Low-topped supercell evolution in association with a mesoscale convective vortex across northern Illinois, August 24th, 2004

A vast majority of tornadoes across northern Illinois produce F0 and F1 type damage, and the storm scale features evident in radar and satellite imagery are often quite subtle providing a challenge to warning operations. An example of such an event occurred on August 24th, 2004 when a total of four F0 tornadoes were reported across northern Illinois during the late afternoon and early evening hours. Several mesoscale factors were noted that played a role in convective initiation. A mesoscale convective vortex (MCV) with its origins from a morning decaying mesoscale convective system across eastern Missouri and west central Illinois moved into northern Illinois by late afternoon. Renewed thunderstorm development occurred in association with this MCV by late afternoon and the convective mode rapidly transitioned to supercellular as the MCV approached an old outflow boundary from morning convection. The first of the four F0 tornadoes occurred shortly after interaction with the outflow boundary, and temporal and spatial changes in the near storm environment associated with this outflow boundary were investigated. Radar data was also examined from the KLOT and KMKX WSR 88-Ds, and storm splitting was noted across northern Illinois as thunderstorm coverage increased in the vicinity of the MCV late in the afternoon. Rotational features in the radar data were often short-lived and occurred shortly after increases in the low level reflectivity suggesting a possibility that vortex stretching played a role in tornadogenesis.