Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 21 2023 12:55:52 ACUS01 KWNS 211255 SWODY1 SPC AC 211254 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0754 AM CDT Thu Sep 21 2023 Valid 211300Z - 221200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE SOUTHERN AND CENTRAL PLAINS... ....SUMMARY... Severe hail and thunderstorm gusts are possible this afternoon into tonight across parts of the southern and central Plains, with slight tornado potential over the central Plains also. ....Synopsis... Across western North America, a split-flow pattern has developed in mid/upper levels, around a cut-off, synoptic-scale cyclone now centered over southeastern OR. The associated mid/upper low is progged to meander erratically southeastward to the area of the OR/ID/NV border junction through the period. A series of shortwave perturbations will pivot through the cyclone's southern semicircle, then eject eastward to north-northeastward across the Great Plains, within strongly difluent streamlines. In the southern branch, one of those perturbations is apparent in moisture-channel imagery over the TX Panhandle, Llano Staccato and southeastern NM. This feature will move roughly eastward astride the Red River Valley in TX/OK today, reaching southeastern OK and northeast TX by 00Z. Another perturbation -- initially over central CO -- will eject in pieces today, toward 00Z positions over western SD, the NE Panhandle, and northeastern CO. Meanwhile, a mid/upper low now near the Quad Cities area of IA/IL will drift slowly near its present position, contributing to general thunder potential over parts of the region, but with shear and instability each too weak for organized severe. A strengthening deep-layer cyclone off the southern Atlantic Coast will stay offshore through this period, along with any associated severe potential. At 11Z, a surface analysis showed a wavy, quasistationary frontal zone extending from a low near AKO northeastward across the western/ northern NE Sandhills to northwestern MN, and southwestward to northeastern NM. Little movement is expected with this boundary, while a lee trough becomes better-defined today over eastern portions of CO/NM. A dryline will sharpen from extreme western KS to southeastern CO and across the TX/NM line today as well. A weak surface trough/convergence zone may shift eastward across southern OK and north TX in response to the leading mid/upper perturbation and its midlevel ascent field. ....Southern High Plains to northeast Texas area... Two main rounds of convection may affect this region, each offering some severe potential: 1. Scattered thunderstorms forming this afternoon over portions of central/southern OK and north TX either side of the Red River, and moving/expanding eastward with hail/gust threats. The early stages of that process may be manifest now as a broken plume of precip and several embedded thunderstorms over the South Plains/Permian Basin regions well to the west. The associated UVV field extrapolates well to the anticipated timing of early/mid afternoon convection not far west of I-35. In addition to deep-layer/large-scale lift related to the trough aloft and its weak surface reflection, diurnal heating of a favorably moist airmass will provide lift and weaken MLCINH. Forecast soundings reasonably depict well-mixed subcloud layers caused by strong heating ahead of the UVV/cloud field, beneath 1500-2500 J/kg MLCAPE. Strong veering of wind with height, and intense upper/anvil-level winds along the northern rim of the subtropical jet, will lead to favorable deep shear for organized multicells and perhaps a few supercells. Weak low-level winds will restrict hodograph size, however, and upscale aggregation into a small MCS may occur fairly soon after initiation. 2. Isolated to widely scattered thunderstorms are expected to develop along and near the dryline/lee trough and higher terrain of northeastern NM during mid/late afternoon hours, as the large-scale DNVA behind the lead trough shifts east of the area, and strong heating removes MLCINH. This round may bridge the previous gap in outlook probabilities over parts of northwest TX and/or southwestern OK; therefore, those areas have been bridged with marginal probabilities for the time being, with some westward extension towards it of the "slight" levels. Small low-level hodographs but favorable deep shear will exist over this area as well, with high-based multicells and supercells offering isolated severe gusts and perhaps hail. As some of this activity moves east of the Caprock this evening into tonight, and eventually along and north of the outflow boundary from the first round, it will pass across a broad, 30-40-kt LLJ contributing moisture and favorable storm- relative flow for increased organization and perhaps upscale growth. Roughly 1000-2000 J/kg elevated MUCAPE and 35-45-kt effective-shear magnitudes are possible. Nocturnal severe potential with this round is more conditional and uncertain, given the presence of diabatic surface cooling and low-level stabilization from prior outflow. However, some associated hail and/or gusts may penetrate to the surface near severe limits, in areas affected by earlier activity. ....Central Plains... An ongoing cluster of thunderstorms and trailing precip -- over northeastern NE and southeastern SD - is leaving behind an outflow boundary southeast of the front, across the northern Sandhills region. This boundary may drift somewhat northward -- and in tandem with the front to the north/west -- focus thunderstorm development over parts of southern SD and northern NE from midday into the afternoon. Some of the earliest convection may be elevated north of the residual outflow, but with time, increasingly surface-based effective-inflow parcels will be available, and activity should build/shift into more of the undisturbed warm sector as the mid/upper trough approaches. Supercells and bowing clusters are possible, with damaging gusts, hail (some greater than 2 inches in diameter), and the potential for a few tornadoes -- the latter depending on supercell maturation/maintenance in an environment with favorably enlarged low-level hodographs. More-isolated convection may form farther south across southern NE or extreme northern KS portions of the outlook area, also with supercell potential. Surface dewpoints in the lower to mid 60s F are expected across the undisturbed warm sector by afternoon, combining with steep low/ middle-level lapse rates to yield areas of MLCAPE in the 2000-2500 J/kg range (locally/briefly higher). A fetch of strong, southeasterly flow in the lowest couple km of the boundary layer will result in well-curved hodographs with effective SRH in the 150-300 J/kg range. Modest mid/upper winds (in an area of substantial difluence aloft) keep effective-shear magnitudes generally 40 kt or less in forecast soundings, despite the backed low-level flow. This indicates heavy-precip character and/or quick transition to messy, clustered modes may occur. However, in the meantime, sounding analogs and 2D hail models suggest significant hail, especially with any mature supercell(s). Clustering will boost local severe-gust potential through cold-pool forcing. Activity should weaken overnight as it moves into less-unstable air over eastern NE; however, slightly more room has been added on the east side of the outlook for that process to finish. ...Edwards/Gleason.. 09/21/2023 $$ = = = 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) .