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.
However, tornado patterns in the U.S. are changing in other ways. Big outbreaks of tornadoes, like the ones in Kentucky and Missouri over the weekend when lots of storms move across a large area in a short period of time, are getting more common.
"In the past we didn't see the kind of large outbreak days that we do now," Widhalm explains. "It used to be kind of unusual to see dozens of them in a day. And now we get that year after year."
She points out that warning people about large tornado outbreaks is labor-intensive, because it requires lots of highly skilled weather forecasters who can use radar data to track storms as they move across entire states and regions. Publishing warnings for one tornado is already complex, and publishing warnings for multiple tornadoes at once over a large area is even more challenging because it requires coordination between forecasters in multiple locations and data from multiple radar instruments.
The increase in large tornado outbreaks is particularly notable as the Trump administration is moving to dramatically cut the budget of the National Weather Service, and weather service offices have many vacant positions that are going unfilled because of a federal hiring freeze.
During the recent deadly tornado outbreak in Kentucky, the local weather service office in Jackson, Ky., had to borrow forecasters from other offices in the region, because they didn't have enough staff, according to a statement from the National Weather Service.
"As planned in advance, neighboring offices provided staffing support to the office in Jackson, KY.," the statement says. "Additionally, the Jackson office remained fully staffed through the duration of the event using surge staffing."
Tornado researchers are still trying to understand why some tornado patterns are changing in the U.S., and what the future could hold.
It's unclear how climate change might affect tornadoes. There's some evidence that the timing of tornadoes in the U.S. might shift, so there are more tornadoes in the fall and winter months. One reason that might happen as the Earth heats up is that moist, warm air is one necessary ingredient for tornadoes to form, and those conditions are getting more likely during times of year that used to be colder and drier.
But, so far, scientists can't say for sure whether that trend will play out, or whether climate change might affect tornadoes in the U.S. in other ways.
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