Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 30 2023 05:59:16 ACUS01 KWNS 300559 SWODY1 SPC AC 300557 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1257 AM CDT Sat Sep 30 2023 Valid 301200Z - 011200Z ....THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF THE UPPER MS VALLEY...CENTRAL AND SOUTHERN HIGH PLAINS...AND WESTERN INTO CENTRAL UTAH... ....SUMMARY... Isolated severe gusts may occur across portions of the southern/central High Plains and western into central Utah this afternoon. An instance or two of severe hail also cannot be ruled out across portions of the Upper Mississippi Valley. ....Synopsis... A broad, pronounced mid-level trough will continue to amplify over the Interior West today as upper ridging builds over the CONUS east of the Rockies. Beneath the mid-level trough, a large surface cyclone should become established over the Great Basin, with the surface cyclonic flow extending as far northeast as the northern Plains, where the northward advection of low-level moisture will be taking place. Strong tropospheric wind fields overspreading a deep, mixed, but marginally buoyant boundary layer will support scattered strong to isolated severe storms across portions of the Great Basin, eastward to the central and southern High Plains. At the terminus of a Great Plains low-level jet, warm-air advection may support isolated strong to severe thunderstorms given the presence of elevated buoyancy. ....Central and Southern High Plains... A mid-level impulse embedded within the broader upper trough will traverse the southern Rockies and approach the central High Plains through the afternoon, providing locally stronger forcing for ascent to support scattered thunderstorms. These storms will develop atop a well-mixed, dry boundary layer extending up to 600 mb in spots. Though hodographs are not overly long, modestly stronger flow aloft, combined with downward momentum transport driven by rainwater evaporation in the deep and dry sub-cloud layer, should support at least isolated severe gusts with the stronger storms during the late afternoon and evening hours. ....Western into central Utah... A belt of 50-70 kt 500 mb south-southwest to north-northeasterly flow will accompany the leading edge of the broad upper trough during the afternoon, and is poised to overspread a dry, well-mixed boundary layer. Through the afternoon, strong southerly flow associated with the aforementioned surface cyclone may encourage dewpoints to approach the 35-40 F range, resulting in marginal (i.e. less than 500 J/kg) SBCAPE. Though buoyancy should be quite scant, strong forcing along a surface cold front will foster a line of low-topped thunderstorms immediately preceded by strong (albeit meridional, unidirectional) speed shear and accompanying elongated hodographs. When also considering the impacts of evaporative cooling in a dry boundary layer with steep low-level lapse rates, organized cells or small bows within the parent convective line may produce a couple of severe gusts by late afternoon. ....Upper Mississippi Valley... By mid to late afternoon, the gradual approach of an upper trough will encourage the strengthening of a low-level jet over the Plains states. A plume of low-level moisture (including low to mid 60s F surface dewpoints) will continue to advect northward to a diffuse surface-850 mb baroclinic zone draped across the northern Plains into the Upper MS Valley. Isentropic ascent along this boundary will be sufficient for scattered thunderstorm development by late afternoon. By this point, 7-8 C/km mid-level lapse rates will overspread the low-level moist axis, boosting MLCAPE to over 2000 J/kg. Forecast soundings depict short hodographs with appreciable low-level curvature, suggesting that any storms that form will be multicellular in nature, capable of producing a couple instances of severe hail by late afternoon or early evening. ...Squitieri/Lyons.. 09/30/2023 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .