Severe Storm Outlook Narrative (AC)
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Issued by NWS
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412 ACUS01 KWNS 261258 SWODY1 SPC AC 261256 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0756 AM CDT Sun May 26 2024 Valid 261300Z - 271200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE EASTERN OZARKS TO PARTS OF THE OHIO VALLEY REGION... ...SUMMARY... Multiple rounds of severe storms are possible from the Ozarks and nearby Mississippi Valley into the Ohio Valley States today, with threats for tornadoes, severe gusts and occasional large hail. ...Synopsis... A progressive mid/upper-level pattern is apparent across the CONUS, perturbed mainly by a synoptic-scale trough initially over the central High Plains, and related cyclonic flow and embedded vorticity maxima/shortwaves over the Central States. The trough should shift eastward to WI, eastern IA, northern MO and central OK by 00Z, then over the Upper Great Lakes, Lower MI, and the lower Ohio Valley by 12Z tomorrow. At the surface, 11Z analysis showed a primary synoptic low over southwestern IA, with warm front across western IL behind convective outflow, and ahead of the outflow from central IL across southern IN to northeastern KY. The warm front should move slowly northward across the Ohio Valley through the afternoon. The cold front was drawn from the IA low southwestward to another low between ICT-PNC, then across western OK and northwest TX. This front will overtake a dryline today over northwest/west-central TX. By 00Z, the cold front should extend from a triple-point low near MSN across central IL, southern MO, southeastern OK, and north-central/central TX. By 12Z, the cold front should reach eastern IN, central KY, and western TN, becoming quasistationary southwestward over the Arklatex to central TX. The cold front will be preceded by an MCS outflow boundary over parts of the Ozarks and Ohio Valley region today. ...Ozarks/Arklatex to Ohio/Tennessee Valleys and south-central Appalachians... A complex, multi-episode severe scenario is apparent, still bearing some important mesoscale uncertainties. However, probabilities supporting an "enhanced" category appear solidly warranted over most of the original area that has not been too convectively processed, and mesoscale foci may yield denser swaths of severe (wind in particular) within that remaining broad corridor. An ongoing severe MCS, with embedded supercells and bowing segments and a history of several overnight tornadoes in the Ozarks, will continue to pose a threat for all severe hazards (including tornadoes) for at least the next several hours past midday. This threat should persist as activity exits the Ozarks, crosses the adjoining Mississippi Valley, and moves into parts of the lower Ohio Valley vicinity. Some northward expansion toward/past the warm front is possible over the Ohio Valley. This activity should only gradually outpace the most favorable buoyancy as it approaches the Appalachians, and may still have a severe threat across the higher terrain. Confidence in its persistence is heightened by the presence of at least one MCV over the eastern Ozarks, which should enhance ascent and lower/middle-tropospheric vertical wind profiles/shear on the mesoscale. Organization of these thunderstorms should continue to take advantage of forced lift with cold pools, as well as lift from internal rotational dynamics of supercells and bookend vortices, to overcome modest MLCINH this morning through midday. Inflow air will contain rich low-level moisture (surface dewpoints commonly in the upper 60s to low 70s F, locally higher), and will be diurnally destabilizing from heating and warm advection near and south of the warm front. For near-term guidance, see SPC Tornado Watch 314 and related mesoscale discussions. Details of subsequent round(s) of convection still remain to be resolved, based substantially on the character and extent of poleward return of the outflow boundary from the leading activity. As such, further modifications (some substantial) may be needed to especially western parts of the outlook areas today. One zone of relatively maximized deep ascent appears to be associated with passage of the convectively reinforced prefrontal boundary over parts of MO and southeastern KS, initially penetrating outflow air from the leading activity. This regime should continue to benefit from large-scale DCVA/ascent aloft, related to the shortwave trough and closely associated convectively induced/enhanced vorticity fields. Activity interacting with the outflow boundary`s theta-e gradient, backed near-surface winds, and maximized mesoscale vorticity in low levels should pose the greatest potential for tornadoes. However the afternoon/evening tornado risk is not yet focused tightly, due to uncertainties regarding the evolution of a boundary still being produced at this time. The general large-scale ascent, along with direct frontal lift and heating of a very moist airmass in the warm sector, also should support at least isolated to widely scattered, near-frontal convective development farther south across parts of AR and eastern OK, southwest of the outflow boundary. This activity, including supercells, should occur in an environment of stronger EML-related MLCINH for much of the day, but also, steep midlevel lapse rates, optimal moisture (surface dewpoints commonly low-mid 70s F), and weakening capping with time this afternoon. Despite some veering of near-surface winds, which may limit convergence, any activity that develops still will occur in an environment of 50-65-kt effective- shear magnitudes, and peak local MLCAPE commonly in the 3500-4500 J/kg range. Forecast hodographs and 2D hail models suggest very large hail of 3+ inches in diameter may occur with any sustained, relatively discrete storm this afternoon into early evening. Additional development is possible farther east in the warm sector, near and east of the Mississippi River and south of the MCS outflow, off the eastern rim of the EML where MLCINH is weaker but moisture and heating are strong. ..Edwards/Goss.. 05/26/2024 $$