NEW : Tornadoes can be spawned from rotating thunderstorms called supercells . These storms are so powerful that they can spin in a circle for hours, and even days at a time. CNN meteorologist : ` Several meteorological conditions combined Wednesday to create a particularly dangerous mix , Morris said . ` It is very rare to have all these ingredients come together '' The worst tornado outbreak in U.S. history occurred in April 1974 .
NEW : Tornadoes can be spawned from rotating thunderstorms called supercells . These storms are so powerful that they can spin in a circle for hours, and even days at a time. CNN meteorologist : ` ` It is very rare to have all these ingredients come together '' The worst tornado outbreak in U.S. history occurred in April 1974 .