SPC AC 251629

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1129 AM CDT Thu Mar 25 2021

   Valid 251630Z - 261200Z

   ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON AND
   EVENING FOR NORTHEAST MS...NORTHERN AND WESTERN AL...AND PARTS OF
   SOUTHERN MIDDLE AND SOUTHEAST TN...

   ...SUMMARY...
   Several long-tracked supercells are expected this afternoon into
   this evening from Mississippi into Alabama and parts of middle and
   eastern Tennessee.  The most persistent/intense supercells will be
   capable of producing strong to violent tornadoes, along with very
   large hail and substantial damaging winds.

   ...An outbreak of tornadic supercells is expected this afternoon
   into this evening from MS into AL and parts of TN, with long-track
   strong to violent tornadoes possible...

   ...Synopsis...
   A 1005 mb surface low in southeast AR will deepen gradually while
   ejecting northeastward to southern IL this evening and northwestern
   OH/southeastern Lower MI by the end of the period.  This will occur
   in conjunction with the northeastward ejection of a midlevel
   shortwave trough from the Red River Valley of TX/OK toward Lake
   Erie.  As the surface cyclone moves northeastward and deepens,
   boundary-layer dewpoints in the mid 60s to lower 70s will be drawn
   northward across MS/AL/TN, in the wake of morning warm advection
   storms (also associated with a subtle lead shortwave trough).  The
   moistening will occur beneath the northeast extent of an elevated
   mixed layer plume with steep midlevel lapse rates of 8-9 C/km, which
   was sampled by the CRP, LCH and JAN 12z soundings.  

   MLCAPE already exceeds 2000 J/kg across much of MS as of late
   morning, and additional destabilization will occur within the warm
   sector as surface temperatures warm into the mid 70s and near 70 F
   dewpoints spread northward.  The ongoing broken band of storms from
   northern AL into central MS, along the low-level jet/moist axis,
   will be a likely zone of intensifying, surface-based storms through
   the afternoon.  Deep-layer vertical shear is quite strong (effective
   bulk shear of 60-70 kt) over the warm sector, and strong low-level
   flow with strong low-level shear (effective SRH of 300-500 ms/s2)
   will be maintained within the warm sector through the afternoon. 
   Given somewhat subtle forcing for ascent along pre-frontal bands
   within the warm sector, scattered discrete or semi-discrete
   supercells appears to be the primary convective mode through the
   afternoon/evening as the warm sector spreads into TN. 
   Intensification of the ongoing and newly developing storms is
   expected from southern/central MS northeastward into AL, from which
   the primary supercell/tornado threat will evolve.

   Given the moderate-strong buoyancy, moist low levels, long
   hodographs with substantial low-level curvature, and primarily
   discrete storms, the scenario favors long-lived, fast-moving
   supercells capable of producing several strong-violent tornadoes
   across parts of MS/AL/TN this afternoon into this evening, before
   diminishing overnight as the primary shortwave trough passes to the
   north. of the main warm sector.

   A few severe storms may also form later this afternoon/evening
   closer to the surface low track with all modes of severe possible,
   though buoyancy and vertical shear will be weaker compared to areas
   farther southeast.  There may also be sufficient destabilization to
   maintain a severe threat into the southern Appalachians along the
   warm front.

   ..Thompson/Moore.. 03/25/2021

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