ZCZC SPCSWOD48 ALL
   ACUS48 KWNS 200858
   SPC AC 200858

   Day 4-8 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0358 AM CDT Tue Apr 20 2021

   Valid 231200Z - 281200Z

   ...DISCUSSION...
   Low-level moisture return appears likely across parts of the
   southern Plains on Day 4/Friday as lee cyclogenesis occurs across
   the southern High Plains ahead of an ejecting upper trough.
   Medium-range guidance is now in reasonably good agreement that this
   shortwave trough will eject eastward over the southern Plains on
   Friday. Increasing low-level moisture to the east of a dryline/cold
   front and steep mid-level lapse rates should support at least
   moderate instability in a corridor across parts of OK and TX by
   Friday afternoon. Strong mid-level flow and related deep-layer shear
   appear favorable for organized severe storms, including supercells.
   The low-level moisture should be somewhat greater in TX compared to
   OK. As mid-level height falls/ascent overspread the warm sector,
   robust convective development appears likely, with at least isolated
   severe storms posing a threat for all severe hazards potentially
   spreading eastward towards east/southeast TX and parts of the lower
   MS Valley Friday evening/night in a strong low-level warm advection
   regime.

   It now appears probable that a surface warm front will lift
   northward across parts of the central Gulf Coast vicinity on Day
   5/Saturday as the shortwave trough continues eastward across the
   Southeast. A strong low-level jet may tend to shift eastward quickly
   during the day across this region, with low-level flow veering more
   to westerly in its wake. Still, increasing low-level moisture south
   of the front, a favorable/strengthening wind profile through mid
   levels, and some eastward advection of an EML supporting potentially
   moderate instability all suggest a threat for organized severe
   storms. A 15% area has been introduced for Saturday from parts of
   southern LA/MS into southern AL/GA and the FL Panhandle where the
   best potential for surface-based storms exists. At least some severe
   threat may continue farther east towards coastal GA/SC late
   Saturday, but confidence was not high enough to include these areas
   in the 15% delineation.

   Deterministic medium-range guidance is in surprisingly good
   agreement in depicting another upper trough ejecting from the
   western CONUS across the Plains around Day 7/Monday into Day
   8/Tuesday. Substantial low-level moisture return may occur ahead of
   this feature across a part of the southern Plains. Assuming
   sufficient low-level moisture does indeed materialize, then the
   presence of steep mid-level lapse rates and favorable timing of the
   ejecting upper trough suggest an organized severe risk may develop
   on Tuesday across some portion of OK/TX. A 15% severe area may need
   to be introduced for next Tuesday as soon as the next outlook update
   if run-to-run model consistency continues.

   ..Gleason.. 04/20/2021

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