Sunday's devastating tornadoes in Ibaraki and Tochigi prefectures were caused by a rare situation attributed in part to a winter that went on for some weeks longer than normal, and a recent sharp rise in surface temperatures, experts said Monday.
One person died and several were injured in the twisters.
"Tornadoes rarely occur in this season," said Yasushi Fujiyoshi, professor at Hokkaido University. "It is rare in the first place for cold air to form in the troposphere in May."