Subj : DAY1SVR: Enhanced Risk To : All From : Mike Powell Date : Tue Jul 18 2023 17:11:00 ACUS01 KWNS 182002 SWODY1 SPC AC 182000 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0300 PM CDT Tue Jul 18 2023 Valid 182000Z - 191200Z ....THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE TENNESSEE VALLEY... ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN/CENTRAL PLAINS... ....SUMMARY... Scattered to numerous damaging winds are expected to continue this afternoon and early evening into the Tennessee and lower Ohio Valleys. Other strong to severe storms should occur across parts of the Northeast and northern/central Plains. ....20Z Update... Severe probabilities have been adjusted from the eastern portions of the central Plains into the mid MS Valley and lower OH Valley in the wake of the ongoing MCS across KY/TN. Scattered to numerous severe/damaging winds remain likely with this MCS as it continues eastward across the TN and lower OH Valleys this afternoon and early evening. See Mesoscale Discussion 1618 for more details on the near-term severe threat across this area. Some potential for redevelopment later this evening and overnight remains across parts of MO and vicinity in a strengthening low-level warm advection regime. Large hail may occur with any supercell that can form in this environment, and severe/damaging winds appears possible with any convection that can grow upscale into a cluster. Have therefore maintained the Slight Risk across portions of this area. A Slight Risk has also been introduced over the Black Hills of western SD into parts of northern NE. While overall thunderstorm coverage may remain fairly isolated across this region, any convection that can be sustained will likely be supecellular given 2000-2500+ J/kg of MLCAPE and 40-50 kt of deep-layer shear. Isolated very large (2+ inch) hail and severe wind gusts should be the main threats. ...Gleason.. 07/18/2023 ..PREV DISCUSSION... /ISSUED 1134 AM CDT Tue Jul 18 2023/ ....Middle/Lower Missouri Valley to Tennessee Valley... Ongoing bowing complex of strong/severe storms continues moving southeast at 40-45 kt across southeast MO. Downstream heating of a very moist environment (dew points 75-80 Deg F) and seasonably strong mid-level northwesterly flow averaging 40-45 kts is expected to result in a continued southeast movement into an increasingly favorable environment for a forward-propagating MCS/potential for significant severe gusts. Please refer to recently-issued MCD 1613 for the latest short-term thinking in this area. Additionally, another round of elevated thunderstorms is expected to develop later today along the convectively influenced northwest/southeast-oriented boundary across Missouri. This may include elevated supercells with a threat for large (isolated very large) hail given strong elevated instability and substantial deep layer shear. With time, upscale growth will be possible and a severe wind threat could materialize. ....Northern Plains... Low-level moisture advection and surface heating will lead to moderate-strong instability across central North Dakota and South Dakota by this afternoon. This instability, combined with convergence from an approaching/sharpening cold front and ascent with a mid-level shortwave trough, should be sufficient for strong/severe thunderstorm development near the front this afternoon. Mid-level winds of 45-55 kt will contribute to favorable wind profiles for supercells and related risk for large hail, some potentially 2+ inches in diameter. ....Northeast/Mid-Atlantic States and New England... Modest height falls will continue to overspread the region, influenced by moderately strong cyclonic mid-level flow with an upper-level trough over northern Ontario/Quebec. Relatively long hodographs during peak heating/maximized instability will support some potential for sustained/organized storms capable of wet microbursts and isolated wind damage. Some hail will also be possible. Refer to MCD 1614 for short-term thinking in this area. ....Central High Plains... A few thunderstorms are possible late this afternoon as low-level upslope flow evolves, with increasing low-level moisture to the north of a surface low. The favorable environment may be relatively short-lived given the late-day increase in moisture, but a favorable environment will exist for a few severe thunderstorms in an environment with 2000+ J/kg MLCAPE and supercell wind profiles. ....Great Basin... A deeply mixed boundary layer is expected today across the Great Basin with ample instability for scattered thunderstorms across much of the region. A belt of stronger mid-level winds of 25-30 kt will exist from east-central Nevada into north-central Utah. This may support some storm organization with the potential for isolated strong/severe wind gusts given the well-mixed sub-cloud layer/steep low-level lapse rates. ....Southern Arizona... Thunderstorms should develop over the Rim/White Mountains and higher terrain of southeast Arizona this afternoon. The region will be nearly coincident with the upper high. While mid-level steering flow will be weak, a westerly component of near-surface winds will likely support propagation into the desert floor, particularly across southeast Arizona within a relatively moist but hot/deeply mixed boundary layer. Some strong, potentially severe outflow winds may occur late this afternoon/early evening. $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .