SPC AC 221730

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1230 PM CDT Thu Apr 22 2021

   Valid 231200Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE SOUTHERN
   PLAINS EASTWARD TO THE CENTRAL GULF COAST STATES...

   ...SUMMARY...
   Isolated to scattered severe thunderstorms are expected Friday into
   Friday night from parts of the southern Plains into the lower
   Mississippi Valley/central Gulf Coast states. Large to very large
   hail, damaging winds, and isolated tornadoes all appear possible.

   ...Synopsis...
   As broad cyclonic flow around a Canadian Maritimes upper low
   gradually departs from the Northeast, the primary feature of
   interest will be a short-wave trough moving out of the Intermountain
   West into/across the Plains states.  With time, the southern portion
   of this trough -- advancing more quickly than farther north -- will
   cross the Ozarks/Arklatex and then the lower Mississippi Valley
   overnight, resulting in a gradually more negative tilt to the
   larger-scale trough.

   At the surface, a low is progged to reside over the southern High
   Plains/in lee of the southern Rockies.  As the upper trough
   advances, surface low pressure will shift eastward in a loosely
   defined manner, while a dryline mixes eastward into central Texas
   during the day and a warm frontal zone extends eastward into the
   central Gulf Coast states.  By the end of the period, a cool front
   will be sweeping across Texas in the wake of the primary surface low
   expected to reside in the vicinity of the Arklatex by 24/12z.

   ...Southern Plains to the central Gulf Coast states...
   A somewhat complex convective scenario is apparent Friday, as upper
   troughing moves into/across the south-central U.S., along with an
   associated weak/rather ill-defined surface pattern.  

   As the upper trough advances, and a surface high shifts eastward
   into the western Atlantic, increasing southerly low-level flow into
   the south-central states will advect Gulf moisture northward beneath
   steep lapse rates aloft overspreading the southern Plains through
   the day.  As a dryline mixes quickly eastward into central Texas
   during the first half of the period, large-scale ascent and an
   increasingly favorable thermodynamic environment should support
   central Texas storm development.  Strong/veering winds with height
   across the area suggests that storms will quickly acquire rotation.
   While CAMs may be a bit early and aggressive initially, with respect
   to storm coverage, isolated supercells -- accompanied by a risk for
   very large hail during the afternoon -- will likely congeal into a
   cluster of more numerous storms, moving eastward across northeastern
   Texas/the Arklatex region by late afternoon/early evening.  

   Meanwhile, a second area of convective development is expected
   farther north, from northwest Texas and eastern portions of the
   Texas Panhandle area east-northeastward across northern Oklahoma and
   southeastern Kansas into Missouri, near a secondary low and its
   associated/weak warm front.  A drier boundary layer across this area
   -- and thus correspondingly lesser CAPE -- should limit severe risk
   a bit.  However, large hail -- particularly over the northwestern
   Texas and Oklahoma portions of the area -- remains apparent.

   By evening, as convection congeals across the Arklatex area, risk
   for damaging winds will likely become a bit more widespread.  CAMs
   suggest that this storm cluster -- aided by very favorable shear --
   could organize into at least a loosely organized bowing MCS during
   the evening, which would shift eastward across the lower Mississippi
   Valley and later the central Gulf Coast states overnight, along a
   warm frontal zone.  While somewhat limited low-level moisture -- and
   thus possibly a very weakly stable boundary layer -- is expected,
   risk for at least locally damaging winds is apparent, particularly
   if convection organizes in an upscale manner as anticipated. 
   Additionally, an intensifying southerly low-level jet implying
   strong warm advection may support a gradual increase in cellular --
   and likely rotating -- convection ahead of the main cluster of
   storms.  Given the presence of the warm front, and increasingly
   favorable veering/increasing of the low-level flow field with
   height, risk for a couple of tornadoes also remains apparent, given
   what should be a not-prohibitively-stable boundary layer in the
   vicinity of, and to the south of, the warm front.

   While CAMs suggest that a fairly well-organized bowing convective
   line may be shifting across Alabama late in the period, a slightly
   more stable boundary layer may preclude more widespread wind risk,
   and thus an eastward expansion of the SLGT risk area will not be
   initiated at this time.

   ..Goss.. 04/22/2021

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