Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 05 2023 12:41:24 ACUS01 KWNS 051241 SWODY1 SPC AC 051240 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0740 AM CDT Tue Sep 05 2023 Valid 051300Z - 061200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE UPPER MISSISSIPPI VALLEY/LAKE SUPERIOR REGION...AND FROM MISSOURI INTO NORTHEASTERN OKLAHOMA... ....SUMMARY... Severe-thunderstorm potential will be greatest today into this evening over parts of the upper Mississippi Valley/Lake Superior region, and from Missouri into northeastern Oklahoma. Severe gusts and isolated large hail are the main concerns. ....Synopsis... In mid/upper levels, a late-summer intrusion of prevailing westerlies south of 40N has taken place across much of the CONUS. The main feature of interest is a strong shortwave trough now apparent in moisture-channel imagery from the western Dakotas southward to eastern CO. This feature should reach the eastern Dakotas, eastern NE and central KS by 00Z, then by 12Z tomorrow, extend from eastern MN to western MO. As this occurs, a weaker shortwave trough initially over parts of MO, AR and LA will shift eastward to near a BMI-MEM-GLH axis by 00Z, reaching southern Lower MI, west-central KY, middle TN, and northern AL by the end of the period. The 11Z surface analysis showed a frontal-wave low near ATY, with cold front trailing across eastern parts of SD/NE, central/ southwestern KS, the northern TX Panhandle and northeastern/north- central NM. The low should migrate to northwestern WI by 00Z, with cold front across central IA, north-central/northwestern MO, southeastern KS, central/southwestern OK, northwest TX, and southeastern/east-central NM. By 12Z, the low should move only slowly eastward across northern WI or westernmost Upper MI, while the cold front reaches eastern WI, southwestern IL, northern AR, and southeastern OK, stalling farther west near the Red River Valley and east-central/southeastern NM. ....Upper Mississippi Valley vicinity... With large-scale and deep-layer ascent increasing through the day, as the mid/upper trough approaches, scattered thunderstorms are possible as early as midday, and more certainly into the afternoon, from parts of MN into WI and Lake Superior. This activity should expand and spread eastward to northeastward over the outlook area through the afternoon, offering sporadic damaging to severe gusts and large hail. A tornado also cannot be ruled out. With this area well-removed from any EML, and the prefrontal boundary layer already moist (surface dewpoints commonly in the 60s F and locally near 70), only a few hours of diurnal heating into midday will be needed to weaken MLCINH to negligible levels. Peak preconvective MLCAPE should grow to around 2000-3000 J/kg. Increasing lift near the front and low should support development in an environment of strengthening flow aloft and deep shear. Maximum vertical shear values should be along the front where effective- shear magnitudes of 40-50 kt are expected, lessening to 25-40 kt in the open warm sector. This will support a mix of multicell and at least transient supercell modes, with upscale clustering potential. The outflow-reinforced front should act as a western delimiter for severe potential, but also, as a dominant focus for storm development. Activity should become more disorganized with eastward extent and time tonight, as it moves into weaker instability and shear. ....Mid Mississippi Valley, MO to southern Plains... Isolated to scattered thunderstorms should develop by mid/late afternoon near the front, and behind the departing zone of DNVA/ subsidence related to the lead trough. A threat for locally severe gusts exists, along with and isolated, marginally severe hail. The gust threat is greater than that for hail area-wide, but the greatest coverage appears to be probable over areas from northeastern OK into MO, matching the upgraded 15% unconditional wind probabilities. The marginal outlook area also has been expanded eastward from that region to allow more room for evening/ early overnight activity to produce strong gusts before weakening. MLCAPE will increase to 1500-2500 J/kg across portions of MO and IA, where dewpoints should remain in the 60s F, with somewhat greater shear and frontal forcing than farther southwest to support somewhat longer-lived and better convective organization. The most favorable low-level lapse rates and deepest mixed boundary layer for severe (perhaps even isolated significant) gusts appears to be across the southern Plains to northeastern OK part of the outlook area, but coverage appears to be lower overall and more uncertain in distribution. Intense diurnal heating ahead of the front will foster minimal MLCINH atop a deep, well-mixed boundary layer. The greatest subcloud depth and hottest surface temperatures are likely across the OK and northwest TX part of the corridor, where deep shear, low-level moisture and overall buoyancy will be less than farther northeast. Still, strong-severe downdrafts can occur in an environment with nearly dry-adiabatic lapse rates from surface to above the 700-mb level, 1000-1500 J/kg DCAPE, and similar values of MLCAPE. Conditional potential exists for significant/65+ kt gusts, but presently appears too isolated, uncertain and nebulously focused for a 10% coverage line. ...Edwards/Leitman.. 09/05/2023 $$ = = = 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) .