Thunderstorm Wind in Clinton County, Illinois

Thunderstorm winds snapped off a large tree at the ground in New Baden. In Aviston, numerous tree limbs were blown down. Also, the tin roof of an outhouse was ripped off.

During the afternoon hours of August 3rd, numerous thunderstorms developed across parts of eastern Missouri and west-central Illinois, triggered by strong surface heating ahead of a surging outflow boundary. These storms developed within a strongly unstable environment, featuring MLCAPE values ranging from 2500 to as much as 4000 J/kg and largely driven by the very warm and humid boundary layer. The initial round of storms struggled to become organized. Eventually cold pool interactions drove upscale growth into a more linear complex. Damaging winds, occasional large hail were reported as well as bursts of very heavy rain rates. By the evening of August 3rd, storms grew upscale, with additional rounds of thunderstorms developing upstream from this initial activity. The primary hazard transitioned from a damaging wind threat to a heavy rain and flash flooding threat, as storms were extremely efficient rain producers. Warm cloud depths were around 4.5km, along with anomalously high precipitable water values of 2 to 2.5 inches, and Corfidi vector orientations that supported westward back-building which enhanced the potential for locally heavy rain, both in terms of instantaneous rates (2+ inches per hour at times) and training of storms. By mid morning on August 4th, widespread rainfall amounts of 1 to 3 inches fell, mainly along and south of I-70 in Missouri and along and east of I-55 in Illinois. Over the St. Louis metro area, as well as Fredericktown, MO, between 4 and 7 inches of rain fell. There were numerous reports of flash flooding in these areas.