An Investigation of Mesoscale Processes and Convective Morphology on 27 April 2011

The 27 April 2011 Super Outbreak, which was part of a multi-day outbreak of severe weather spanning 25-28 April 2011, produced 199 tornadoes and resulted in more than 320 fatalities and 2700 injuries. This event comprised three tornadic systems, with the afternoon supercell outbreak being the most notable for producing long-track, strong tornadoes. However, a morning QLCS was also especially severe and produced 76 tornadoes of up to EF3 intensity from multiple bow echoes and line-embedded mesovortices. Additionally, a weakly-tornadic midday QLCS occurred over northern Mississippi and Alabama, which produced a residual thermal boundary that influenced the evolution of the northern supercells later during the day.

This goal of this study is to determine how mesoscale dynamical processes resulted in the formation of these three systems and to evaluate which environmental factors led to the differing organization, morphology, and severity of convection on 27 April. While several studies have examined the roles of surface boundaries in the initiation and organization of convection, relatively few studies have thoroughly investigated the roles of mesoscale forcing and organizational mechanisms for convection located within the warm sector (i.e., not along a surface boundary). Such processes are important in this case owing to the high-degree of mesoscale organization found during the supercell outbreak, which was manifested as parallel lines of convection ahead of the surface dryline.

In additional to surface boundaries, some potentially important mesoscale phenomena for convective organization within tornado outbreaks found in past studies are cold fronts aloft, mesoscale instabilities, and gravity waves generated both by convection and by the geostrophic adjustment process in response to unbalanced flow aloft. Many of these mesoscale processes are highly influenced by the environmental response to previous convection, which warrants a thorough investigation into the influence that earlier convection may have had on the eventual evolution of the supercell outbreak. An investigation into such processes and their hypothesized influence in the evolution of this event will be discussed.