NEW : tornadoes can be spawned from rotating thunderstorms called supercells . Authorities are investigating damage from the storms . With the latest outbreak , April may turn out to be a historic month for tornadoes , he said . 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 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 .