Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 29 2022 12:56:02 ACUS01 KWNS 291255 SWODY1 SPC AC 291254 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0654 AM CST Thu Dec 29 2022 Valid 291300Z - 301200Z ....THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF EAST/SOUTHEAST TEXAS...WESTERN/CENTRAL LOUISIANA AND THE ARKLATEX REGION... ....SUMMARY... Isolated damaging gusts or a tornado are possible mainly this afternoon and tonight, across parts of east/southeast Texas, western/central Louisiana and the Arklatex region. ....Synopsis... In mid/upper levels, synoptic troughing was apparent over the Rocky Mountain States. As they pivot through the associated cyclonic-flow field, a series of shortwave troughs and related vorticity lobes will contribute to the slow eastward progress of the synoptic trough. Among those perturbations: 1. A strong shortwave trough -- evident in moisture-channel imagery from a vorticity max over southeastern CO southwestward to central/ east-central NM. This feature is forecast to eject northeastward across the central Plains today while elongating, and reaching southern MN, western IA, southeastern NE, and central KS by 00Z. 2. A weaker, southern-stream trough initially over far west TX and northern Chihuahua, forecast to eject northeastward to central/ north-central TX and weaken somewhat by 00Z, then cross northeast TX, eastern OK and western AR overnight. 3. A subtle shortwave trough -- now over coastal northwestern CA -- forecast to move southeastward and amplify, becoming the basal perturbation for the larger-scale trough across northern MX by 12Z tomorrow. At the surface, 11Z analysis showed surface lows over northern MN and between HLC-GBD, connected by a surface trough and weak cold front curving across northwestern IA and eastern NE. The front then extended southwestward from the KS low across extreme southeastern CO. A dryline was drawn over southeastern KS to south-central OK, central TX in the Hill Country, and southward to northeastern Coahuila. The front should move eastward through the period, reaching western WI, northern MO, southwestern OK, and northeastern NM by 00Z, overtaking the KS/OK dryline segment. By 12Z, the front should reach northern Lower MI, southern IL, and north-central/west- central TX, with a weak surface low/trough in the warm sector over southeast TX between HOU-SHV. The bulk of surface-based convective potential described below should occur ahead of the cold front and near the developing low/trough through this period. ....Arklatex to West Gulf Coastal Plain... Multiple episodes of convection are likely across the outlook area through the period, beginning with a plume of widely scattered showers now apparent in satellite and composited radar imagery over the Sabine Valley region of TX/LA, south-southwestward offshore from GLS. Modified 12Z LCH RAOB and near-term model soundings suggest the effective inflow parcels are surface-based, for dewpoints in the 65-68 F range now observed south of I-20 in this corridor (i.e., south of SHV). A shallow layer of stable lapse rates near the surface will erode slowly through the morning into midday in the midst of continued theta-e advection and weak diabatic heating. This should contribute to gradually strengthening/deepening convection and increase in coverage of thunderstorms. Deep shear will not be as strong with this round as later activity, though sufficiently large low-level hodographs already are apparent in regional VWP/RAOBs to support 100-200 J/kg effective SRH. A brief tornado or damaging gust would be the main concern. Scattered to locally numerous thunderstorms are possible tonight in one or two plumes, offering isolated locally strong to low-end severe gusts and a marginal, conditional tornado potential. To the west of this plume and east of the dryline, also in the east TX warm sector, greater afternoon moisture/heating and a broad plume of low- level convergence will combine to support increasing thunderstorm development/coverage by mid/late afternoon. This process will continue into evening as theta-e advection offsets diabatic cooling effects to maintain surface-based parcels and keep CINH minimal. Modest midlevel lapse rates atop a deep layer of near-saturated low-level RH will support MLCAPE around 500-1200 J/kg, while lack of CINH fosters considerable coverage by evening, and likely messy storm mode. Strengthening mid/upper winds ahead of the advancing synoptic trough (and southern-stream shortwave perturbation), will lead to somewhat greater bulk shear, with effective-shear magnitudes around 30-40 kt in western parts of the area, weaker eastward. This activity (perhaps blended with initially separate near-frontal convection on the north end) should shift eastward, evolving into a loosely organized QLCS, and potentially merging with a continuation or redevelopment of the field of early-day convection in LA. Overnight, additional convection -- much of it elevated -- should form above the LLJ in AR, perhaps with some surface-based inflow southward into the Arklatex and northern LA. This activity -- some of which may be associated with the front -- would occur amidst greater deep-layer winds/shear (up to 45 kt effective-shear vectors) but weaker instability, also posing a marginal wind/tornado threat. ...Edwards/Mosier.. 12/29/2022 $$ = = = 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) .