An Overview of the 27 April 2011 Super Outbreak over North Alabama and Southern Tennessee

The largest tornado outbreak in the NWS Huntsville Weather and Forecast Office's County Warning Area began during the early morning hours of 27 April 2011, with the first of three successive waves of tornadic thunderstorms. A quasi-linear convective system (QLCS) and an embedded mesoscale convective vortex during the early morning hours were responsible for 21 tornadoes producing EF-0 and EF-2 damage during the first wave. A second QLCS occurred during the late morning hours, producing seven tornadoes of EF-0 to EF-1 intensity. The most deadly and powerful wave occurred in the afternoon with the “march of the supercells”. This wave produced 11 tornadoes, six of which were of EF-4 to EF-5 intensity. During these waves, NWS Huntsville issued 92 tornado warnings and 31 severe thunderstorm warnings.

This event was unique in its meteorological setup and maintenance that lasted from 0900 UTC on 27 April until 0210 UTC on 28 April across the NWS Huntsville County Warning Area. To begin understanding the unique characteristics of this event, this presentation will include a discussion of the synoptic and elements of the mesoscale environment for each of the three waves of severe weather. In addition, an analysis of radar data, associated with the NWS WSR-88Ds and data from the University of Alabama at Huntsville's Advanced Radar for Meteorological and Operational Research dual-polarimetric radar, will also be presented.