NEW : tornadoes can be spawned from rotating thunderstorms called supercells . Authorities are investigating damage from the storms . Chief meteorologist : ` ` It is very rare to have all these ingredients mixed together '' The worst tornado outbreak in U.S. history occurred in April 1974 .
NEW : Tornadoes can be spawned from rotating thunderstorms called supercells . Authorities are assessing damage from the storms . 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 .