Date: April 3-4, 1974
Fujita scale intensity: F0-F5
During the worst tornado outbreak in U.S. history, nearly 150 tornadoes touched down in 13 states. Over a 16-hour period, more than 300 people were killed and more than 5,000 hurt.
Spawned from severe thunderstorms, the twisters varied from F0 to F5. One whirlwind was five miles (eight kilometers) wide. At one point 15 tornadoes were on the ground at the same time. One twister lasted for more than two hours. Another two tornadoes swirled around each other, caught in a violent dance.
Thirteen states Alabama, Georgia, Illinois, Indiana, Kentucky, Michigan, Mississippi, North Carolina, Ohio, South Carolina, Tennessee, Virginia, and West Virginiasaw twisters.
The most damaging and deadly touched down in Xenia, Ohio, where the F5 tornado leveled half of the town in just nine minutes. Winds topped 260 miles (418 kilometers) an hour. Four schools, nine churches, and more than 1,300 homes and businesses were totaled. Nearly half of the town's population of 25,000 were left homeless.
Although National Weather Service forecasters saw the potential for twisters to develop, they had to wait for a tornado sighting before they could issue a warning.
Today more sophisticated technology lets forecasters see storms evolving, and warnings are often issued before tornadoes even form. The result is an average warning time of 11 minutes.