SPC AC 210728

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0228 AM CDT Wed Apr 21 2021

   Valid 231200Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
   SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY...

   ...SUMMARY...
   Isolated to scattered severe thunderstorms should occur Friday into
   Friday night from parts of the southern Plains into the lower
   Mississippi Valley. Large to very large hail, damaging winds, and
   isolated tornadoes all appear possible.

   ...Synopsis...
   An upper trough should eject eastward from the Four Corners region
   and Southwest across the southern/central Plains on Friday,
   eventually reaching the lower/mid MS Valley by the end of the
   period. An EML with associated steep mid-level lapse rates will
   likely be present over much of OK/TX. A low-level jet should
   continue to transport low-level moisture northward through the day
   across the southern Plains, with a warm front potentially reaching
   as far north as the OK/KS border vicinity. At the surface, a weak
   low over the TX Panhandle Friday morning is forecast to develop
   southeastward to the vicinity of southwestern OK/western north TX by
   Friday evening while gradually deepening. A dryline should extend
   southward from this low across portions of west/central TX. The
   surface low should continue developing eastward to the lower MS
   Valley late Friday into early Saturday morning, with a composite
   dryline/cold front sweeping eastward across central/east TX. Rich
   low-level moisture should advance northward ahead of the low/front
   across much of LA and southern MS late in the period.

   ...Southern Plains...
   Large-scale ascent associated with the shortwave trough should
   overspread the warm sector across OK/TX by late Friday morning into
   the early afternoon. As the cap erodes, robust convective
   development appears likely along the eastward-mixing dryline. There
   is still some uncertainty regarding the position of the dryline by
   Friday afternoon across the southern Plains. Regardless, the
   presence of rather steep mid-level lapse rates, increasing low-level
   moisture, and diurnal heating should support moderate to locally
   strong instability across the warm sector in OK/TX by Friday
   afternoon. Deep-layer shear across this region appears strong enough
   to support organized severe thunderstorms, including supercells.
   Current expectations are for convection to initially develop along
   or just east of the dryline, and subsequently move
   east-northeastward. Large hail appears likely with this initial
   activity, and isolated very large hail (2+ inch diameter) may also
   occur. A veering/strengthening wind profile suggests some threat for
   isolated tornadoes with any storms that can remain semi-discrete
   through early Friday evening. Eventually, upscale growth may occur
   along the cold front, with a damaging wind threat potentially
   continuing into parts of east TX late Friday. Other isolated severe
   storms may develop in the low-level warm advection regime in the
   open warm sector along parts of the middle/upper TX Coast into east
   TX through the day, posing a risk for all severe hazards.

   ...Sabine River Valley into the Lower Mississippi Valley...
   A south-southwesterly low-level jet is forecast to strengthen
   considerably from east TX into LA and southern MS Friday
   evening/night. Surface-based convection appears increasingly
   probable across this area as low-level moisture quickly increases.
   The EML may advect eastward over this region, and NAM forecast
   soundings show a favorable thermodynamic and kinematic environment
   to support organized severe thunderstorms along/south of the warm
   front. All hazards, including tornadoes, will be possible through
   Friday night into early Saturday morning. The best severe threat may
   be focused with the eastward-developing low-level jet, but
   additional severe storms along the cold front may also occur. Even
   with this seemingly favorable setup for severe thunderstorms, the
   potential exists for afternoon storms with low-level warm advection
   to slow the northward advance of the warm front and limit
   instability. Once uncertainty regarding the evolution of these
   afternoon storms is better resolved, greater severe probabilities
   for tornadoes may need to be included for parts of the Sabine River
   Valley vicinity.

   ..Gleason.. 04/21/2021

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