Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 14 2023 12:53:57 ACUS01 KWNS 141253 SWODY1 SPC AC 141252 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0752 AM CDT Mon Aug 14 2023 Valid 141300Z - 151200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF KENTUCKY AND TENNESSEE...EXTENDING EASTWARD TO THE MID-ATLANTIC REGION... ....SUMMARY... Strong/locally severe thunderstorms are expected today and into this evening across portions of the Midwest, Kentucky and Tennessee, extending eastward to the Mid-Atlantic Region. ....Synopsis... In mid/upper levels, ridging will persist from the Pacific Northwest to the southern High Plains, then eastward over the Gulf Coast States. However, highest heights will shift from the Gulf and Pacific Northwest regions into areas of the central Rockies to northern Great Basin, where a col has been noted between lows over the Upper Midwest and Pacific near southern CA. This will occur as the former low -- apparent in moisture-channel imagery over southwestern MN -- continues digging southeastward across IA, reaching northwestern IL by 00Z. By then, the accompanying through should extend across portions of WI, MO and OK, before reaching an axis near MBS-EVV-PAH-JBR around 12Z. At the surface, 11Z analysis showed an occluded low over southern MN, related to the northern-stream trough, with separate triple- point low near STL. A cold front was drawn from there southwestward over southern MO, southeastern OK, parts of north-central/west- central TX, and southeastern NM. A warm front extended over southern parts of IL/IN to southern WV, becoming quasistationary over central VA. The warm front will move northward over more of WV/VA through the afternoon. By 00Z, the triple point low should reach the PKB area, with cold front across eastern parts of KY/TN, northern parts of AL/MS, northern LA, and south-central/southwest TX. By 12Z, with a low in the corridor from northeastern PA to near NYC, the cold front should extend across southeastern PA, northern/ western VA, western NC, northern GA, and southern MS, while decelerating over south TX. ....Mid-Atlantic to TN/KY... Several episodes of strong-severe thunderstorms are possible across this corridor, with enough spatial overlap between them to warrant maintaining a continuous area of severe probabilities similar to those outlined in the previous outlook. A broad area of ongoing precip with embedded thunderstorms -- initially over portions of KY/TN -- largely should remain non-severe while shifting eastward into the central Appalachians today. However, increasing deep-layer flow and favorable low-level moisture will precede the associated UVV plume, along with muted diurnal heating of a moist airmass with weak CINH from parts of WV across the nearby Blue Ridge and Piedmont of VA, eastward toward Chesapeake Bay and over the Delmarva Peninsula. This may support strong-severe gusts and hail with thunderstorms forming during the afternoon over higher terrain and shifting eastward to northeastward toward the coast. Of note: progs with RAP basis -- including HRRR -- appear to be mixing and drying the boundary layer excessively over the Piedmont and coastal plain, leading to potential underproduction and/or muting of convection. After several hours of warm advection and increasing surface heating behind the morning activity, the best organized overall severe threat should develop this afternoon from parts of middle/eastern TN to southern OH/WV, and shift eastward over the adjoining Appalachians. MLCAPE should range from around 2000-3000 J/kg across middle/eastern TN -- where strongest heating/recovery is probable but deep flow/shear somewhat weaker -- to around 500-1000 J/kg near the Ohio River and northern WV. Effective-shear magnitudes around 40-50 kt should be common over eastern KY and WV, with modest 0-1-km SRH but around 150-250 J/kg effective SRH. This suggests an eastward-moving corridor of convection containing blend of supercells, multicell clusters and quasi-linear segments with sporadic bow/LEWP features will be possible, with accompanying threat for damaging to severe gusts, isolated hail, and at least marginal/conditional tornado potential. The northern part of this convective plume may persist overnight into an airmass east of the mountains -- across parts of the Piedmont and coastal plain of the Mid-Atlantic -- supportively recovered via theta-e advection from any earlier activity not far to the south and east. The parameter space should be favorable for severe well after dark, with 40-50 kt effective-shear magnitudes, locally enlarged hodographs with 200-300 J/kg effective SRH, and around 1000-2000 J/kg MLCAPE. Convective coverage is uncertain, but all hazards will be possible. ....Portions of the Midwest... Scattered thunderstorms should develop from midday through the afternoon, in a broad arc extending from just southeast of the occluded low toward the eastward-moving triple point, along/ahead of a combined occluded front/inverted trough. This activity should shift eastward to northeastward across the northwestern spur of the "marginal" outlook area, offering isolated severe hail, damaging gusts, and perhaps a tornado or two. With abundant clouds and some areas of precip possible across the region, diabatic heating will be present but considerably muted. Nonetheless, a combination of warm advection just ahead of the occluded front, and midlevel DCVA/cooling preceding the strong shortwave trough, will contribute to development of modest but adequate surface-based buoyancy, prior to frontal passage. Modified forecast soundings suggest surface dewpoints in the 60s F supporting MLCAPE generally in the 500-1000 J/kg range. Mid/upper-level flow and deep shear will increase southeastward across the region, away from the immediate vicinity of the mid/upper trough, despite the presence of weak low-level flow. Still, effective-shear magnitudes mostly below 35 kt should limit organized severe potential to marginal levels by the time the boundary layer destabilizes enough for surface-based parcels. ...Edwards/Dean.. 08/14/2023 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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