Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jun 20 2023 12:54:39 ACUS01 KWNS 201254 SWODY1 SPC AC 201253 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0753 AM CDT Tue Jun 20 2023 Valid 201300Z - 211200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN PLAINS...AND EAST TEXAS TO THE CENTRAL GULF COAST... ....SUMMARY... Isolated to scattered severe thunderstorms appear possible today across parts of the northern Plains, and east Texas to the Southeast. Damaging winds and large hail should be the main threats. ....Synopsis... A quasistationary, positively tilted large-scale wave train is entrenched across the CONUS, featuring a mean trough from west- central Canada southwestward across the Pacific Coast, ridging from northern MX across the lower Missouri and upper Mississippi Valleys to James Bay, and troughing from Atlantic waters east of Canada southwestward across the Tidewater area to the central Gulf Coast. Within the latter, a broad, deep-tropospheric cyclone -- initially centered near the eastern TN/KY line -- is progged to retrograde slowly westward by the end of the period, orbited by several convectively induced/enhanced vorticity lobes of varying sizes. A swath of somewhat enhanced northwesterlies aloft (around 40 kt at 500 mb and 50-70 kt at 250 mb) will extend around the southwest side of the cyclone, and over the Arklatex and Mississippi Delta regions. Farther northwest, a broad fetch of southwesterlies east of the mean trough will extend from CA across the Great Basin and northern/ central Rockies, to the northern Great Plains. Most of the embedded perturbations that cross the Rockies will remain behind the front described below. However, a weak shortwave trough -- evident in moisture-channel imagery over the Four Corners region -- is forecast to eject over the central High Plains by 00Z. This evening and overnight, this perturbation will gain vorticity from convective generation, and reach from the east-central Dakotas to central NE by 12Z. The 11Z surface analysis showed a wavy, slow-moving frontal zone from coastal NC west-southwestward to a triple-point low near ATL (with a surface reflection of the deeper low farther northwest on the TN/KY line). The front then extended southwestward to southern AL, then across northern LA, western parts of OK/SK, central NE, and the eastern Dakotas, to another low just north of the international border in southwestern MB. A cold front was drawn from that low to another over northwestern SD, then southwestward across southern WY. The northern Plains cold front should move slowly eastward, reaching eastern ND, south-central SD and western NE by 12Z tomorrow. The northern Plains to LA frontal segment -- which had been drifting northward as a warm front -- may shift westward/southwestward again as a "back-door" cold front, while large-scale forcing and mass fields respond to the retrograding Tennessee Valley cyclone and associated convective processes near the Gulf Coast. ....East TX, LA/Southeast... A wide area of at least marginal severe-thunderstorm potential is evident from east TX to FL and southern GA. Within this swath, the best parametric overlap of favorable shear and instability should be in and near the "slight" area, where a mix of multicellular and supercellular storm modes is expected to move southeastward with all severe types possible. This activity may evolve as early as the next few hours over parts of the Arklatex or east TX, from a surface-based manifestation of initially elevated convection now rooted in a zone of low-level warm advection across north-central/northeast TX. Refer to SPC mesoscale discussion 1173 for near-term details on this process. This, or additional episodic activity along and south of the front near a parallel surface moist axis, should become severe as it accesses a strongly destabilizing diurnal boundary layer, where mid/upper 70s F surface dewpoints underlie steep midlevel lapse rates. Resulting MLCAPE in the 3000-4500 J/kg range will support rapid growth of surface-based convection and strengthening to severe levels, with large hail and severe gusts possible. Weak low-level flow and related small hodograph size may mitigate areal tornado threat, but localized potential can develop near favorable oriented boundaries. ....Northern Plains... Widely scattered to scattered thunderstorms are expected to develop initially in two regimes that may merge late this evening in northern parts of the outlook area: 1. Post-frontal/upslope-enhanced lift regime near the Bighorns of northern WY, with resultant activity potentially becoming supercellular for a time across downshear areas of northern WY and southeastern MT, then evolving upscale as it moves northeastward into part of the western Dakotas, reaching the frontal zone tonight. Strong veering of wind with height, and somewhat stronger post- frontal flow aloft compared to the warm sector, will combine to yield supercell-favoring low-level and deep shear, with large hail and isolated severe winds possible. 2. Near the western cold front, in a zone of relatively maximized low-level lift, moisture, buoyancy and diurnally weakened MLCINH. A narrow corridor of 60s F surface dewpoints and steep tropospheric lapse rates will support peak/preconvective MLCAPE in the 2000-3000 J/kg range, atop a well-mixed subcloud layer conducive to maintenance of strong-severe gusts to the surface. Though deep shear will be weak this far east, enough convective coverage is forecast to warrant these unconditional wind/hail probabilities. ...Edwards/Jewell.. 06/20/2023 $$ = = = 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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