SPC AC 161620

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1020 AM CST Mon Dec 16 2019

   Valid 161630Z - 171200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS NORTHEAST LA TO
   CENTRAL MS...

   ...SUMMARY...
   Numerous to potentially widespread severe thunderstorms are
   anticipated across parts of the Lower Mississippi Valley and Deep
   South with tornadoes and damaging winds as the primary hazards. The
   peak tornado risk is expected through early evening, centered on
   northern and central Louisiana into central Mississippi.

   ...Lower MS Valley and Deep South...
   Primary changes this outlook are to increase tornado and wind
   probabilities across parts of LA/MS. In addition, the SLGT risk has
   been expanded north for wind into middle TN near the expected
   surface cyclone track.

   Gradual expansion of surface-based thunderstorms is expected across
   the Sabine and Lower MS Valleys, along/ahead of the surface cold
   front. Activity should grow in coverage and move across the
   Mid-South and MS/AL regions through this evening into tonight. 
   Several tornadoes will be possible across the entire region, but
   with greatest potential in a corridor from northern/central LA
   across central MS where semi-discrete supercells are most probable,
   and should access the most favorable CAPE/shear parameter space. 
   Damaging gusts should be the most common convective event area-wide.
   Severe hail is most probable over southwestern parts of the outlook
   area, essentially collocated with greatest tornado probabilities.  

   12Z soundings sampled the pronounced elevated mixed layer overlying
   the rich low-level moisture plume (characterized by upper 60s to low
   70s surface dew points) emanating northeast from the western Gulf.
   Abundant cloud coverage will be a limiting factor, but moderate
   surface-based buoyancy characterized by MLCAPE of 1000-2000 J/kg
   will persist in the ambient environment ahead of the front across LA
   through parts of MS. Relatively backed surface winds, beneath a
   40-60-kt southwesterly LLJ, will contribute to enlarged hodographs
   and 300+ effective SRH.  

   Given potential for sustained warm-sector supercells, such as
   progged by the HRRR, HRRR-P and WRF-ARW, such an environment will
   support a risk for multiple significant tornadoes. Otherwise,
   regardless of warm-sector convective density, a band of
   thunderstorms should evolve close to the front and shift across the
   remainder of the outlook area through the period. This activity will
   offer a risk for numerous to widespread damaging winds and embedded
   QLCS tornadoes. It will eventually encounter gradually weaker
   low-level and mid-level lapse rates and less inflow-layer moisture
   content with northeast/eastern extent. This should result in a
   somewhat lesser threat for damaging winds and tornadoes tonight.

   ..Grams/Wendt.. 12/16/2019

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