SPC AC 281243

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0643 AM CST Sun Feb 28 2021

   Valid 281300Z - 011200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTHEAST
   TEXAS TO MIDDLE TENNESSEE AND SOUTHERN KENTUCKY...

   ...SUMMARY...
   Damaging thunderstorm gusts, a few tornadoes and isolated large hail
   are possible from northeast Texas to Middle Tennessee and southern
   Kentucky.

   ...Synopsis...
   In mid/upper levels, a positively tilted trough is apparent in
   moisture-channel imagery from central Canada across the northern/
   central Plains and central Rockies to AZ and Baja.  A shortwave
   trough initially  over SD/NE will eject/stretch northeastward across
   the Upper Great Lakes through the period, losing amplitude, but
   remaining connected to a very lengthy vorticity banner extending
   southwestward around the northern/western sectors of a developing
   closed cyclone over parts of NM/AZ.  The associated 500-mb low
   should reach southern NM by the end of the period at 01/12Z. 
   Farther equatorward, a weak but potentially influential southern-
   stream perturbation over Chihuahua should eject northeastward across
   west-central/north-central TX today.

   At the surface, 11Z analysis depicted a surface low near the IA/MN
   border with cold front southwestward over southeastern KS,
   southwestern OK, and the TX South Plains.  By 00Z, the low should
   reach ON, just north of Lake Huron, as the center of a well-
   developed and deepening synoptic cyclone, with cold front trailing
   across Lower MI, IN, the lowest segment of the Ohio Valley, the
   Arklatex region, and portions of south-central/southwest TX.  By
   12Z, the cold front should reach eastern NY, central VA,
   northwestern GA, central MS, central LA, the mid/upper TX Coastal
   Plain, and deep south TX.

   ...North TX to TN/KY...
   Scattered thunderstorms may develop along/ahead of the cold front as
   early as mid/late morning over parts of north TX and southeastern
   OK, increasing in coverage and extending northeastward through the
   day as the regime both backbuilds southward, shifts eastward and
   expands northeastward.  An organized belt of scattered to numerous
   thunderstorms is expected to evolve as a result, by mid/late
   afternoon, from the Ozarks across southeastern OK to central/
   northeast TX.  The foregoing warm sector will destabilize and build
   buoyancy through the day from a combination of warm advection,
   moisture advection/transport, and modest/erratic surface heating
   under broken cloud cover.  Surface dew points in the 60s to near 70
   F should be common near and south of a decaying warm front now
   located over parts of TN, AR and eastern OK, and forecast to become
   more diffuse with time.  This will contribute to a pre-cold-frontal
   plume of MLCAPE 1500-2000 J/kg over the TX part of the outlook,
   where midlevel lapse rates will be steepest, and 800-1500 J/kg
   elsewhere.

   Low-level and deep shear will be favorable, with effective SRH
   commonly in the 150-250 J/kg range despite somewhat veered flow just
   above the surface, and 60-80 kt effective-shear vectors.  The latter
   will be aligned with a substantial component parallel to the main
   belt of convective forcing, such that a combination of embedded
   supercells and QLCS mesocirculations are possible.  Those will yield
   potential for tornadoes and damaging to severe gusts.  Hail will be
   more of a concern relatively early in the convective cycle and in
   western parts of the outlook area, while modes still are less messy,
   and amidst greater lapse rates.  The severe potential may continue
   through much of the evening before weakening late tonight, except
   for the next regime described below.

   ...South TX tonight...
   Isolated to scattered thunderstorms are possible overnight -- mostly
   from about 06Z onward -- across portions of the Edwards Plateau, and
   adjoining parts of the Rio Grande Valley, northeastward over the 
   Hill Country.  This activity may develop on either side of the cold
   front, as strong moist advection via easterly flow occurs mainly in
   the 850-950-mb layer.  Forecast soundings indicate this process,
   underlying steep low/middle-level lapse rates, will yield weak
   MUCINH, with MUCAPE ranging from around 1500-2000 J/kg over southern
   parts of the outlook area to 500 J/kg in the north.  The substantial
   easterly component in the inflow layer will contribute to strong
   deep shear, with effective-shear magnitudes 60-80 kt possible. 
   Activity developing on or just south of the front will be undercut
   quickly, but may persist long enough to produce large hail.  At this
   time, coverage of the severe convection is uncertain, so the
   unconditional outlook will be maintained at marginal probabilities.

   ..Edwards/Jewell.. 02/28/2021

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