SPC AC 170552

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1252 AM CDT Wed Mar 17 2021

   Valid 171200Z - 181200Z

   ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS FROM NORTHEAST
   LOUISIANA ACROSS NORTHERN MISSISSIPPI INTO NORTHWEST ALABAMA...

   ...SUMMARY...
   A regional outbreak of severe storms is expected today and tonight
   across portions of the lower Mississippi Valley into Alabama.
   Widespread severe storms capable of producing tornadoes (several of
   which may be intense), very large hail and intense damaging wind
   gusts are expected. More than one round of severe storms are
   possible across parts of Mississippi into Alabama during the
   afternoon into the overnight hours.

   ...Lower Mississippi Valley to Alabama...

   Early-morning water-vapor imagery depicts a well-defined upper low
   over central NM moving east toward the TX Panhandle. This feature
   will eject across OK into the Ozark Plateau by the end of the
   period. Intense large-scale forcing for ascent is currently
   spreading across the southern High Plains region where deep
   convection is currently expanding in areal coverage along the dry
   line/Pacific front as it encounters western edge of moisture plume.
   This boundary will remain active leading into the start of the day1
   period.

   Strongly diffluent flow aloft will overspread the lower MS
   Valley/central Gulf States during the day as 80kt+ 500mb flow
   overspreads increasingly moist/buoyant warm sector. Latest model
   guidance suggests several LLJ segments will increase ahead of the
   trough, each likely modulating bouts of organized severe
   thunderstorm clusters/supercells. The primary synoptic low is
   forecast to track across northeastern OK into southern MO.
   Significant elevated convection will be noted north of the warm
   front but ample surface-based buoyancy/shear south of the front
   should prove favorable for supercells as one LLJ segment focuses
   across AR into southeast MO ahead of this feature. At the same time,
   a secondary LLJ will strengthen across MS during the day and become
   the primary LLJ into the overnight hours over northwestern AL/middle
   TN. This secondary LLJ appears to be responding to a mid-level speed
   max that will translate across south-central TX into southern LA
   during the late evening hours.

   Of particular interest is the boundary-layer moisture that is now
   present along the Gulf Coast where lower 70s surface dew points are
   observed. This air mass will advance north during the day and
   diurnal heating will allow moderate instability to develop along a
   corridor from southern LA through central MS into western AL where
   SBCAPE should be on the order of 2500 J/kg. While the primary 500mb
   speed max will likely maintain convection along the surging Pacific
   front over southeast TX, an uncapped, buoyant air mass downstream
   over MS/AL should prove favorable for discrete supercell development
   ahead of the primary short wave. Forecast soundings across the HIGH
   Risk exhibit profiles favorable for strong, long-track tornadoes,
   and this is especially true from northeast LA through northern MS
   into northwest AL.

   ..Darrow/Lyons.. 03/17/2021

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