Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 20 2023 12:51:04 ACUS01 KWNS 201250 SWODY1 SPC AC 201249 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0749 AM CDT Sun Aug 20 2023 Valid 201300Z - 211200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE DESERT SOUTHWEST REGION... ....SUMMARY... Some tornado potential should develop today over the Imperial/lower Colorado River Valleys and vicinity, as what remains of Hurricane Hilary approaches then passes the area. ....Synopsis... The large-scale, mid/upper-level pattern will remain dominated by a large anticyclone covering most of the CONUS from the Rockies eastward and south of about 45N, with a 600+ decameter 500-mb high meandering around the MKC region. To its west, a long-lived, cut-off, cold-core cyclone -- its center apparent in moisture- channel imagery about 150 nm south-southwest of SFO, will move slowly and erratically northward through the period, with the low itself likely remaining offshore. Embedded in the intervening, broad, southerly flow belt, Hilary is forecast to weaken rapidly through the period as it accelerates across northern Baja, southern CA and NV. See latest NHC advisories and discussions for more information on tropical watches/warnings and forecast track/ intensity guidance for Hilary. ....Desert Southwest... A threat for supercells, with potential for a couple tornadoes, is expected to develop through the local mid/late morning hours over southern parts of the outlook area and shift/expand northward through the afternoon. Isolated severe gusts also may occur in convection away from the closed circulation of Hilary (and thus not treated as damaging wind with the tropical cyclone). Rapid cell motions and fast flow not far above the surface will support some downward momentum transfer and downburst potential, despite the very moist environment. This pattern superimposes the meso-alpha scale wind enhancements related to Hilary with ambient/gradient flow between the major Plains anticyclone and the Pacific cut-off low. The result is a broad fetch of strong, southerly to southeasterly flow in low/middle levels. Associated favorably enlarged low-level hodographs are already spreading northward into the region, per VWP data from Yuma radar, and will continue to do so. As such, and despite the rapidly weakening state of the tropical cyclone itself through the day, at least isolated supercells are possible across a broad area from southern parts of UT/NV southward to the international border -- wherever even weak surface heating can foster enough buoyancy to sustain individual, deep moist convective plumes for an hour or more to maturity. The greatest potential concentration of such activity still appears to be over parts of the lower Colorado River Valley region westward through the Imperial Valley and into parts of the Mojave Desert, where a 5%/"slight" outlook is maintained, with some expansion to include similar nearby environments. The most-favorable corridor should begin near the MX border in a few hours and shift/expand northward through the afternoon. A combination of theta-e advection from the northern Gulf of California, and occasional cloud breaks ahead of Hilary's core precip region to foster modest diurnal heating, should offset nearly moist-adiabatic middle-level lapse rates and yield 200-500 J/kg MLCAPE (locally higher). Effective SRH in the 300-700 J/kg range should be common where enough instability develops to support it. Potential is expected to diminish rapidly this evening, as the cyclone weakens drastically and associated/ remaining favorable wind fields move over substantially more-stable boundary-layer air. ...Edwards/Jewell.. 08/20/2023 $$ = = = 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) .