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Purdue University: "A study of the severe weather of 2 June 1990"

Final Report

On 2 June 1990, 42 tornadoes occurred in Indiana, four of which produced F4 damage. Radar imagery indicated that several of the cells were isolated and were, consequently, easily tracked. Mesoanalyses were conducted of the surface fields of temperature, moisture and wind over Indiana, Illinois, and adjacent areas in order to clarify the roles played by the high degree of thermodynamic instability and the strong convergence of surface moisture noted that day by forecasters. The roles played by the strong upper-level diffluence that was present and the positive vorticity maximum that tracked from the Missouri-Iowa border across Indiana were also investigated in order to relate the upper-level flow and associated kinematic fields to observed storm motions.

Detailed analyses of the evolution of the radar reflectivity patterns were examined for several tornadic cells, using conventional WSR-74C (5cm) data from Indianapolis and WSR-57 (10 cm) from Evansville. Both low-level and elevated presentations were used in the analysis. A subset of this component of the research focused on testing the current conceptual High-Precipitation (HP) supercell models. Two new three-dimensional HP storm conceptual models were also derived from the study. A limited sample of 15 storms from 9 cases (1988 through 1992), including the June 2 event, were used to construct the conceptual models.

A combination of radar, satellite, and lightning data were analyzed to evaluate the usefulness of 15-minute lightning data overlaid on radar and satellite imagery. Analyses of the lightning produced by these storms showed characteristics similar to those previously observed in Texas and Oklahoma supercells. There tended to be a sharp increase in the number of flashes produced during the period a cell exhibited supercell characteristics (as seen in the radar PPI display). In contrast to the Plainfield, IL supercell of 28 August 1990, the number of positive strikes were very few. No clear pattern could be discerned between the lightning characteristics and the production of tornadoes. Lessons learned from the lightning analysis that may be of interest to the forecaster are as follows: 1) availability of lightning data can aid in identifying when a radar echo has reached thunderstorm stage; 2) by monitoring the lightning reports simultaneously with radar imagery, the establishment of a squall line can be confirmed (the fast-moving nature of the line is reflected in the streak-like nature of the lightning strike locations); 3) lightning data can confirm that a storm has entered the supercell stage; and 4) it does not appear that any one-to-one correspondence exists between lightning characteristics and tornado production. However, the flash rate does appear to be correlated with overall storm intensity, suggesting that severe weather is likely associated with the storm.

In summary, the lightning data were shown to provide valuable supplementary information. A mapping of strikes over the past 3 to 6 hours, coupled with an ability to obtain some limited quantitative information on a particular storm's "lightning history", can be a valuable aid in real-time nowcasting and the decision to issue warnings.