The worst tornado swarm ever hit within a 24-hour period on April 3 and 4, 1974. A powerful upper-air trough (cold air aloft) ran across a very warm, humid flow out of the Gulf of Mexico. The resulting instability caused massive thunderstorms to boil upward. The rotational energy for tornadoes came from wind shear, a product of changing wind direction and speed with altitude. When the rotation was entrained into the super-cell thunderstorms, numerous tornadoes developed, many of them being F4s or F5s on the Fujita Scale. In all, 148 tornadoes touched down from the Great Lakes to the Gulf Coast. The storms killed 330 and injured over 5,000. The horrific Xenia, Ohio and Brandenburg, Kentucky tornadoes were part of this severe weather outbreak.

Story by AccuWeather.com Expert Senior Meteorologist John Kocet.