Thursday, 19 June 2014 | Weather
The tornadoes that occurred in Madison and Verona late Monday night and early Tuesday morning were quite unusual in that they were fairly strong (EF2 and EF3 respectively, as estimated by preliminary observations) which spun up on the lead edge of a “QLCS” (quasi-linear convective system), what’s typically referred to as a squall-line.
QLCSs are structured quite differently to the sorts of supercell thunderstorms that are the more common progenitors of tornadoes, with long, linear and somewhat chaotic stretches of fast-moving updraft out ahead of them as opposed to rather slow-moving, geographically-confined regions of intense updraft with strong wind-shears above that allow stable, long-track tornadoes to form. Squall-lines can produce quite dangerous straight-line winds from “congealed” downdrafts pushing outward ahead of them, but they rarely induce enough consistent or uniform rotation to spin up more than what are usually referred-to as “gustnadoes,” evanescent, incidental spin-ups that may reach EF0 or EF1.
Tuesday morning’s QLCS was notable, however, both for its intensity and the interactions that occurred between its “bowing line-segments,” or regions of the QLCS that had temporarily pushed forward ahead of the surrounding line. In this retrospective that was published on the National Weather Service’s Sullivan site, the radar review of the squall-line’s passage across southern Wisconsin shows with remarkable clarity the cyclonic (leftward) rotation that is induced at the north end (“comma head”) of the bowing segment that was bringing strong straight-line winds to southern Dane County. Other cyclonic comma-heads can be seen in the line as well, including one that produced a tornado vortex signature (TVS) over southern Juneau County (the hook visible in the middle of the image before the animation begins). The Dane County comma-head seems to have its origins in the earlier rotation seen near the beginning of the animation over Lafayette County, which probably goes on to produce the Hay Hollow Road spin-ups in northwestern Green County around 11:50 PM. Upstream imagery from the LaCrosse radar site seems to indicate a rotational connection as well with the parental comma-head of the two Platteville tornadoes which occurred about an hour earlier.