SPC AC 071251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0751 AM CDT Wed Apr 07 2021

   Valid 071300Z - 081200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
   OF ARKANSAS...NORTHERN LOUISIANA...WESTERN/NORTHERN
   MISSISSIPPI...AND SOUTHWESTERN TENNESSEE...

   ...SUMMARY...
   Scattered severe thunderstorms are expected from this afternoon into
   the evening across parts of the lower Mississippi Valley, Mid-South,
   Arklatex and Ozarks.  Wind damage, hail and tornadoes will be
   possible.

   ...Synopsis...
   In mid/upper levels, a split-flow pattern will persist across the
   central CONUS, downstream from a strong trough digging southeastward
   from the AK Panhandle across BC and the Pacific Northwest.  The
   feature responsible for that split is a meridionally elongated
   cyclone -- now apparent in moisture-channel imagery over the central
   High Plains -- with a basal vorticity max over southwestern KS.  The
   primary 500-mb low should proceed east-northeastward across KS to
   the EMP/TOP area by 00Z, then eastward to near COU by 12Z.

   At the surface, 11Z analysis showed an elongated area of low
   pressure along a frontal zone from western IA to eastern KS, with
   cold front southwestward across south-central OK and west-central/
   southwest TX.  A primary surface low should consolidate and deepen,
   reaching southwestern/south-central IA by 00Z, with cold front
   arching across eastern MO, northern to southwestern AR, and
   northeast to south-central TX.  The low should move little overnight
   while becoming more deeply occluded. The cold front should reach
   western KY, middle TN, eastern MS/western AL, and southeastern LA by
   12Z. 

   ...Arklatex/Ozarks to lower/mid Mississippi Valley...
   See SPC mesoscale discussion 320 for coverage of a marginal,
   near-term hail concern ongoing in the Ozarks and vicinity.

   Scattered to numerous thunderstorms are forecast to form today in an
   arc near the cold front -- initially from portions of western MO to
   the Arklatex region, then backbuilding/shifting over the Mid-South
   and Delta regions into this evening.  Damaging winds, several
   tornadoes, and sporadic severe hail all will be possible in a broad
   swath from IA to parts of LA/MS, with the greatest severe potential
   in and near the "Enhanced" area. 

   The earliest development is expected from extreme eastern portions
   of KS/OK into near the MKC-FSM/northern I-49 corridor around midday.
   This should occur as the cold front encounters the northern fringe
   of diurnal boundary-layer destabilization and continuing warm-sector
   moisture advection.  This will be overlain by increasing large-scale
   ascent related to a vorticity lobe pivoting around the southeast
   side of the mid/upper cyclone, preferentially eroding already weaker
   EML-related MLCINH compared to father south.  As the front
   encounters even stronger boundary-layer destabilization (needed to
   overcome greater capping) southward into the Arklatex region,
   additional convection should develop into early/mid afternoon,
   perhaps extending as far south as east TX and western/northern LA by
   late afternoon.  

   The resulting, essentially continuous arc of thunderstorms will
   encounter a favorable inflow region, with surface heating and dew
   points in the 60s F contributing to a prefrontal corridor of
   1000-1500 J/kg peak MLCAPE (locally/briefly higher).  Buoyancy will
   be supported somewhat more by cold air aloft in the north and higher
   boundary-layer theta-e in the south.  While the flow aloft remains
   difluent, height gradients will tighten over the near-frontal warm
   sector with the eastward shift of the cyclone aloft, along with some
   backing with time of midlevel flow.  The net effect should be for
   strengthening deep shear (e.g., effective-shear magnitudes reaching
   35-50 kt), but also, a more front-parallel component to winds aloft,
   encouraging quasi-linear storm mode with embedded supercells and
   bow/LEWP features.  

   The resulting MCS will encounter veering flow with height in low
   levels and favorable effective SRH for occasionally tornadic
   mesocirculations.  The warm sector will be narrower with northward
   extent, leading to convection's outrunning favorable inflow sooner
   this evening in the mid Mississippi Valley region, and lasting well
   into the overnight hours across portions of MS, TN, LA and AL.

   ..Edwards/Kerr.. 04/07/2021

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