SPC AC 071932

   Day 3 Convective Outlook AMEND 1
   NWS Storm Prediction Center Norman OK
   0232 PM CDT Wed Apr 07 2021

   Valid 091200Z - 101200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR NORTHEAST
   LOUISIANA AND CENTRAL MISSISSIPPI...

   AMENDED FOR UPGRADE TO ENHANCED RISK.

   ...SUMMARY...
   Scattered severe thunderstorms are forecast on Friday from parts of
   Oklahoma and Texas eastward through the lower Mississippi Valley. 
   The greatest risk area appears to be the lower Mississippi Valley
   where large hail to very large hail, severe gusts, and tornadoes are
   possible.

   An enhanced risk was added to portions of northeast Louisiana and
   central and southern Mississippi. A southern stream mid-level
   shortwave trough is forecast to amplify during the evening hours on
   Friday across east Texas and Louisiana. In response, the low-level
   jet is expected to increase during the evening/early overnight hours
   across Louisiana and Mississippi. This strengthening low-level jet
   should increase the threat for damaging winds and tornadoes during
   the late overnight Friday and early morning hours Saturday across
   mainly central and southern Mississippi. 


   ...Previous Discussion...

   ...Synopsis...
   The southern branch of stronger westerly mid-level flow will reside
   across the Desert Southwest eastward to the northern Gulf Coast
   states.  Embedded within this belt of flow, a disturbance is
   forecast to move from the southern High Plains to the lower MS
   Valley by late evening.  Farther north a mid-level trough over WY/CO
   will move southeastward to the lower MO Valley/Ozark Plateau by
   Saturday morning.  A previously stalled west-east frontal zone will
   slowly advance northward during the day and be draped from the TN
   Valley westward to the southern part of the Ozark Plateau.  A cold
   front will push southeast across OK and north and central TX
   primarily after sunset.  

   ...Parts of OK/TX east through parts of the South...
   Considerable variability is evident in the latest deterministic
   model suite with the NAM an outlier compared to the ECMWF showing
   some recent consistency.  Regardless, an area of low pressure is
   forecast over central TX during the day with a dryline extending
   southward across central TX.  Strong heating over north-central TX
   with relatively cool 500 mb temperatures (-15 to -16 deg C) will
   result in 700-500 mb lapse rates in excess of 8 deg C/km.  Coupling
   the lapse rates with 40+ kt effective shear, organized storms
   including supercells are likely.  Very large hail may accompany the
   stronger updrafts.  Additional storms are likely to develop during
   the evening as a LLJ develops over the lower MS Valley.  Upscale
   growth into one or more bands or MCS/s is possible with the risk for
   damaging gusts increasing.  As the low-level shear strengthens
   overnight, the risk for a couple of tornadoes may increase.  

   Farther east across the Gulf Coast states into the southern
   Appalachians, isolated to widely scattered diurnal storms are
   possible as the boundary layer destabilizes.  Although the focus for
   storms will probably be nebulous, several clusters of storms seem
   probable across a large area from the lower MS Valley into the
   western Carolinas.  Ample mid-level flow and more than adequate
   moisture/buoyancy will aid in storm organization.  Damaging gusts
   and hail are the primary threats with this activity.

   ..Bentley/Dial.. 04/07/2021

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