15th Year Anniversary of Mid-South Severe Weather Outbreak

Nearly 15 years have passed since a prolific severe weather outbreak that took place across central Kentucky and middle Tennessee during the late afternoon of 16 April 1998. Unfortunately, these devastating events resulted in 42 tornadoes, 143 injuries, 12 fatalities, and a catastrophic hail event accumulating over $500 million in damages. The purpose of this study is to provide a reanalysis of the synoptic and mesoscale environments that were conducive to these devastating outcomes, with a particular focus on the record hail event that occurred over Bowling Green, KY and the urban tornado that struck downtown Nashville, TN. The data used in this study comes from the North American Regional Reanalysis dataset and analyzed using the Interactive Data Viewer. The large-scale weather system was characterized by a broad open wave trough over the western U.S. and a closed upper-level low over Manitoba/Ontario Canada. The Mid-South was positioned beneath the exit region of 250 and 500 hPa jet streaks and diffluent circulation from the Polar and Subtropical jetstreams. At 850 hPa a southwesterly low-level jet in excess of 45 kts helped spread anomalous precipitable water values in the 99th percentile across the region. At the surface, a moisture axis extended alongside the Mississippi River with dew points ranging from 17-22˚C, with a low-level moisture convergence maximum over south-central KY and central TN. Moreover, a surface based instability axis was in place, with a strong SBCAPE gradient of 900-2400 J kg-1 with little convective inhibition across the region. It was this conditionally unstable large-scale environment, along with the advancement of a strong surface cold frontal boundary that provided sufficient forcing for vertical ascent within an anomalous moist, warm sector. Subsequently, the development and maintenance of clustered supercells produced widespread record hail and tornadoes across KY and TN.