The Tornadic Quasi-Linear Convective System over East-Central Wisconsin on 6-7 August 2013. Part 2: Radar and Mesovortex Analysis

Detailed radar analysis of the second of two severe bow echoes embedded within a larger quasi-linear convective system (QLCS) on the late night of 6 August 2013 are presented. The first severe bow echo moved across parts of central Wisconsin while the second bow echo moved across east-central and northeast Wisconsin. Detailed damage survey analysis was completed with the second bow echo. This bow echo formed west of Green Bay, Wisconsin and produced narrow swaths of wind damage within a 50-km wide corridor. Careful damage survey analysis and Weather Surveillance Radar-1988 Doppler (WSR-88D) data from Green Bay, Wisconsin revealed the genesis of seven low-level mesovortices within the second convective system. Damage assessments revealed that six of these mesovortices were tornadic reaching EF1/EF2 intensity. The remaining mesovortex was non-tornadic. The first two mesovortices formed when the reflectivity line segment was nearly oriented in a “north-south” pattern within the larger QLCS and became tornadic within ten minutes after their genesis.  These two vortices merged approximately fourteen to sixteen minutes after their genesis resulting in one mesovortex which continued to remain tornadic. This differs from previous studies in which tornadic activity terminates after merger of two mesovortices.  

Approximately twenty minutes later, subsequent mesovortices formed from the apex of the developing bow northward to a nearly east-west surface boundary produced from an earlier convective line. Swaths of damaging winds were co-located with all seven mesovortices. Damage survey analysis showed a straight-line to convergence-like damage pattern from several tornadic mesovortices. Careful analysis of the radar data suggests that it was difficult to distinguish between tornadic and non-tornadic vortices.  This differs from earlier observations during the BAMEX project and other recent studies.  All of the tornadic vortices initially formed at low-levels then rapidly deepened just prior to tornado formation. This was also true for non-tornadic vortices.  One feature that did stand out was the very rapid deepening and increased low-level rotation of several tornadic mesovortices just prior to tornado formation. This is consistent with previous studies. A few of the tornadic mesovortices exceeded depths of 4 km which is remarkable for late-night tornadic QLCS events. Similar to previous studies the mesovortices (both tornadic and non-tornadic) appeared to form coincident or after the intensification of the mesoscale rear-Inflow jet (RIJ). This paper will focus on two primary aspects: 1) the relationship between an intensifying RIJ and mesovortex genesis and 2) characteristics of the tornadic and non-tornadic mesovortices compared to previous work. The detection and warning implications are also discussed from these results.