Eagle, Wisconsin Tornado - Waukesha County

Jun 24, 2010
The tornadoes that struck Eagle, and other parts of Southern Wisconsin were a surprise to many as the forecast called for limited severe weather earlier in the day. So, what happened? How did the atmospheric environment change so rapidly as to produce severe thunderstorms? Below I explain in further detail.

Weather Synopsis: 3:00 P.M. - 7:00 P.M.

There was an outflow boundary positioned across Southern Iowa into Illinois. The storms erupted along this boundary during the late afternoon, requring the Storm Prediction Center to issue a tornado watch for Southeast Iowa and North Central Illinois. To the north was an area of showers and isolated thunderstorms that developed across eastern Iowa in the late afternoon marching eastward into Wisconsin. These storms were not severe, and looked meager for several hours on radar.

The Positives:
There was a strong shortwave trough across Minnesota and Wisconsin that promoted lift for the storms to develop. Another ingredient that played an integral part in the formation of the storms was wind shear and instability. Deep layer shear (0-6 km), was more than adequate to help the storms sustain themselves as they traveled east. Enough moisture and instability were present for the storms to become severe.

The Negatives:
One reason forecasters thought severe weather would be minimal was the presence of weak lapse rates (decrease in air temperature with height). This might explain why the storms were not impressive when they formed in Iowa and as they traveled into southern Wisconsin. However, by the time the storms traveled further east into the area the lapse rates improved. Around 7:00 p.m., the storms were starting to appear slightly more organized across far South Central Wisconsin, but there was nothing that illustrated severe weather.

Storms Become Tornadic! 7:00 P.M. - 11:00 P.M.

Around 8:00 p.m., I noticed an interesting anvil overhead to my west and quickly checked the radar. At this time, there were numerous tornado warnings across southern Wisconsin. A particular area of concern was a supercell thunderstorm north of Janesville. The storm continued to strengthen and move east toward Whitewater and Palmyra, and produced the tornado that caused damage in Eagle. Once the tornado dissipated east of Eagle, the supercell began growing in size and produced a large amount of damaging downburst/straight-line winds across Genesee, Vernon, Big Bend, and Muskego, Wisconsin. Additionally, another tornado formed south of Vernon and moved east into Muskego. Milwaukee and Racine counties were spared from the most intense winds, but did experience sporadic tree damage.

The meteorologists at the National Weather Service and Milwaukee news stations did a great job issuing and relaying warnings. Thankfully, this prevented deaths and injuries.

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