Subj : DAY1SVR: Day 1 Convective To : All From : Mike Powell Date : Mon Jun 19 2023 09:40:00 ACUS01 KWNS 191255 SWODY1 SPC AC 191254 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0754 AM CDT Mon Jun 19 2023 Valid 191300Z - 201200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF SOUTHWESTERN SOUTH CAROLINA TO NORTHERN FLORIDA AND SOUTHEASTERN LOUISIANA... ....SUMMARY... Severe thunderstorms are possible today over parts of the Southeast, central Texas, Arklatex, and northern Plains. ....Synopsis... Generalized pattern amplification is expected in mid/upper levels, as ridging aloft strengthens from northern MX across the southern High Plains, lower/mid Missouri Valley, Lake Superior area, James Bay, and northern QC. West of the ridge, an elongated, positively tilted cyclone is apparent in moisture-channel imagery over southwestern Canada and the U.S. Northwest, with dual centers: over southern AB and the southern WA Coast. The cyclone will devolve into a substantial open-wave trough, but move little or none eastward through the period, as the AB center ejects across northern SK. East of the ridge, a well-defined, convectively enhanced vorticity max anchored a mid/upper cyclone centered over the lower Ohio Valley. The 500-mb low should drift southeastward to middle/eastern TN, while the cyclone at large remains cut off from increasingly distant troughing over the Northeast and Atlantic Canada. The surface analysis at 11Z showed a low over southwestern KY with cold front across middle TN and central MS, becoming quasistationary over northern parts of LA/TX. This low, surrounded by more baroclinicity from convective than synoptic/subsynoptic processes -- should drift erratically through central/eastern parts of KY/TN through tomorrow morning. The cold front will move slowly southeastward through parts of MS/AL, while the western part becomes a warm front and shifts northward across the Arklatex region and into southern OK. A dryline -- drawn from the southern TX Panhandle to the Big bend region, will mix eastward into parts of west-central/central TX today. A quasistationary front was drawn across western SD, extreme southeastern MT, central WY, western UT and southern NV. The front may move slowly southeastward across the Great Basin and western WY, but with little movement from WY northeastward. This evening through tonight, a low should consolidate along the front over central WY and ripple northeastward to western SD by the end of the period. ....Southeast... Ongoing convection across portions of southeastern MS may pose a threat for damaging gusts, isolated hail and/or a brief tornado in the next couple hours while moving southeastward toward the coast. Severe potential may maximize with this activity near an outflow boundary; see SPC Mesoscale Discussion 1163 for near-term coverage of that activity. A band of thunderstorms responsible for that outflow is moving southeastward over portions of southwestern GA and the FL Panhandle, and has been producing subsevere gusts at observation sites it crosses. Still, damaging subsevere winds or an isolated, marginally severe gust cannot be ruled out in the strongest internal surges for another hour or two, until the activity moves into an environment of stronger MLCINH and lower theta-e across southern GA and northern FL. The associated outflow boundary is expected to stall and weaken across the central/eastern Gulf Coastal Plain through the day. This will lead to a round of renewed severe potential this afternoon into evening near and to its east across GA, westward to the central Gulf Coast. Scattered thunderstorms should develop amid diabatically weakened MLCINH -- inland near that boundary, other outflow/differential-heating zones, and perhaps sea-breeze boundaries along immediate coastal areas. Rich low-level moisture and seasonally steep low/middle-level lapse rates should drive peak MLCAPE into the 2500-3500 J/kg range over western parts of the outlook area, and 1000-2000 J/kg over eastern parts where weaker lapse rates and more persistent cloud/outflow influences should temper diurnal destabilization somewhat. As the mid/upper cyclone to the north slowly digs southeastward, flow over the central/eastern Gulf Coast region will become more difluent and northwesterly. Strengthening mid/upper winds and some increase in directional change (veering with height) are expected, leading to 35-45-kt effective-shear magnitudes over most of the Gulf Coast region, supporting potential for organized multicells and a few supercells. All severe hazards will be possible before activity moves offshore -- and/or weakens as early evening cooling and spreading outflow reduce areal extent and magnitude of instability. ....Northern Plains... Isolated to widely scattered thunderstorms should initiate this afternoon near and east of the front, most likely over elevated terrain near the WY/MT border, as gradual diurnal heating under broken cloud cover preferentially erodes MLCINH, in the presence of regionally maximized low-level convergence. This activity should move northeastward across the outlook area, offering occasional severe gusts/hail. The greatest severe threat appears to be over parts of southeastern MT into western SD with a sustained supercell or two possible late afternoon into early evening. As the Northwest cyclone transitions to a trough, flow aloft will back to more southwesterly, with a series of mostly low-amplitude/embedded vorticity lobes crossing the area. This will maintain strong gradient flow in mid/upper levels, contributing to effective-shear magnitudes around 45-55 kt. Surface dewpoints generally in the 50s F, and a deep boundary layer characterized by nearly dry-adiabatic lapse rates, will contribute to MLCAPE in the 500-1000 J/kg range. Strongly veering winds with height will lead to fairly large/curved hodographs supporting 150-300 J/kg effective SRH, though modest speeds will limit shear/SRH in the lowest km. ....Central TX and vicinity... Isolated to scattered thunderstorms are possible along/ahead of the dryline and southwest of the front this afternoon into early evening. Severe hail and gusts will be possible with any sustained convection. Though temperatures aloft are generally warning with the amplification of the ridge, and a growing EML off the Mexican plateau, hot surface conditions (exceeding 100 deg F) near convective temperature and weak low-level convergence are expected to combine to erode MLCINH. Surface dewpoints initially in the 70s F may mix into the 60s -- still enough to support strong instability through a deep troposphere, with moist-sector MLCAPE potentially reaching the 3000-4000 J/kg range. A conditional potential exists for significant hail or localized gusts near sig-severe levels, given the supercell-favorable vertical wind profiles (e.g., 35-45 kt effective-shear magnitudes) and hot/deep/well-mixed subcloud layer expected. However, considerable uncertainty exists regarding storm coverage and duration, given the presence of ridging aloft and related lack of mid/upper support. Accordingly, and unsurprisingly, vast differences exist among models on thunderstorm coverage and longevity. For now, will maintain marginal unconditional probabilities, with the understanding that a pocket of this area may need an upgrade today if mesoscale trends lead to a better-focused convective potential. ...Edwards/Jewell.. 06/19/2023 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .