The Devastating 27 April 2011 Tornado Outbreak: Initial Scientific Assessment

By some metrics, the tornado outbreak on 27 April 2011 is among the most significant tornado outbreaks, rivaling the infamous “Jumbo Outbreak” of 3-4 April 1974. The number of tornadoes over a 24 h period (midnight to midnight) was 199 (Fig. 1), the tornado fatalities and injuries were 319 and more than 2,400, respectively, and the insurable loss was aroundexceeded four billion dollars. In this paper, we provide an meteorological overview of this outbreak, and illustrate some unique features documented by WSR-88D radars; instruments operated by the University of Alabama in Huntsville, including the Advanced Radar for Meteorological and Operational Research (ARMOR) C-band dual polarization radar, the Mobile Alabama X-band(MAX) dual polarization radar, the Mobile Integrated Profiling System(MIPS); and a mix of surface measurements. The initial results have stimulated a number of research questions: • Why were storms so efficient in producing tornadoes? • Why were so many tornadoes long tracked, intense, and wide? • What mechanisms produced an impressive mesoscale convective vortex and the flurry of 16 tornadoes associated with it over Marshall and DeKalb counties? • Why was debris effectively lofted to relatively high altitudes? • Why did the violent tornadoes exhibit horizontal vortices along their periphery? • Did external influences, such as boundaries, gravity waves, and topography play a role in tornadogenesis and tornado intensity change? The primary motivation for this preliminary research is to document the diverse characteristics of this outbreak, and summarize the detailed analyses that are worthy of additional research on this case.