Wisconsin experiences tornadoes, which are columns of violently rotating air extending from a thunderstorm to the ground. The state’s position in the Midwest frequently aligns atmospheric conditions to produce these severe weather events. Wisconsin is susceptible because it sits at the convergence point of warm, moist air from the Gulf of Mexico and cooler, drier air masses from Canada. This clash creates the instability and wind shear necessary for the formation of supercell thunderstorms that spawn tornadoes.
Frequency and Historical Context
Wisconsin averages about 23 tornadoes annually, though the yearly count can vary significantly, such as the 62 recorded in 2005. The majority of these events are on the lower end of the Enhanced Fujita Scale (EF Scale), which classifies intensity based on wind speed and damage. Most tornadoes in Wisconsin are classified as EF0 or EF1, causing light to moderate damage.
However, the state has experienced destructive, violent tornadoes, demonstrating the potential for extreme weather. The village of Barneveld in Iowa County was nearly destroyed by an F5 tornado (now EF5) in June 1984, killing nine people and injuring 200 others. The Oakfield tornado in Fond du Lac County in July 1996 was also rated F5 and became one of the costliest in state history. These EF3, EF4, and EF5 storms, while less common, highlight the need for preparedness.
Geographic Distribution and Seasonality
The period between May and August is Wisconsin’s peak tornado season, with the highest frequency occurring in June. This seasonality aligns with the time when warm, humid air is consistently available to interact with passing cold fronts and upper-level disturbances. Tornadoes most commonly strike during the late afternoon and early evening, between 3:00 p.m. and 7:00 p.m., when daytime heating maximizes atmospheric instability.
Geographically, the southwestern and western portions of Wisconsin experience a higher frequency of tornadoes compared to the northern counties. This distribution difference is due to the higher likelihood of severe storms tracking from the plains states and more favorable terrain for air mass interaction in the south. The area near the Mississippi River averages about one tornado day per year, a rate that decreases significantly toward the north.
Safety and Alert Systems
Understanding the difference between a Tornado Watch and a Tornado Warning is foundational to safety during severe weather. A Tornado Watch means conditions are favorable for tornadoes to develop, signaling residents to review emergency plans and remain alert. A Tornado Warning is a more urgent alert, indicating a tornado has been sighted or detected by radar, meaning there is imminent danger.
Alerts are delivered through several mechanisms, including NOAA Weather Radio, which provides continuous information directly from the National Weather Service. Many communities utilize outdoor warning sirens, though these are primarily intended to warn people who are outside. When a warning is issued, seek shelter in the lowest level of a sturdy building, such as a basement. If a basement is unavailable, the safest place is an interior room on the ground floor, away from windows, under a sturdy table to protect against falling debris.