SPC AC 180548

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1248 AM CDT Thu Mar 18 2021

   Valid 181200Z - 191200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF COASTAL SOUTH CAROLINA AND NORTH CAROLINA...

   ...SUMMARY...
   Scattered severe thunderstorms are expected today from portions of
   eastern Georgia, through the Carolinas into extreme southeast
   Virginia. All severe hazards are possible. Other isolated severe
   storms are possible from southern Ohio into the central
   Appalachians.

   ...Southeast...

   Early this morning an organized squall line was advancing across
   northwest GA, arcing southwest into southern AL. This MCS is
   supported by a progressive trough with a strong 500mb jet that
   should translate across central GA into the western Carolinas by
   18z. Latest trends suggest this convection should spread across much
   of western/northern GA by the start of the day1 period. Given the
   speed/movement of this squall line, the western edge of severe
   probabilities will be adjusted to reflect an earlier arrival.

   Latest short-range model guidance and HREF members are not
   particularly aggressive in destabilizing the Carolinas later today.
   This is reflected in several HREF individual members struggling to
   develop organized mesocyclones. While environmental shear will
   increase during the day, forecast soundings suggest thermodynamics
   may be a bit weaker than earlier expected. There is some concern
   this may influence storm coverage if the air mass does not
   destabilize sufficiently. Latest thinking is coastal southeast SC
   into southwest NC may exhibit better instability by early afternoon.
   03z/RAP for ILM at 17z suggests SBCAPE may approach 2000 J/kg with
   strong, deep-layer shear, more than adequate for strong/robust
   updrafts. Will maintain Moderate Risk across this region where
   higher buoyancy may reside. Otherwise, strengthening wind fields
   suggest any storms that form ahead of the early-day squall line
   could certainly evolve into supercells that would pose a risk for
   tornadoes, wind, and hail. This activity should develop early in the
   period, then track/spread northeast across the Carolinas into
   southeast VA by late afternoon.

   ...Oh Valley/Central Appalachians...

   Exit region of upper jet will encourage vertical motion ahead of the
   surface low over northern KY early in the period. Models suggest
   strong boundary-layer heating across KY into western WV will
   generate steep lapse rates such that convective temperatures will be
   breached by 18z. Scattered supercells will likely develop ahead of
   the surface low, then track east toward the higher terrain of the
   central Appalachians. Hail, wind, and perhaps a few tornadoes can be
   expected with this strongly diurnal activity.

   ..Darrow/Lyons.. 03/18/2021

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