker given overall
   poorer-quality boundary layer moisture and a cooler environment.
   However, steeper midlevel lapse rates with colder air aloft should
   result in weak elevated instability. Stronger forcing and favorable
   shear profiles may compensate somewhat for meager instability, and
   at least a narrow line of near-surface-based convection closer to
   the surface low and cold front is expected during the
   afternoon/evening across parts of the lower OH Valley. Strong low
   level wind field associated with low/midlevel jet streak will favor
   bowing segments capable of isolated strong/damaging gusts.

   ..Leitman.. 11/24/2020

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