SPC AC 220558

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1258 AM CDT Thu Apr 22 2021

   Valid 231200Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS 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.

   ...Southern Plains to the Lower Mississippi Valley...
   An upper trough will eject eastward Friday from the Southwest and
   northern Mexico across the southern Plains, eventually reaching the
   lower/mid MS Valley by the end of the period. At the surface, a weak
   low initially centered over the TX Panhandle should develop
   southeastward through the day, likely reaching the southwestern OK
   and western north TX vicinity by Friday evening. A dryline extending
   southward from this low should mix eastward across west/central TX
   before stalling near the I-35 corridor by the early evening. From
   Friday evening into early Saturday morning the surface low should
   develop eastward towards AR, while an attendant cold front sweeps
   southeastward across OK/TX. A marine warm front should also lift
   northward across parts of east TX, LA, and southern MS/AL from
   Friday afternoon and continuing into the overnight as a strong
   (40-50 kt) south-southwesterly low-level jet moves quickly eastward
   across these areas.

   There is still a fair amount of uncertainty how much convection will
   develop along the dryline in central TX/southwestern OK through
   Friday afternoon. Most guidance suggests the possibility of a
   bimodal distribution of storms across the southern Plains, with the
   triple point in southwestern OK late Friday afternoon and a
   low-level warm advection regime in east/coastal TX through the
   daytime hours being favored zones of initiation. Still, there
   appears to be at least some chance for additional robust
   thunderstorm development posing mainly an isolated large to very
   large hail threat farther west along the dryline by peak afternoon
   heating, as modest large-scale ascent with the upper trough
   overspreads the southern Plains. Based on latest guidance being in
   better agreement on the placement/eastward extent of the dryline,
   severe probabilities have been adjusted slightly eastward in central
   TX.

   Any thunderstorms that form near the triple point in southwestern OK
   will likely become severe supercells quickly. The main threat
   initially should be large to perhaps very large (2+ inch diameter)
   hail given steep mid-level lapse rates around 8 C/km, sufficient
   low-level moisture and diurnal heating to support 1500-2500 J/kg of
   MLCAPE, and effective bulk shear of 40-50 kt. By early Friday
   evening, a modest strengthening of the low-level flow and gradually
   increasing low-level moisture may support an isolated tornado threat
   with any storms that can remain discrete. Although details remain
   unclear, there also appears to be some potential for upscale growth
   into a line or bow Friday evening/night as the cold front advances
   southeastward. Damaging winds would become a greater risk across
   parts of central OK into north TX if this upscale growth occurrs.

   Farther east, convection may develop by early Friday afternoon from
   parts of coastal/east TX into the ArkLaTex in association with a
   30-40 kt low-level jet. This activity would occur in tandem with the
   return of rich Gulf moisture across these regions. Thunderstorms may
   have the potential to become elevated north of the marine warm front
   as they move northeastward. Even so, boundary-layer instability is
   expected to gradually increase through the day along/south of the
   warm front, and deep-layer shear will remain strong. The potential
   for large hail will exist with any supercells that can develop
   initially, and isolated very large hail may occur as steep mid-level
   lapse rates overspread this region. With time and continued
   low-level warm advection occurring, storms will probably form into
   one or more clusters as they move eastward into LA and southern MS
   Friday evening/night. The damaging wind threat should increase as
   this mode transition occurs, but isolated tornadoes also appear
   possible with any convection that can remain at least semi-discrete,
   as 0-1 km shear associated with the low-level jet is forecast to
   remain quite strong.

   Some consideration was given to greater hail probabilities
   along/east of the dryline in OK/TX, but confidence in more than
   isolated coverage of thunderstorms remains low. The best potential
   for initiation appears to be near the triple point in southwestern
   OK, but it may be that only one or two supercells form over this
   area. Across east TX into the lower MS Valley, there may be a
   greater and somewhat conditional threat for tornadoes given the
   forecast low-level shear, but the probable mode transition from
   supercells to a messy line may limit this potential somewhat. There
   may need to be a higher categorical outlook with greater hail and/or
   tornado probabilities introduced when these mesoscale uncertainties
   become a bit clearer.

   ..Gleason.. 04/22/2021

   CLICK TO GET WUUS02 PTSDY2 PRODUCT

   NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 1730Z