On April 19, 1996, the largest single-day total of tornadoes ever struck the state of Illinois. 36 tornadoes were documented, some as strong as F-3 on the Fujita tornado intensity scale. We are studying the development of tornadic storms in this day, by examining National Weather Service Doppler data and through simulation of the event. Tornadoes also were reported in parts of Missouri, Iowa, Indiana and Kentucky. Beyond its importance as a severe midwest event, the early storm initiation, evolution, and association with mesoscale boundaries are of particular interest.

In addition to detailed study of the available observations from this event, we are also using MM5, WRF and COMMAS to study the convection on this day. With high-resolution numerical studies of the outbreak, we are investigating:

• Storm initiation. Low-level (drytrough) and mid-troposphere (cold front aloft) forcing were present on this day. Their respective roles in the formation and movement of the storms will be documented and explained.
• The pre-tornadic cell behavior. The western IL storms were observed to split and merge prior to becoming tornadic and sweeping across central Illinois. The significance and long-term effects of the merging, both on movement and storm rotation, will be ascertained. As part of this study, soundings from the MM5 results will be used in further idealized simulations (with COMMAS) to study storm development with and without cell merging, and to study isolated cell evolution in the buoyancy and shear environments found along the active boundaries on this day (warm front, drytrough in eastern MO, CFA over central IL).

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