Large tornado outbreaks are becoming more common. But it's unclear why

There have been a lot of tornadoes so far this year. Recent tornadoes killed dozens of people in Kentucky, Missouri and Virginia. Earlier this spring, deadly tornadoes tore across eight midwestern states.

An above-average number of tornadoes in the Midwest have occurred so far this year, according to data collected by the National Weather Service. Usually, by this time in May, a little over 600 tornadoes have been reported in the U.S. This year, it's more than 850 tornadoes.

But that isn't evidence of a long-term trend. "There's not really an increase or a decrease in the overall number of tornadoes that we see," says Melissa Widhalm, the associate director of the Midwestern Regional Climate Center at Purdue University. "That number's been pretty stable," going back to the 1950s or so.