ZCZC SPCSWOD48 ALL
   ACUS48 KWNS 130958
   SPC AC 130958

   Day 4-8 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0358 AM CST Sat Mar 13 2021

   Valid 161200Z - 211200Z

   ...DISCUSSION...
   Plenty of spread/differences continue between the GFS and ECMWF for
   the medium-range period -- even as early as Days 4-5.  While the
   broad/overall picture remains fairly clear that a notable increase
   in severe threat will occur perhaps as early as late Tuesday
   afternoon (Day 4) near a southern Plains dryline, and then
   continuing Day 5/Wednesday (lower Mississippi and possibly the lower
   Ohio Valleys east across the central Gulf Coast states/Mid South),
   and potentially Day 6/Thursday (southern Appalachians/Carolinas).

   The differences amongst the models are centered around evolution of
   the next upper system moving out of the western U.S. at the start of
   the period.  The ECMWF depicts a closed low -- strengthening with
   time as it ejects, and thus the feature advances more slowly than
   the GFS, which depicts a more progressive, open wave.  This has
   substantial implications for timing/location of surface features
   through the first few days of the period -- with these differences
   becoming great enough by Day 6 that no meaningful assessment of
   areas of possible risk can be confidently highlighted.

   With the ECMWF's pattern evolution very similar to the EC's ensemble
   mean, and also similar to the UKMET solution, will lean a bit toward
   a slower/less progressive evolution and thus a slightly more
   westward forecast.  Day 4 (Tuesday), some conditional risk for
   mainly hail may exist as far west as a northwestern Texas/western
   Oklahoma dryline during the early evening, with some hail risk
   spreading across parts of Oklahoma and North Texas overnight. 
   However, with this risk more conditional, will opt not to include a
   risk area for Tuesday at this time.

   By Day 5/Wednesday, a more substantially unstable environment,
   encompassing a large area, is likely, as a moist Gulf airmass
   streams northward, beneath rather steep lapse rates advecting
   eastward.  With an eastward moving surface cyclone and attendant
   warm and cold fronts focusing ascent, and the likelihood for broad
   warm advection/QG ascent within the expanding warm sector, scattered
   severe storms appear likely even given timing/location differences
   of surface features amongst the models.  While a more concentrated
   area or areas of severe weather may evolve Wednesday, those details
   -- and any associated outlook highlights -- will require discernment
   in subsequent outlooks nearer the event.  For now, a large/broad 15%
   risk area will be introduced, representing a large envelope of
   all-hazards severe potential.

   Day 6, model differences become substantial enough that -- while
   severe weather risk may continue, no areas will be highlighted due
   to the predictability concerns.

   ..Goss.. 03/13/2021

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