It was unusually hot in the Midwest on April 11, 1965 -- Palm Sunday. Temperatures climbed into the 80s. Storms began to develop as a strong low-pressure system sliding through Wisconsin drew warm, humid air into southern Michigan. Cooler and drier air high in the atmosphere mixed with moist, warm air near the surface.
The conditions were ripe for tornadoes.
40 years ago this date was the worst in U.S. history, according to NOAA, parent organization of the National Weather Service. By the end of the day, 250 people would die and 1,500 would be injured.