SPC 0600Z Day 2 Outlook
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
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