SPC AC 091626

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1126 AM CDT Fri Apr 09 2021

   Valid 091630Z - 101200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   INTO TONIGHT FROM SOUTHERN AR ACROSS NORTHEASTERN LA...CENTRAL MS
   AND WEST CENTRAL AL...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS
   AFTERNOON/EVENING NEAR THE NC/VA BORDER...

   ...SUMMARY...
   Multiple episodes of severe thunderstorms are expected today from
   parts of eastern Oklahoma and northeast Texas across the Mid-South
   and Mississippi Delta regions, to parts of Alabama and Georgia. 
   This includes a threat for a swath of severe wind from  Arkansas
   across the lower Mississippi Valley.  Large, damaging hail and 
   tornadoes also are possible.

   ...OK to MS/AL through tonight...
   A rather complex pattern and convective evolution is expected,
   beginning early this afternoon and continuing through Saturday
   morning.  A lead shortwave trough moving over the Red River Valley
   of TX/OK will continue eastward today and will encounter a
   moistening/destabilizing environment by early afternoon near the
   Ark-La-Tex.  Boundary-layer dewpoints in the upper 60s and surface
   temperatures warming into the lower 80s, beneath midlevel lapse
   rates near 9 C/km, will result in MLCAPE in excess of 3000 J/kg. 
   Ascent (as evidenced by the midlevel convection now over
   north/northeast TX) will help erode the cap noted in 12z soundings,
   and surface-based storm development is likely by early afternoon
   along the warm front from northeast TX into northwestern LA. 
   Low-level shear will not be particularly strong through mid
   afternoon, but the steep lapse rates/large CAPE and effective bulk
   shear in excess of 40 kt will favor a mix of supercells and upscale
   cluster growth, with attendant threats for very large hail and
   damaging winds as the convection spreads eastward toward the
   Ark-La-Miss through the afternoon/evening.  The northern extent of
   this convection should reach as far north as western TN, and the
   storms could pose a damaging-wind threat as far east as northwest AL
   before weakening late evening. 

   There will be some potential for storm development along the warm
   front and/or remnant outflow boundary later this afternoon across
   northern LA and northeast TX, in the wake of the initial Ark-La-Tex
   convection.  Gradual increases in low-level flow/shear suggest some
   potential for supercells with very large hail, damaging winds, and a
   few tornadoes.  The evolution of convection this afternoon/evening
   across northern LA and vicinity still appears rather
   complex/uncertain, and details will not likely be resolved until
   this afternoon.

   Farther west, additional storm development is likely later this
   afternoon into this evening along the north edge of the warm sector
   in northeast OK, and along a cold front into central OK.  MLCAPE
   will increase to 2000-2500 J/kg as surface temperatures warm into
   the 70s north to the lower 80s closer to the Red River, with
   boundary-layer dewpoints ranging from near 60 north by mid afternoon
   to the mid 60s near the Red River.  The combination of
   moderate-strong buoyancy and sufficient deep-layer shear will
   support both organized clusters and some supercells, with fairly
   rapid upscale growth into a line expected by this evening along the
   cold front.  Storms will subsequently surge southeastward from
   eastern OK into western/southern AR and northeast TX tonight,
   potentially along the outflow-reinforced front in the wake of the
   afternoon Ark-La-Tex convection.  Large hail and swaths of damaging
   winds, some significant, appear likely, as well as the potential for
   a few tornadoes with embedded circulations as convection continues
   overnight across LA/MS, and potentially into AL before 12z.

   ...Southern VA/northern NC this afternoon into early tonight...
   Around the southeastern periphery of a closed midlevel low over WI,
   a subtle embedded speed max will translate east-northeastward from
   the southern Appalachians to VA by this evening.  Surface heating
   and some low-level moistening along a slow-moving front across
   southern VA could provide a focus for thunderstorm development later
   this afternoon into early tonight.  MLCAPE near or above 1500 J/kg
   and effective bulk shear of 35-40 kt will favor a mix of organized
   clusters and supercells capable of producing large hail and damaging
   winds, as well as a brief tornado with slightly stronger low-level
   shear along the boundary. 

   ...Central and north TX this afternoon/evening...
   Regional 12z soundings showed a very strong cap across TX, with
   convective temperatures generally in the mid 90s.  Large CAPE is
   expected east of the dryline, but there is substantial uncertainty
   that surface temperatures will get sufficiently warm to eliminate
   convective inhibition.  Will maintain a conditional severe risk,
   with somewhat higher probabilities in north TX compared to central
   TX, given substantial uncertainty regarding convective initiation.

   ..Thompson/Goss.. 04/09/2021

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