Storm mode evolution from a quasi-linear convective system to a discrete tornadic supercell during the historic Wisconsin tornado outbreak of 18 August 2005: a radar perspective

The state of Wisconsin experienced its largest one-day tornado outbreak in recorded history during the afternoon and evening hours of 18 August 2005, with 27 tornadoes reported across the central and southern portions of the state. A particularly challenging aspect of this tornadic episode for the operational forecaster developed early in the event over Vernon County, in southwestern Wisconsin. A complex change in storm mode, from a quasi-linear convective system (QLCS) into a more discrete classic supercell, was coincident with rapid tornadogenesis as the storm crossed Vernon County. Two tornadoes (producing F1 and F2 damage, respectively) touched down in Vernon County during this change in storm mode.

During the early afternoon hours of 18 August 2005, a 75-km long QLCS formed over extreme southeastern Minnesota, west of Vernon County, Wisconsin, in response to cold frontal convergence and support aloft from a midtropospheric shortwave trough moving across central Minnesota. As the QLCS crossed the Mississippi River into Vernon County, environmental shear and convective instability over Wisconsin were augmented on the mesoscale by a retreating outflow boundary from overnight convection. In addition, cell mergers (with weak convection forming ahead of the QLCS) appear to have instigated a rapid change in storm mode from the linear system into a cyclic, classic supercell across Vernon County.

Conceptual models of the evolution of linear convective systems don't typically include such a rapid change of storm mode into discrete tornadic supercells. However, Burgess and Curran (1985) documented a similar event in Oklahoma (26 April 1984) where a mature linear system transitioned into discrete supercells due to changes in the overall vertical shear profiles as the storms moved east.

The close proximity of the La Crosse (KARX) WSR-88D provided a well-sampled, high resolution dataset of associated wind fields and convective processes, which is presented here. This radar-centric presentation will focus primarily on the QLCS-to-supercell storm mode change which took place across Vernon County. Cell mergers immediately prior to both instances of tornadogenesis, are theorized to have played a major role in the change in storm mode.