Anomalous Cloud-to-Ground Lightning in an F5-Tornado-Producing Supercell Thunderstorm on 28 August 1990.

An F5 tornado that devastated Plainfield, Illinois, and environs on 28 August 1990, killing 29 people, is shown to be preduced by a thunderstorm characterized by highly anomalous could-to-ground (CG) lightning activity. Unlike typical summertime convective in which the majority of CG flashes lower negative charge to ground, the Plainfield storm produced predominantly positive-polarity CG flashes during development. Changes in storm structure revealed by radar imagery appear tied to distinct patterns in the CG flash parameters of polarity, flash frequency, first stroke peak current, flash multiplicity, and flash location relative to the parent cumulonimbus. The primary findings are 1) the anomalous predominance (91%) of positive-polarity CG flashes during development; 2) positive CG flashes anomalously occurring mainly within the region of the storm's radar reflectivity core; 3) the onset of a major downburst coinciding with a sudden increase in CG flash rate, from 4 to 17 flashes min¹, and positive percentage, from 91% to 100%; 4) a reduction in flash rate from 17 to 3 flashes min¹ in 3 min, coinciding with the rapid development of a front-flank mesocyclone; 5) a 20-min span of reduced CG activity (1-2 flashes min¹) coinciding with tornado formation and intensification; 6) the reversal in dominant CG flash polarity from positive to negative over the entire thunderstorm domain at the time of tornado touchdown, an occurrence previously undocumented in any other tornadic thunderstorm; 7) steadily weakening mean peak current values (from +100 to +38 kA) leading to the reversal, and steadily strengthening values (from 20 to 40 kA) following the reversal; and 8) the temporary clustering of all CG flashes within 10 km of the tornadic mesocyclone at maximum (F5) tornadic intensity. These findings suggest the possibility of a relationship between this tornadic thunderstorm's dynamics and electrical activity.