Issued 550 pm CDT Mon Mar 21 2011
Previous articles (click here) have discussed the winter temperatures and snowfall when an El Nino winter is followed the next year by a La Nina Winter. 2010-2011 was such a case. Now let's concentrate on what happens the following warm season regarding severe weather. The primary severe weather season is April to September in southern Wisconsin, so we looked back at the following years during those warm season months in 1965, 1971, 1974, 1989, 1996, 1999, and 2008.
It is interesting to note that several significant events did happen in or near southern Wisconsin during those years. In 1965, the Palm Sunday Outbreak occurred, which was one of the more significant tornado outbreaks during the 1960s. In 1974, the SuperOutbreak of tornadoes took place, with roughly 150 tornadoes in one day. In 1996, the violent Oakfield F5 tornado struck in Fond du Lac County, the last F5 tornado to impact the NWS Milwaukee/Sullivan forecast area.
For the Milwaukee Forecast area, here are the tornado tallies for those years.
Year Tornadoes Strong Violent
1965 11 3 F2
1974 6 2 F3 1 F4
1989 10 1 F2
1996 13 1 F5
2008 26 1 EF2
Looking over this data, 5 of the 7 years had strong tornadoes in southern Wisconsin and 2 of the 7 years had violent tornadoes.
The following maps show all the severe weather reports for the southern Wisconsin area from April 1 to Sep 30 on each of those 7 years. The Tornado F-scale, wind gusts in knots, and hail size is shown. This images are courtesy of the Storm Prediction Center (click here for interactive map).
You probably noticed that there is a general trend for more and more severe weather reports, especially in 1996 and 2008. In the 1980s, there was a significant push toward verfication of severe weather in the National Weather Service. In the 1990s, the WSR-88D Doppler Radars became widely used, and severe weather spotter networks grew very large, and video cameras and storm chasers were on the upswing. So severe weather reports became easier to come by and documentation has been better.
Jeff Craven, Science and Operations Officer
National Weather Service Milwaukee Sullivan, WI