Supercell differentiation and organization for the 19 April 1996 Illinois tornado outbreak

On April 19, 1996, an outbreak of tornadic supercells struck portions of Illinois and adjacent states. In Illinois alone, 27 tornadoes were documented, setting a single day record while additionally matching the average number of tornadoes for an entire year. An estimated $30 million in damage was reported in 31 counties in Illinois with 1 fatality. This tornado outbreak occurred with an intensifying surface cyclone (992 mb) in southern Iowa positioned under the left exit region of an approaching 65 m s^-1 upper-level jet streak. Storms initiated and became severe between 2000 and 2100 UTC with nearly simultaneous tornado occurrence from storms initiated on the warm front, dry trough and dry trough/warm front occlusion. Most of the tornadoes on this day were short-lived, but a few persisted for over 20 min. and had damage paths of nearly 25 km with peak intensity rated at F3.

A fascinating aspect of this case to be presented involves the early evolution, interaction and merger of cells that initiated along the dry trough just west of the Mississippi River in Missouri. Radar analysis of this evolving convection reveals a complex pattern of storm splits and mergers. Of more than a dozen initial cells along the dry trough, only 2 large long-lived supercells remained that tracked across central Illinois, spawning numerous tornadoes along their paths. It appears that differential motion due to the rotational character and phase of development of the storms led to the numerous mergers that produced the single 2 central Illinois supercells. Evidence exists that mergers may have influenced the intensification and increased the rotation of the primary supercells. Cell interactions also exist between storms that initiated on the warm front and those that approached from the dry trough to the southwest (especially those anticyclonically rotating storms resulting from storm splits). The connection between these merger events and tornadogenesis onset will be presented. A companion presentation by the authors (lead author Jewett) will present modeling results that address the role of the prominent boundaries on this day in terms of their local environment influence on storm intensity, structure and organization.