Despite myths that Lake Michigan or skyscrapers shield the city, Chicago is highly susceptible to tornadoes, having been struck by notable events like the devastating 1967 Oak Lawn tornado, an 1876 multiple-vortex tornado in the Loop, and the region's strongest-ever tornado in Plainfield in 1990. The city is also a cornerstone of tornado science, as University of Chicago professor Ted Fujita developed the famous Fujita scale there and pioneered research on downbursts. According to the National Weather Service, a violent (F4–F5) tornado hits the Chicago metro area on average once every 9.8 years, leading experts to conclude the region is "overdue" for a major event. Most fatalities stem from F4 twisters, with three catastrophic days—March 28, 1920, April 21, 1967, and August 28, 1990—accounting for a large share of deaths. Severe weather peaks in May and June, typically between 4 and 8 p.m., though only about 5% of regional tornadoes reach F3 strength or higher. Interestingly, the first tornado ever recorded in Illinois, which struck modern-day Des Plaines, prompted correspondence that ultimately helped establish the National Weather Service.