An epidemic of deadly tornadoes this week likely has been spawned by the rare collision of hot, humid air from the Gulf of Mexico with frigid Arctic winds pushed down across five Southern states by global climate patterns, weather experts said.

These forces have helped produce about 800 twisters so far this month, four times the average for April, meteorologists said. The National Weather Service reported 165 tornadoes on Wednesday alone. The last comparable spate of destructive storms occurred in April 1974, when 148 twisters...