Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 18 2023 12:55:27 ACUS01 KWNS 181255 SWODY1 SPC AC 181253 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0653 AM CST Sat Nov 18 2023 Valid 181300Z - 191200Z ....NO SEVERE THUNDERSTORM AREAS FORECAST... ....SUMMARY... Organized severe thunderstorms are unlikely through tonight. ....Synopsis... A substantial pattern change is underway in mid/upper levels, related to the progressive nature of high-amplitude synoptic troughing now found over eastern North America and just off the Pacific Coast. A strong shortwave trough -- apparent in moisture- channel imagery just offshore from Cape Mendocino -- will move slowly inland today and either dissipate or be absorbed by an even stronger and amplifying upstream perturbation, now located about 500-600 nm west of the WA/OR coastline. By 12Z tomorrow, this should result in a negatively tilted trough and prominent 500-mb vorticity banner from just west of Vancouver Island southeastward across central OR to near LAS. Preceding corridors of large-scale ascent (DCVA aloft and/or low-level warm advection), amidst modest low/middle-level moisture inland, will support a large, irregular area of isolated thunder potential over the West. A southern-stream belt of enhanced and difluent cyclonic flow -- containing a few low-amplitude shortwaves -- will spread southeast of the trough across AZ and NM, reaching the southern Plains by 12Z. In the low levels, offshore flow will continue across at least the eastern 1/2-2/3 of the Gulf through the period, following the cold- frontal passage related to the current Eastern mid/upper trough. While surface onshore flow will occur by the end of the period over the Texas Coast, short marine trajectories will allow little time for substantial modification. Still, enough moist advection will occur above the surface to support elevated thunderstorms developing eastward from NM across portions of the south-central Plains tonight. There, increasing (but still marginal) return moisture will be superimposed with coupled ascent from low-level warm advection and mid/upper lift, located leftward of the cyclonically curved jet max aloft. ...Inland northern CA... Isolated shallow, but perhaps at least briefly rotating, cells may occur this afternoon over the Sacramento Valley. Such activity would follow closely behind earlier precip/convection now progged to greatly limit diurnal destabilization (MLCAPE 200 J/kg or less in forecast soundings, with weak low-level lapse rates). Though any associated severe threat still appears strongly conditional, veering flow with height (as already evident in the 12Z OAK RAOB) and up to about 75-125 J/kg of 500-m SRH appear possible over the area into late afternoon. If greater destabilization potential becomes apparent, informed by 12Z and later guidance and mesoscale trends, marginal unconditional tornado probabilities may be needed in a succeeding outlook. ...Edwards/Leitman.. 11/18/2023 $$ = = = 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) .