SPC AC 182126

   Day 1 Convective Outlook AMEND 1
   NWS Storm Prediction Center Norman OK
   0426 PM CDT Thu Mar 18 2021

   Valid 182000Z - 191200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM NORTHERN
   SOUTH CAROLINA THROUGH CENTRAL AND EASTERN NORTH CAROLINA AND
   SOUTHEAST VIRGINIA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER THE SOUTHERN
   APPALACHIANS AND A PORTION OF NORTHERN FLORIDA...

   AMENDED FOR TO EXPAND ENHANCED RISK FARTHER NORTH INTO SOUTHEAST
   VIRGINIA FOR TORNADO AND DAMAGING WIND THREAT

   ...SUMMARY...
   Scattered severe storms remain possible this afternoon and evening
   mainly from a portion of South Carolina through central and eastern
   North Carolina and southeast Virginia. Other strong to severe storms
   are possible this afternoon over the southern Appalachian region as
   well as northern Florida.

   ...South Carolina through central and eastern North Carolina and
   southeast Virginia...

   Cluster of storms over western SC and southwestern NC is expected to
   continue developing northeast through central and eventually eastern
   NC this afternoon and evening. Vertical wind profiles with 50+ kt
   effective bulk shear and sizeable 0-2 km hodographs will remain
   supportive of organized severe storms including supercells. However,
   trend has been for a pocket of dry air in the low levels to advect
   northward through northern SC into southeastern NC, and this has at
   least temporarily reduced boundary layer instability. Nevertheless,
   some low-level theta-e advection will resume in wake of the dry
   pocket contributing to 1000-1500 J/kg MLCAPE. The best chance for
   severe storms are expected as activity develops northeast along
   instability gradient through a portion of central and eastern NC and
   possibly southeast VA this afternoon and evening with damaging wind,
   large hail and a few tornadoes possible.

   ...Southern Appalachian region...

   Storms are developing within corridor of ascent just east of the
   progressive shortwave trough. Cold temperatures aloft, steep lapse
   rates and diabatic heating will continue to foster widely scattered
   storm development this afternoon. Isolated large hail and a few
   locally strong wind gusts are the main threats. Activity should
   diminish toward late afternoon or early evening.

   ..Dial.. 03/18/2021

   .PREV DISCUSSION... /ISSUED 1133 AM CDT Thu Mar 18 2021/

   ...Southeast...
   Overall severe threat will likely focus in two corridors which are
   encompassed by the cat 3/ENH risk for tornado and wind. Initial
   broken band of non-severe convection is ongoing from south-central
   GA through the eastern FL Panhandle. This activity has struggled to
   intensify despite strong low to deep-layer shear as it impinges on a
   relative minimum in buoyancy sampled by the 12Z JAX sounding and
   evident in mesoanalysis time-series. However, destabilization is
   occurring downstream from north FL to the coastal Carolinas and
   redevelopment of severe storms is anticipated at some point this
   afternoon. The eastward translation of a 50-55 kt LLJ will support
   an enlarged hodograph favorable for a few supercells. Modest
   mid-level lapse rates will remain a limiting factor, though MLCAPE
   will be large enough given to support a few tornadoes and scattered
   damaging winds, with a strong tornado or two still possible.

   The separate area of potential severe should be coincident with the
   track of a mesolow from the northeast GA/northwest SC border across
   the Piedmont towards south-central VA. Here, MLCAPE will likely
   remain weak but cloud breaks just ahead of the low near the
   mid-level dry slot may prove sufficient for an intensifying area of
   convection. Where sufficient buoyancy overlaps the more
   favorable/curved hodographs east of the mountains, a couple
   supercells capable of a strong tornado, damaging winds, and isolated
   large hail will all be possible this afternoon/evening.

   ...OH Valley...
   East and southeast of the weakening occluded cyclone, pockets of
   stronger boundary-layer heating are underway on the nose of the
   mid-level dry slot. In conjunction with cool mid-level temperatures,
   500-1000 J/kg of MLCAPE will be sufficient for a few strong-severe
   storms later this afternoon. This area will be on the northwest edge
   of the stronger flow/shear, where a mix of multicell clusters and a
   few low-end supercells are probable. A tornado or two, large hail,
   and damaging winds will be possible.

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