Thunderstorm Wind — Rock, Wisconsin

2000-08-05 · near Evansville, Rock, Wisconsin

Wider weather episode

This summer's most powerful downburst wind event to affect south-central and southeast Wisconsin pummeled portions of Dane, Green, and Rock counties with damaging hurricane-force winds, large hail, and urban-flood rains. As the 35 mile long line of thunderstorms moved southeast through Dane, northern Green, and Rock Counties, it increased in strength, ultimately producing straight-line winds estimated up to 100 knots (115 mph), heavy rains at a rate of 1 to 3 inches per hour, and hail stones 1 inch in diameter. Due to downed power lines and poles, ultimately about 18,000 customers had electrical power disrupted for 1 to 2 days. Descriptions of weather-related events follow in a county-by-county breakdown.Dane County:Powerful thunderstorm winds, estimated at 56 knots (65 mph), toppled several large trees in and near the city of Belleville. There were no reports of injuries or deaths. Minor urban-type flooding was noted due to intense rain accompanying the storm.Green County:The squall-line intensified as it clipped the northeast quarter of Green County. Based on inflicted damage, winds gusts were estimated to be 74 knots (85 mph). The worst-hit area extended from a subdivision west of the village of Dayton (Town of Exeter), east/southeast to the county line. About 50 homes sustained varying amounts of damage caused by felled trees, tree branches, or the wind outright. Damage to home siding and roofs was also noted. Hundreds of trees were toppled, about 2 dozen farm buildings were damaged or destroyed, several silos were damaged, and two semi-tractor trailers were overturned. Many power lines were either blown over or knocked down by tree debris.Rock County:This county bore the brunt of the storm as its straight-line winds intensified to an estimated 100 knots (115 mph). The macroburst damage path was 3 to 6 miles wide, extending from southwest of Evansville in the Town of Magnolia (Highway 59 and A) east/southeast through the city of Janesville to southeast of Avalon near Caver Roehl Park. A newspaper headline nicely stated the effects: Rain! Wind! Wow! The powerful winds leveled thousands of trees (some 200 years old); tore roofs off homes; businesses, and farm buildings; blew tops of silos off, flipped over vehicles; blew in windows; damaged or peeled off home siding; knocked over billboards and road signs; closed many roads due to debris, and flattened corn and soybean fields. Based on newspaper photographs and eyewitness accounts, maximum wind gusts were estimated to be in the 87 to 100 knots range (100 -115 mph). The worst of the damage appeared to be equivalent to that caused by tornado winds of about 115 mph, or at the bottom of the F2 category. Many people noted that the sky grew very dark as the storm blew in with a wall of white rain mixed with tree leaves and branches. The city of Janesville was at ground zero, as a 3.7 square mile looked like a bomb had hit its downtown area and the near east and west sides. A roof was torn off one business, and bricks were pealed off another business. It took at least 2 weeks to remove the tree debris from city streets, sidewalks, and backyards. Dozens of vehicles were damaged by felled trees and branches. It is estimated that 400 to 500 city homes sustained some degree of damage (6 destroyed). Traxler and Bond Parks looked like disaster areas and were closed. About 50 electrical poles were snapped by tree debris or the winds. One person in the city was injured when a tree fell on their pickup truck. Just north of Janesville on Interstate-90, another person was injured when their SUV was flipped over by the winds. In addition, 5 semi-tractor trailers were overturned on I-90 in the same vicinity. A new sub-division 3 miles south of Janesville experienced extensive roof or siding damage due to the winds or tree debris. Several vehicles were damaged as well. Just to the south of the city, the wastewater treatment plant recorded 1.48 inches of rain as the storms moved through, resulting in urban flooding. There were other unofficial reports of 2 to 3 inches in the city area.The civil Town of Magnolia, in the western part of the county, was also hard hit by the macroburst, especially near and along Highway 59 and County Trunk Highway A. About 40 power poles in this Town were toppled by the winds or tree debris.A little further north on Croft Road, just southwest of Evansville, a local resident noted this sequence of events: the initial, powerful winds and blinding rains lasted for about 5 minutes and were followed by hailstones up to 1 inch in diameter. Then the rain and hail stopped. After a couple minutes he noticed across the road in a field a ground-based vortex pulling corn stalks up to about 200 feet above the ground, while above it at cloud base was a funnel cloud. This weak vortex then hit a barn and caused slight damage before dissipating. Shortly thereafter, another round of rain, and gusty, but much weaker winds occurred. Based on this resident's account, and their prior experiences with tornadoes while living in Texas, this vortex was classified as a tornado. The macroburst continued to inflict wind damage to a point about 3 miles east/southeast of the Janesville city limits along Highway 14, at which point it weakened. However, the thunderstorms briefly intensified once again southeast of the village of Avalon near Carver Roehl Park. At this location, a barn's roof was lifted and dropped by powerful winds, resulting in a collapsed structure which trapped 40 steer. Fifteen of the steer died from injuries, and some of the hay bales were destroyed. Collectively across Rock County, about 4500 acres of corn and soybean crops were badly damaged or destroyed. Thirty-five farm buildings had minor damage, 8 had major damage, and 19 were destroyed by the winds. Walworth County:The remains of the squall-line generated downburst winds which leveled large trees in and near the city of East Troy.Racine County:The remains of the squall-line generated downburst winds which leveled large trees west of Waterford.Kenosha County:The remains of the squall-line generated downburst winds which leveled large trees southwest of the village of Twin Lakes.Milwaukee County:The remains of the thunderstorms dumped rain amounts of 2.25 to 3.1 inches across Milwaukee County based on Ham reports. Moderate urban flooding resulted. A West Allis motorist was rescued from their stalled vehicle due to water depths of 1.5 to 3 feet in low spots on roads. Similar water depths on strees were noted elsewhere in the county, especially in the southern half. No damage was reported. Synoptically, an upper level trough was moving across Wisconsin on August 5th, with a weak vorticity maxima over the southern part of the state moving east/southeast. The south winds of a low-level jet fed the storms as west-northwest flow at 18,000 feet intensified to 60 knots based on profiler data. Thunderstorm echo tops were 40 to 42,000 feet.

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Source: NOAA Storm Events Database, event_id 5172599. Narrative written by the NWS forecast office that issued the report.