Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jun 03 2023 12:55:26 ACUS01 KWNS 031255 SWODY1 SPC AC 031254 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0754 AM CDT Sat Jun 03 2023 Valid 031300Z - 041200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF SOUTH TEXAS IN AND NEAR THE RIO GRANDE VALLEY... ....SUMMARY... Scattered thunderstorms with strong-severe gusts may move out of northern Mexico tonight and across parts of deep south Texas. Isolated strong to locally severe thunderstorms are possible today across parts of the southern Plains and north-central Appalachians vicinity. ....Synopsis... In mid/upper levels, net retrogression of the messy, central North American omega block is forecast, with the 500-mb high shifting westward from northwestern ON to southern MB. Slow-moving mean troughing characteristically will flank the anticyclone to the southwest (Intermountain West and southern High Plains) and southeast (East Coast to eastern Gulf). As a part of the eastern mean trough, a compact cyclone -- now developing from a strong shortwave trough over northern ME -- will dig southward and strengthen, with its center over or just east of Cape Cod by 12Z tomorrow. Farther west, assorted vorticity maxima should meander erratically through the troughing between the northern Great Basin and the southern High Plains. A subtle perturbation now over the southern Sonora/Sinaloa region of northwest MX should move east-northeastward toward the mountainous terrain of northern Coahuila through the day. At the surface, a cold front was apparent at 11Z over eastern and southern NY. The overland part should shift southwestward across parts of the coastal and interior Mid-Atlantic through the period as the mid/upper cyclone to its northeast organizes and moves southward. Farther southwest a dryline was drawn from a weak low near ROW southward across far west TX, the Big Bend area, and south-southeastward over the western slopes of the Serranias del Burro and nearby ranges in Coahuila. This dryline should mix slightly eastward through the day, but leave favorable moisture over higher elevations, near a dewpoint axis now drawn from south of BRO northwestward between LRD and Monterrey, to eastern parts of the Big Bend area of TX. An outflow boundary from prior/overnight convection was drawn near CLL-VCT-COT-DRT to eastern Brewster County TX. ....Deep south TX, southern Plains... Scattered thunderstorms should develop this afternoon over northern Coahuila, including the Serranias del Burro and nearby elevated terrain, and perhaps farther north across the international border into the lowest Pecos River region of TX. Large hail and severe gusts are possible in the early stages of the event, transitioning to a dominant severe-wind threat. Significant hail cannot be ruled out on the TX side, but is more conditional than over nearby parts of MX where certainty is greater for sustained supercell(s). The primary forcing mechanisms for this convection should include the western limb of the remnant boundary, and even more crucially, strong diurnal heating of higher terrain amidst an upslope component of a very moist boundary layer. The modified 12Z DRT RAOB and model soundings suggest a corridor of 3000-4000 J/kg MLCAPE is possible this afternoon near the surface moist axis, which should move little through the day. 40-50 kt effective-shear magnitudes and long hodographs are possible, beneath the northern fringes of the subtropical jet. Early-stage supercells and organized multicells over MX may aggregate, forming a cold-pool-driven, forward-propagating MCS with embedded bow/LEWP formations and perhaps lingering/embedded supercell(s). Given the ambient pattern, and positioning of the theta-e gradient related to the remnant outflow boundary, such a complex would move southeastward and obliquely across the Rio Grande Valley (RGV), this evening into the overnight parts of the period. Convection should be maintained by forced ascent of a favorably moist airmass undisturbed by the prior day's MCS. Even with some nocturnal near-surface cooling ahead of the activity (slowed somewhat by the moist/high-theta-e conditions), a remnant diurnal mixed layer below the weak EML inversion should support maintenance of damaging gusts to the surface. The outlook still appears somewhat conditional; however, the favorable environment combined with a preponderance of synoptic- scale and CAM guidance indicating this potential compels an upgrade at this time. An even more concentrated/intense corridor of wind potential may organize across some part of northeastern Coahuila into the lower RGV region, but that level of mesobeta-scale threat is still to uncertain to warrant even larger unconditional probabilities this soon. Otherwise, a more randomized, poorly defined, lower-moisture, but also weakly capped environment will exist across most of the southern Plains, with several lingering mesoscale outflow/ differential-heating boundaries potentially concentrating afternoon/ early-evening convection locally. Isolated large hail and strong- severe gusts would be the main concerns, amid weak deep-layer wind profiles and lack of substantial shear. ....North-central Appalachians and vicinity... Widely scattered, predominantly afternoon thunderstorms are expected to form along/ahead of the front, atop a favorably moist, yet weakly capped and well-mixed boundary layer destabilized by diurnal heating. This activity should move generally southwestward across the outlook area, offering damaging gusts approaching severe limits, and small to marginally severe hail cannot be ruled out. Deep-layer wind profiles and shear in the warm sector are forecast to remain weak, with the strongest height gradient aloft relegated above the post-frontal sector in the Northeast. Despite the weak shear and modest midlevel lapse rates, however, a diurnally heated and well-mixed boundary layer will support potential for localized damaging downdrafts. Peak preconvective MLCAPE near 1000 J/kg is possible, with 500-1000 J/kg DCAPE. The threat should diminish sharply after dark as the near-surface layer cools/stabilizes, and buoyancy weakens. ....FL Keys... Through the period, NHC forecasts marginal T.S. Arlene -- already small and rather skeletal in convective structure -- to move southeastward and degenerate to a depression, then remnant low. As such, local wind profiles/hodographs and low-level shear are expected to remain too weak to support an eastern-semicircle tornado risk that would affect the Keys, even as what is left of the system approaches northwestern Cuba. Please see latest NHC advisories for details on the expected track and intensity of Arlene. ...Edwards/Smith.. 06/03/2023 $$ = = = 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) .