The Tornadic Quasi-Linear Convective System over East-Central Wisconsin on 6-7 August 2013. Part 1: Environmental Setting and Radar Overview.
Eugene S. Brusky Jr., Mike Cellitti and Scott Berschback NOAA / National Weather Service Green Bay, Wisconsin 54313
A rare early-morning tornadic quasi-linear convective system (QLCS) struck east-central Wisconsin shortly after midnight on 7 August 2013. This was the largest early-morning tornado outbreak in east-central Wisconsin since 1950. The tornadic nature of this damaging QLCS was not well anticipated. During the evening, several thunderstorm clusters extending from north-central through west-central Wisconsin congealed to form two large bowing segments over central and east-central Wisconsin by 0500 UTC. The bowing segments organized within an increasingly supportive synoptic-scale setting characterized by an approaching shortwave trough, increasing deep-layer shear associated with an unseasonably strong upper-level jet, weak low-level warm advection and low-level moisture advection. The setting within which the QLCS evolved could generally be categorized as a moderate shear and low instability environment. According to the SPC RAP-based mesoanalysis, 0 - 6 km bulk shear values of 30 m s -1 and MLCAPE values around 500 J kg -1 were representative of the pre-storm conditions.
Preliminary radar analysis indicated that the QLCS generated numerous mesovortices as it accelerated eastward across central and east-central Wisconsin. Two primary episodes of mesovortex development were observed. The first occurred over central Wisconsin and the second a few hours later over east-central Wisconsin. Based on storm reports from emergency managers and the public, only isolated wind damage occurred with the first episode. Several tornadoes occurred with the second episode based on NWS damage assessments. No formal NWS damage assessments were conducted with the first episode. In both episodes, a portion of the QLCS was observed to surge eastward, with the leading updraft-downdraft convergence zone (UDCZ) becoming orientated nearly normal to the 0 - 3 km bulk shear vectors. The second phase of mesovortex development was considerably more impressive, spawning six tornadoes of EF1 - EF2 intensity within 45 minutes. The tornadic mesovortices were observed to race eastward at speeds of 30 to 32 m s -1 in response to a mature rear-inflow jet (RIJ). All known tornadic mesovortices evolved within a 50-km wide corridor bounded by the apex of the bowing line segment to the south, and an east-west thunderstorm outflow boundary to the north. Preliminary radar and environmental analysis suggest conditions were favorable for mesovortex genesis and strong intensification.
Given the climatological rarity of early morning tornado outbreaks in northeast Wisconsin, the fact that an impending tornado threat was not well anticipated, the rapid evolution of the tornadic mesovortices, and extreme forward propagation speed of the QLCS, the event offered numerous short-term forecast and warning decision challenges.