Storm Record · Dane, Jefferson, Walworth, Waukesha Counties
June 21, 2011: Straight-line wind across four counties
What the National Weather Service recorded
A warm front lifted into southern Wisconsin ahead of slow-moving low pressure in Iowa. Moderate wind shear with 0-6km shear values of 35 to 45 knots and mixed layer (ML) CAPE in the 1500 to 2500J/kg range in place over the region combined with enhanced lift from an upper disturbance rotating through the base of an approaching upper trough to produce strong to severe supercell thunderstorms during the afternoon into the evening of June 21st. The storms produced two EF1 tornadoes in the vicinity of the nearly-stationary warm front that tracked along the border between Fond Du Lac County and Green Lake County. The severe thunderstorms also produced large hail and damaging wind gusts, including a 2-mile wide microburst across sections of Waukesha County that damaged homes, barns and felled thousands of trees and numerous power lines. In addition, the thunderstorms fed off a moisture-rich air mass moving into the region, with precipitable water values around 1.8 inch, that also produced heavy rainfall and flash flooding in portions of Waukesha and Milwaukee counties.
Source: NOAA Storm Events Database · episode 54490
Every report, in the Weather Service's words
Where It Hit
| Location | County | Hazard | Size | Report |
|---|---|---|---|---|
| Eagle (4 NW) | Waukesha | Wind | 110 mph wind | A 2-mile wide swath of damage was caused by a microburst associated with a bowing line of severe thunderstorms that tracked from southwest to northeast across a large portion of Waukesha County. Wind gusts estimated up to between 100 to 110 mph (87 to 96 knots) dropped thousands of trees, damaged homes and barns and blew down power lines that left 10,000 customers without power in Waukesha County. One person was injured by airborne tree debris near the Pewaukee and Delafield town borders. A semi-trailer truck was blown over on Interstate-94 in Waukesha County, while two other high-profile vehicles were nearly blown over by the strong winds, but concrete construction barriers saved the vehicles from being completely blown over. |
| Eagle (2 SE) | Waukesha | Wind | 79 mph wind | |
| Mukwonago | Waukesha | Wind | 70 mph wind | Part of a line of severe thunderstorms with damaging wind gusts up to 70 mph (60 knots) that blew down numerous trees moved through northeast and eastern Waukesha County during the middle afternoon of June 21st. |
| Palmyra (2.7 SE) | Jefferson | Wind | 64 mph wind | A National Weather Service employee reported 12 inch and larger tree limbs blown down by thunderstorm wind gusts estimated at 65 mph (56 knots). |
| East Troy (4 SE) | Walworth | Wind | 60 mph wind | A trained spotter reported large tree branches knocked down by thunderstorm wind gusts estimated to be as high as 60 mph (52 knots). |
| Sun Prairie (4 SSW) | Dane | Hail | 1" hail |
Service areas this storm touched