SPC AC 270551

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1251 AM CDT Sat Mar 27 2021

   Valid 271200Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM SOUTHERN
   ARKANSAS TO MIDDLE TENNESSEE...

   ...SUMMARY...
   Severe storms are expected across the lower Mississippi through
   Tennessee Valley regions. All hazards are possible including very
   large hail, damaging wind, and tornadoes. Potential for a strong
   tornado exists from eastern Arkansas into western Tennessee. Other
   severe storms are possible across a portion of the Midwest with
   isolated damaging wind and large hail the main threats.

   ...Lower MS Valley/TN Valley...

   Large-scale heights will not change appreciably until late in the
   day1 period across the lower MS and TN Valley region. Water-vapor
   imagery depicts an upper low digging southeast across AZ early this
   morning. This feature is forecast to translate across far West TX
   during the evening before deamplifying as it ejects into
   north-central TX late. Prior to the influence of this feature, one
   LLJ branch will extend across the TX Coast-LA-middle TN. Low-level
   warm advection in association with this feature is expected to
   encourage a considerable amount of convection that should be ongoing
   at the start of the period. Several members of the HREF suggest a
   possible MCS may evolve over middle TN, then propagate east during
   the day. Areal extent of this activity may greatly influence
   convective development later in the day as rain-cooled boundary
   layer may establish an outflow boundary that could drape west across
   TN.

   Strongest low-level heating will be noted south of this rain-cooled
   air mass, and buoyancy should be greatest from northeast TX into
   northern MS where SBCAPE will exceed 2000-2500 J/kg. With
   temperatures warming into the mid-upper 70s, it appears surface
   parcels will reach their convective temperatures by 20z and
   scattered discrete storms will likely develop from the Arklatex,
   northeast toward the early-day outflow boundary over western TN.
   This corridor will likely experience significant convective coverage
   by the end of the period.

   Forecast wind profiles favor supercells and lapse rates should be
   sufficiently steep for very large hail with storms across the
   western half of the ENH Risk. Tornadoes are also possible, a few
   potentially strong, with discrete storms, though QLCS-type spin ups
   are also possible as a squall line matures later in the evening. As
   the squall line matures, the threat for strong, damaging winds will
   become more likely as deep-layer westerly flow increases across the
   mid-South Region.

   Farther north, strong mid-level height falls will overspread the
   upper Midwest region in advance of a northern branch short-wave
   trough that will dig into WI/IA/MO by 28/12z. It's not entirely
   clear how much moisture/instability will be drawn north ahead of the
   surface low into eastern MO/IL prior to convective initiation. 00z
   NAM suggests mid 50s surface dew points will advance across this
   region which will help destabilization; however, nearest mid 50s
   surface dew points are roughly 300 mi south across southeast
   AR/northern MS. If early-day convection disrupts this moisture flow,
   then somewhat less buoyancy may be available for convection
   extending ahead of the low into central IL. At this time wind/hail
   appear to be the primary risks with robust storms along the front.

   ..Darrow/Dean.. 03/27/2021

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