Initiation and evolution of severe convection in the 19 April 1996 Illinois Tornado Outbreak

On 19 April 1996 an outbreak of tornadic storms occurred in central and northern Illinois and parts of Iowa, Indiana and Wisconsin. The single- day total of 27 tornadoes in Illinois matched the number expected in an average year. The storms formed along both a surface dry trough and warm front. The convection initiated along the dry trough was observed, after a period of splitting and merging, to move off the boundary into central Illinois and become tornadic.

We are studying this case with the NCAR/Penn. State MM5 model. The modeled convection also moves off the dry trough and remains at the leading edge of the mid-tropospheric dry slot and 600-mb theta-e gradient consistent with a weak cold front aloft (CFA). The initiation near the surface boundary and propagation consistent with the CFA resembles behavior noted by Locatelli et al. (1998). We will discuss the role of low- and mid-tropospheric boundaries on the initiation, intensification and propagation of convection on this day. Comparisons will be made with isolated storm simulations to further reveal the role of boundaries and the complicated environment in which these storms formed. A companion paper by the authors (lead author Lee) will present results of an observational study emphasizing radar analysis.