A Case of Prolific QLCS Tornado Production: The Lower Mississippi River Valley Tornado Outbreak of 30 April, 2017

On the morning of Sunday, April 30th 2017, an intense quasi-linear convective system (QLCS) advanced through northeast Louisiana and into Mississippi. As this system evolved, it accelerated to the northeast and became an efficient producer of meso-vortices, which resulted in numerous tornadoes. In all, 29 tornadoes occurred across the NWS Jackson forecast area during a seven-hour period. Of these, five tornadoes were rated strong (EF2). Historically, this extreme number of tornadoes for an event ranks in the top five for the NWS Jackson forecast area.

This study will investigate the complex evolution of the system, and the overall background environment that was in place to support QLCS tornadoes. Throughout the development of the QLCS, two potentially critical mass-field adjustments were ongoing at separate spatial scales, to facilitate an environment more favorable for line-embedded tornadoes: (1) adjustments to the synoptic-scale mid-level flow and associated deep shear vectors concurrent with the development of a convectively-induced cold pool, and (2) the development and evolution of a line-embedded mesoscale convective vortex (MCV) that further augmented vertical kinematic fields. This study will emphasize the influence of these phenomena on favoring sustained, upright convective plumes along the QLCS’s updraft-downdraft convergence zone that ingested ambient and convectively enhanced vorticity in support of QLCS tornadoes. The relationship between evolving vertical shear orientations relative to the leading convective line and their support of intense leading-line convection and tornadoes will be addressed in the context of previous research. Additionally, such a complex system resulted in challenges during the public warning phase as 19 tornado warnings were issued during the seven-hour period, including some in the pre-dawn hours. Tornadic debris signatures (TDSs) were also prevalent during many of the tornadoes. These signatures played an important role in messaging along with the post analysis and damage survey process.