SPC AC 252226

   Day 1 Convective Outlook AMEND 1
   NWS Storm Prediction Center Norman OK
   0526 PM CDT Thu Mar 25 2021

   Valid 252220Z - 261200Z

   ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS NORTHERN AND CENTRAL
   ALABAMA...FAR SOUTHERN MIDDLE TENNESSEE...AND FAR EASTERN
   MISSISSIPPI...

   AMENDED FOR EXPANSION OF HIGH/MDT/ENH RISKS IN EAST-CENTRAL AL AND
   MRGL RISK IN CAROLINAS

   ...SUMMARY...
   Several long-tracked supercells are still expected this afternoon
   into this evening from far eastern 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.

   ...2220Z Update...
   A long-track, strong to potentially violent tornado is ongoing
   across central AL and will likely persist towards the GA border
   given a favorable environment. Have expanded the categorical risks
   southeast to account for this extremely intense supercell.

   Also expanded MRGL risk in the Carolinas downstream of a long-lived
   supercell.

   ...20Z Update...
   Primary change with this update was to begin trimming severe
   probabilities from the west across the lower MS Valley. As low-level
   flow continues to veer over this region, the best severe potential
   should shift into AL and vicinity through the remainder of the
   afternoon and early evening. For more information on the near-term
   severe threat across far eastern MS into northern/central AL, see
   Mesoscale Discussion 256. Strong, long-track tornadoes still appear
   likely across this region through the evening.

   Farther north, additional convection should form later this
   afternoon and evening across parts of western/middle TN and the mid
   MS Valley as a warm front continues to lift northward and a
   shortwave trough moves over these regions. All severe hazards appear
   possible, including the potential for strong tornadoes given the
   strength of the low-level flow/shear. A severe threat should
   continue this evening into tonight across the lower OH Valley. For
   more information on the short-term severe threat across
   western/middle TN, see Mesoscale Discussion 257. Additional
   information on the increasing severe threat over the mid MS Valley
   can be found in Mesoscale Discussion 258.

   ..Grams/Gleason.. 03/25/2021

   .PREV DISCUSSION... /ISSUED 1129 AM CDT Thu Mar 25 2021/

   ...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.

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