ZCZC SPCSWOD48 ALL
   ACUS48 KWNS 220846
   SPC AC 220846

   Day 4-8 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0346 AM CDT Mon Mar 22 2021

   Valid 251200Z - 301200Z

   ...DISCUSSION...
   ...Day 4/Thu - Eastern TX to the Central Gulf Coast States...

   An upper shortwave trough over the Rio Grande/northern Mexico will
   strengthen and become negatively tilted as it ejects northeast
   across the Arklatex through Thursday evening, and to the Ohio Valley
   by Friday morning. Intense shear will overspread the south-central
   and southern U.S. ahead of the trough. Furthermore, strong low-level
   warm advection will result in a broad warm sector ahead of a
   deepening surface low and eastward-advancing cold front from the
   Sabine Valley eastward across the central Gulf coast/TN Valley
   vicinity.

   Differences in the evolution of the surface low across the lower MS
   and OH Valleys are still apparent in medium-range guidance. This is
   mainly resulting in uncertainty in the position of the surface low
   and cold front Thursday morning, and how far east each of these
   features progresses by Friday morning. As a result, changes in
   severe probabilities, especially on the western and northeastern
   edges, are likely in the coming days. Nevertheless, weak to moderate
   instability will overlap with favorable shear parameters and an
   overall supportive pattern for severe convection. A couple of rounds
   of severe storms could be possible, as some warm-sector development
   may occur across the Lower MS Valley before a QLCS develops along
   the surging cold front during the evening/nighttime hours. All
   severe hazards will be possible with discrete warm-sector
   supercells. Potential for damaging gusts and tornadoes will becoming
   preferential with any upscale development along the cold front.

   ...Day 5/Fri - Southeast to Mid-Atlantic...

   Timing and intensity differences continue with respect to the upper
   shortwave trough and surface features on Day 5/Fri. This will impact
   severe potential from parts of the FL Panhandle into southern GA and
   the SC/NC/VA Piedmont vicinity. Some severe potential will exist,
   but confidence is low. The ECMWF maintains a negatively tilted,
   compact shortwave trough lifting northeast from the OH Valley into
   the Northeast, while the surface cold front shifts east of the
   Appalachians during/just after peak heating. Meanwhile the GFS is
   much faster and lower-amplitude, resulting in poorly aligned
   thermodynamics and kinematics. While some severe threat is possible,
   confidence is too low to include probabilities at this time.

   ...Days 6-8/Sat-Mon...

   A larger-scale upper trough should develop eastward from the
   northern/central Plains through the eastern U.S. during the weekend.
   The cold front from Day 4-5/Thu-Fri will stall across southern GA to
   the central Gulf coast and southern TX. Low level warm advection
   associated with continued southwesterly deep-layer flow will allow
   the position of the boundary to fluctuate through the weekend. This
   could possibly bring richer Gulf moisture back inland across the
   coastal plain from TX to AL northeastward across southern GA and the
   Carolinas, before a developing coastal low near the Mid-Atlantic
   sweeps another cold front through the Southeast. Some severe
   potential could accompany this frontal passage, but confidence in
   this scenario is low, precluding probabilities at this time.

   ..Leitman.. 03/22/2021

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