Day 1 Convective Outlook
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Summary
Severe thunderstorms capable of producing large hail, damaging wind gusts, and occasional tornadoes are expected across the northern and central Plains and the lower Ohio Valley through this evening and into early Thursday morning. Affected areas include parts of Wyoming, South Dakota, Nebraska, Iowa, Illinois, Indiana, Kentucky, and the lower Ohio Valley.
Product Text
000 ACUS01 KWNS 122002 SWODY1 SPC AC 122000 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0300 PM CDT Wed Aug 12 2026 Valid 122000Z - 131200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE NORTHERN AND CENTRAL PLAINS AND LOWER OHIO VALLEY... ...SUMMARY... Scattered severe thunderstorms with occasional wind damage remain likely this afternoon/evening from Illinois/Indiana into Kentucky. Initial supercells with very large hail across the central/northern High Plains are expected to evolve into a cluster or two overnight while spreading eastward across Nebraska/South Dakota with the potential for severe gusts. ...20z Update OH Valley... Several clusters of strong to severe storms remain ongoing in proximity to the outflow from the prior complex this morning. Expectations are for additional storm development to occur along the southern portions of the outflow across southern IN/IL and western/northern KY where the air mass remains quite warm and unstable (MLCAPE 300-4000 J/kg). Deep-layer shear of 25-35 kt would support a mixed mode of clusters and short line segments, with severe gusts (some 70-75 mph), isolated hail and a tornado or two possible into this evening. Have trimmed probabilities behind the current storms/outflow and adjusted CIG1 wind probabilities farther southwest. ...WY/SD/NE... Initial supercells have developed and are maturing within a broad post-frontal upslope flow regime across eastern WY/MT, western SD and northwest NE. These storms are expected to remain mostly supercellular and should continue to pose a threat for large to very large hail (2-3 inch), severe wind gusts and potentially a tornado or two into this evening as they track eastward along a diffuse frontal boundary. Additional, likely higher-based storms should develop across northeast CO and western NE before spreading northeastward with a risk for severe gusts and some hail. Eventually, upscale growth into one or more clusters/bowing segments appears likely as a 40-50 kt low-level jet and strong low-level warm advection is forecast to develop across the central/northern Plains. This would support a risk for severe gusts of 65-80 mph across parts of central and eastern NE/SD, likely reaching western IA by early Thursday morning. 2% tornado probabilities have been added to western SD and eastern WY where the supercellular mode could support some tornado risk. ..Lyons.. 08/12/2026 .PREV DISCUSSION... /ISSUED 1130 AM CDT Wed Aug 12 2026/ ...OH Valley area through this evening... An MCS is ongoing across northern IL with the primary cold pool surge and bowing segment progressing from northeast IL into northwest IN. Elevated convection is ongoing in the zone of warm advection atop the cold pool in IN. This elevated convection will pose a threat for isolated large hail into this afternoon, while a feed of steeper midlevel lapse rates and low-level moisture from the west will help maintain the IL/IN MCS into this afternoon with a continued threat for occasional wind damage with gusts of 60-75 mph. The eastern extent of the severe threat into OH is uncertain given the ongoing/elevated convection and limited recovery time. Thus, the eastward extent of any wind threat associated with the ongoing IL/IN MCS is likewise uncertain. Additional thunderstorm development is possible this afternoon along the trailing outflow from central IL into southern IN and KY, where occasional wind damage will be possible into this evening. ...Northern Plains this afternoon through tonight... A subtle midlevel speed max over southwest WY will progress eastward around the northwest periphery of the southern Plains ridge. Weak ascent preceding this wave, in combination with surface heating in cloud breaks and low-level upslope flow with boundary-layer dewpoints in the 60s, will support scattered afternoon/evening thunderstorm development from northeast CO into western SD. Lingering steep midlevel lapse rates on the fringe of the monsoonal moisture plume in WY and effective bulk shear near or above 40 kt (with long hodographs aloft) will favor supercells initially with the potential for very large hail (2-3 inches in diameter). There will likely be some upscale growth into a cluster or two this evening into tonight on the nose of a 30-40 kt low-level jet across NE/SD, with a transition to severe outflow gusts of 65-80 mph as the main threat. The storm cluster(s) will likely reach western IA early Thursday morning. ...Lower MI into western NY/PA through late evening... Subtle speed maxima will move east-southeastward from Lower MI to NY/PA through this evening. Midlevel lapse rates of 6-7 C/km and boundary-layer dewpoints in the mid 60s will support MLCAPE near or above 1000 J/kg, in an environment with sufficient deep-layer shear/hodograph length for storms with supercell structures. Isolated large hail and wind damage will be the main threats, though a more discrete mode would be less favorable for wind damage, and storm coverage could remain somewhat isolated. Thus, will maintain a MRGL risk, though there was some consideration for an upgrade to SLGT across western NY and vicinity. ...South GA/north FL this afternoon... Scattered thunderstorm development appears probable from south GA into north FL this afternoon along a subtle confluence zone with large buoyancy immediately southwest of a remnant MCV over southeast GA. A few multicell clusters will be capable of producing isolated strong outflow gusts/wind damage with downbursts. $$