Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 23 2023 12:46:36 ACUS01 KWNS 231246 SWODY1 SPC AC 231244 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0744 AM CDT Sat Sep 23 2023 Valid 231300Z - 241200Z ....THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF WESTERN MISSOURI...EASTERN KANSAS AND NORTHEASTERN OKLAHOMA... ....SUMMARY... Large to very large hail, wind damage and tornadoes will be possible today across eastern parts of the Plains States to the mid/upper Mississippi Valley. A tornado or two may occur in parts of the coastal Mid-Atlantic. ....Synopsis... In mid/upper levels, the flow over the CONUS splits around two prominent cyclones: a cold-core vortex now centered over northeastern WY, and the warm-core midtropospheric portion of T.S. Ophelia over eastern NC. The WY cyclone's 500-mb low is forecast to reach central SD at 00Z, then turn northeastward toward the eastern ND/SD border by 12Z tomorrow. As that occurs, a surface low -- analyzed at 11Z near BBW -- will move to southwestern MN by 00Z. The western part of a warm front -- initially drawn from the low across southern MN to southern WI -- should follow the surface low across the mid Missouri Valley and reach from central MN to southern Lake Michigan by 00Z. By then, the trailing cold front -- analyzed across western KS and southeastern CO -- should reach northwestern IA, southeastern NE, eastern KS, north-central to southwestern OK, and northwest TX. By 12Z, the low should occlude and wrap northwestward under the mid/ upper center, with the occluded/cold front across central/ southeastern MN, central/eastern IA, western MO, and northeastern OK, to a weak low over the Red River Valley between SPS-CDS. ....Eastern Plains States to mid/upper Mississippi Valley... An ongoing band of thunderstorms over central NE, near the cold front, should proceed east-northeastward into a narrow channel of prefrontal destabilization already evident in objectively analyzed thermodynamic fields. This activity may persist through the morning, with either its lateral/north-south expansion or newer/ nearby development evolving into the main band of severe thunderstorms today for the northern part of the outlook area. Warm/moist advection and diurnal heating will underlie large-scale ascent/cooling aloft that preceded the mid/upper cyclone, in a northward-narrowing prefrontal corridor characterized by MLCAPE ranging from 2000-2500 J/kg near the MO/IA line to 500-1000 J/kg in eastern SD and western/southern MN. Backed low-level winds near the warm-frontal zone and east of the low, as well as an intense LLJ developing in the low-level mass response to the approaching cyclone, should contribute to enlarged and elongated low-level hodographs over this region. Effective SRH over 300 J/kg may be common. To the extent relatively discrete supercells can form and mature before moving out of the buoyancy corridor, tornadoes are possible, along with occasional large hail and damaging to severe gusts from all convective modes. From the northwest MO area southward/southwestward down the front, convective initiation will be later, reflecting weaker large-scale lift and the presence of greater capping. Nonetheless, from late afternoon into evening, scattered thunderstorms should develop as the cold front impinges on diurnally minimized MLCINH, in a favorably unstable and moist environment. A deep troposphere, surface dewpoints in the mid 60s to near 70 F, and steep low-level lapse rates will contribute to a prefrontal field of MLCAPE in the 2500-3500 J/kg range from parts of western KS/western MO into eastern OK. Enlarged hodographs, with about 35-45 kt effective- shear magnitude, suggest supercell potential with very large, damaging hail possible, along with occasional strong-severe gusts and some tornado threat. CAPE will lessen somewhat with southwestward extent from there into north-central/northwest TX where greater boundary-layer mixing and lesser dewpoints will be found, but with greater DCAPE and downdraft-acceleration potential in support of local wind threat. Quasi-linear/upscale growth of the activity along/ahead of the front may continue a wind threat and at least marginal tornado potential well into this evening. ....Mid-Atlantic/Tidewater... T.S. Ophelia's center has moved inland over eastern NC, and the cyclone is forecast by NHC to weaken through the remainder of the period as it continues mostly northward over the coastal plain of NC/VA. The tornado threat appears conditional and marginal at this time, but still exists for the remainder of the day mainly northeast of the center. See SPC Mesoscale Discussion 2179 for near-term guidance. Convection in this system has been rather skeletal, widely scattered in character, and not particularly deep in the sector from north through southeast of center. So far, the most-favorable parameter space -- in terms of buoyancy and low-level shear, with around 500-1000 J/kg MLCAPE and 300-800 J/kg effective SRH -- has been located under a midlevel dry slot, readily apparent in satellite imagery. The drying is wrapping around the southern and eastern sectors of the circulation from source region of dry air aloft sampled well by last night's 00Z JAX and CHS soundings. Surface analyses have indicated that most of the outer convection north through northeast of center has moved quickly to the cool, stable side of a buoyancy-limiting, surface warm-frontal zone, before it could organize to substantial supercellular character. Still, either inner or outer bands on the periphery of the dry slot -- or any rogue supercell that can form in the slot and survive to maturity -- may pose a tornado threat today, before instability and wind fields related to the system weaken even further. Cells interacting with the warm-frontal zone should have the greatest rotational potential. See NHC advisories for the latest track/ intensity forecast information on Ophelia, as well as tropical watches/warnings. ...Edwards/Kerr.. 09/23/2023 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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