SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1117 AM CDT Sat Oct 18 2025 Valid 181630Z - 191200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THROUGH TONIGHT FROM THE ARK-LA-TEX TO THE LOWER OHIO AND MISSISSIPPI VALLEYS... ...SUMMARY... Scattered severe thunderstorms are possible mainly this afternoon into tonight across parts of the Ark-La-Tex, Ozarks, mid to lower Mississippi and lower Ohio Valleys. Damaging winds, a few tornadoes and hail may occur. ...Synopsis... A southern stream shortwave trough will eject quickly east-northeastward from TX/OK to AR/MO, prior to phasing with an amplifying northern stream trough by tonight across the mid MS Valley. Downstream from the phasing troughs, a deepening surface cyclone will develop northeastward tonight from the mid MS Valley to Lower MI, as a trailing cold front surges southeastward from the Plains into the mid-lower MS Valley. ...TX/OK today into the MS/OH Valleys through early Sunday... Regional 12z soundings revealed a plume of 7.5-8 C/km midlevel lapse rates (FWD-OUN-LMN) atop low-level dewpoints in the 60s, which is already contributing to moderate buoyancy (MUCAPE up to 1500 J/kg) as of mid morning. As continued forcing for ascent with the lead shortwave trough overspreads the moisture/buoyancy plume, ongoing thunderstorms are expected to increase in coverage through the afternoon from southern MO/IL to eastern OK/western AR and north/northeast TX. Initial storms will be in the form of clusters/short line segments with upscale growth into a more extensive squall line expected by later this afternoon/evening. Buoyancy will increase some in the moisture plume through the afternoon with surface heating in cloud breaks, and deep-layer vertical shear will increase with the approach of the ejecting midlevel trough. Thus, the stronger embedded cells will have the potential to produce large hail (1-1.75 inches in diameter) along with an increase in the threat for wind damage as convection becomes more linear. The tornado threat is a bit more uncertain given the early/messy start to convection this morning, and since low-level shear will not be strong this afternoon in the warm sector as a result of primary cyclogenesis not occurring until tonight from IL into lower MI (north of the richer low-level moisture). Still, a couple of tornadoes may occur with embedded circulations. There will be the potential for more discrete supercells and a few tornadoes ahead of the larger-scale line of storms from far east TX into LA/southern AR and MS overnight where boundary-layer dewpoints exceed 70 F and low-level shear increases. Though buoyancy will be weak, rapidly strengthening wind fields overnight and forcing for ascent along the synoptic cold front could support a low-topped line of convection with the potential for wind damage and possible a couple of embedded tornadoes from the OH Valley into southeast Lower MI ~06-12z. ..Thompson/Supinie.. 10/18/2025
