May 28, 2012: Straight-line wind across five counties

The Memorial Day holiday became turbulent during the late afternoon and evening hours as a line of severe thunderstorms ahead of a cold front crossed southern Wisconsin, producing damaging winds gusts and large hail up to 1.25 inches in diameter. An unstable air mass was in place, with high temperatures in the upper 80s to around 90, and dew points in the lower 60s, that produced mixed-layer CAPE values of 1000-1800 J/kg, with forcing along the front enhanced by weak upper level divergence and weak differential vorticity advection. While directional shear was minimal, with above-surface winds unidirectional from the west, there was sufficient speed shear to support organized updrafts, and for the individual cells to merge into bowing line segments. Numerous reports of downed trees and power lines were received, as well as some minor structural damage. There were also many large hail reports, mainly along and south of a line from northern Sauk County, near Reedsburg and the Wisconsin Dells, to northern Ozaukee County near Port Washington. Fire officials in Edgerton, in northeast Rock County, and in the city of Jackson in Washington County reported structural fires caused by lightning strikes.