Cloud-to-ground lightning characteristics associated with tornadoes on 15 May 1998

On 15 May 1998, a large-scale thunderstorm complex moved north from Central Texas to Canada in 24 hours. This storm produced over 52 thousand cloud-to-ground (CG) lightning flashes with almost 60 percent lowering positive charge to the ground. This compares with approximately 8 percent of all flashes lowering positive charge to ground annually in the continental United States. At least 25 tornadoes were reported during this storm in Nebraska, Iowa, Minnesota, Illinois, and Wisconsin. During the longer lived tornadoes, the peak current for positive flashes increased in magnitude. While this characteristic of lightning does not permit improvements in forecasting these severe weather events, it does imply a change in the storm structure during the tornadoes. The change in the CG flash characteristics is shown relative to the tornadoes’ positions and times of occurrence using time-series plots and radar images for this storm system.