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 massivethunderstorms to boil upward. The rotational energy for tornadoes came fromwind shear, a product of changing wind direction and speed with altitude. Whenthe rotation was entrained into the super-cell thunderstorms, numeroustornadoes 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 stormskilled 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.