Western Wisconsin Derecho
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| Shelf cloud as it approached Downsville, WI | |
| Date(s) | July 15, 1980 |
|---|---|
| Peak wind gust | 112 mph |
| Tornadoes caused | 10 |
| Maximum rated tornado1 | F3 tornado |
| Fatalities | 3 |
| Damage | $240 million (1980) |
| Areas affected | Wisconsin |
| 1Most severe tornado damage; see Fujita scale | |
The Western Wisconsin Derecho was a derecho, or severe weather system, that occurred through several counties of Western Wisconsin on July 15, 1980. It caused $240 million in damage (1980 dollars) -- the largest storm damage total in Wisconsin history to that point. Three people were killed. To this day, the storm is still referred to as "The July 15th (1980) Storm," "July 15th, 1980," or simply "The Storm."
The storm developed during the devastating heat wave of 1980. Eight of the previous nine days in the Twin Cities had highs above 90 °F (32 °C). The high on 11 July was 100 °F (38 °C). On 14 July, the day before the storm, the high was 99 °F (37 °C); the overnight low was a sticky 79 °F (26 °C).
At 2 p.m. on the afternoon of July 15, a weak area of low pressure was stationed over eastern South Dakota, adjacent to a warm front that extended southeast through southern Minnesota into eastern Iowa. Ahead (north) of the warm front was an area of hot, dry air: the high temperature for the day at Eau Claire was 93 °F (34 °C) and 93 °F (34 °C) in the Twin Cities. South of the front was very humid, hot air: Cedar Rapids, Iowa had a high temperature of 98 °F (37 °C).
As the afternoon progressed, the warm front moved into the drier air to the north, pulling with it the moist air over the dry air. Regional meteorologists were aware of the potential for thunderstorms because of the low pressure system and the intense heat and moisture. The storm system formed in eastern Minnesota during the early evening of July 15, when a boater was killed on Prior Lake. Winds were estimated at 110 mph. Storm damage in Minnesota approached $43 million.
Entering Wisconsin around 7:30 p.m., the storm devastated approximately 4,800 square miles of St Croix, Pierce, Dunn, Eau Claire, Chippewa, and Clark counties; the band of damage was more than 40 miles wide. The greatest destruction occurred from Menomonie through Eau Claire. A maximum wind speed of 112 mph (180 km/h) was recorded at the Chippewa Valley Regional Airport at 8:48 p.m., blowing away the airport's anemometer; 100 mph wind was recorded in the city of Eau Claire. At 9:39 p.m., WEAU-TV broadcast an alert from the National Weather Service regarding one of several tornado warnings that evening for Eau Claire County; moments later, the station lost power and went off-the-air. Seven of the city's eight radio stations were still off-the-air the next morning. Up to 97 percent of Northern States Power customers in Eau Claire were left without power following the storm. After seven days, 20,000 customers still had no power.
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