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A Statistical Evaluation of Tornado-Production Tendencies of Southernmost Supercells Compared to Adjacent Supercells in a North–South-Oriented Line

Supercell events in which adjacent storms were oriented in a north-south, linear manner, with at least one storm producing a tornado, were identified to determine if southern-end supercells were favored for tornado production over other cells in the line. Data included the official Storm Prediction Center severe-weather database and Level II WSR-88D data for the full 2016 and 2013 calendar years, with select cases from 2011. A total of 568 supercells, 243 tornadic, were sampled in association with 193 north-south-oriented lines. The Χ2 statistic was used to test for independence between storm location and tornado production and to analyze other possible sources of dependency, including number of supercells in the line, month of occurrence, geographic region, type of surface boundary initiating the storms, and tornado destruction potential index. There was no statistically significant evidence to indicate that southernmost supercells are more prolific tornado producers than other supercells in the line, although certain subgroupings had larger observed frequencies of occurrence than expected. Among the other, more conclusive results were: 1) in March, April and October, fewer southernmost supercells produced tornadoes than expected; 2) more north-south-oriented lines occurred in the Southern Plains than any other U.S. geographic region, and these lines were commonly initiated by the dryline; 3) there was no dependency between southern-end storm tornado production and geographic location; and 4) warm fronts resulted in more southernmost tornadoes than expected values, but not at statistically significant levels.