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.

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.

Transcript

STEVE INSKEEP, HOST:

Kentuckians are recovering from last weekend's tornadoes, and they might be in for more dangerous weather today, according to the National Weather Service. Much of southern Kentucky is at high risk for a significant tornado just days after a burst of storms killed more than two dozen people in that state, as well as in Missouri and Virginia.

LEILA FADEL, HOST:

Earlier this spring, another cluster of deadly tornadoes tore across eight states in the Midwest. So is climate change making tornadoes more frequent?

INSKEEP: It's a question we pose from time to time, and NPR's Rebecca Hersher is here to talk through the evidence. Good morning.

REBECCA HERSHER, BYLINE: Good morning.

INSKEEP: First, has this been a particularly bad spring for tornadoes?

HERSHER: Well, yes, it actually has. There have been more tornadoes than average, and that's according to data from the National Weather Service. So usually, by this time in May, there have been a little over 600 tornadoes reported in the U.S. But this year, there have been more than 850 tornadoes, so significantly more.

INSKEEP: OK. So is this just a bad year, or is there evidence that the frequency is increasing in a real way over time?

HERSHER: It's just a bad year. You know, the annual number of storms varies a lot year to year, so it's not surprising to have a really active year or two like this year. I spoke to a tornado expert about this. Her name is Melissa Widhalm, and she helps lead the Midwestern Regional Climate Center at Purdue University.

MELISSA WIDHALM: There's not really an increase or a decrease in the overall number of tornadoes that we see. That number's been pretty stable.

HERSHER: You know, year after year. That's going back to the 1950s. But here's what is changing. She says big outbreaks of tornadoes, like the ones in Kentucky and Missouri over the weekend - those are getting more common where lots of storms move over a really large area in a short period of time.

INSKEEP: Oh, this is interesting, and this is a thing that people have said about climate change - that you end up with more extreme weather events. So not more tornadoes total, but more of these clusters of tornadoes.

HERSHER: Yeah, exactly. And Widhalm says those clusters or outbreaks are a new phenomenon.

WIDHALM: In the past, we didn't see the kind of large outbreak days that we do now. It used to be kind of unusual to see dozens of them in a day, and now we get that year after year.

HERSHER: Now, she points out that warning people about those big clusters is really labor-intensive. You know, it requires a lot of weather forecasters and radar data to track the storms as they move across multiple states, which is notable at a time when the Trump administration is cutting weather forecasting budgets, and there are a lot of vacancies in the National Weather Service. You know, during the outbreak over the weekend the local weather service office in Jackson, Kentucky, had to borrow forecasters from other offices in the region because they didn't have enough staff on their own, and that's according to a statement from the National Weather Service.

INSKEEP: Appreciate that last detail. It is the National Weather Service acknowledging this in a statement. So let's try to figure out what's going on. We understand that climate change can lead to more extreme weather events. We have these more extreme weather events. But can we be a little more precise about the mechanics? Do scientists understand why big outbreaks of tornadoes are getting more common?

HERSHER: I wish I could say yes, but unfortunately, tornadoes are very complex, and this is just one of many unanswered questions about how they're changing, especially as the climate changes. You know, in general, climate change is causing more severe weather, as you said. One of the ingredients for a tornado is moist, warm air. So obviously, as the atmosphere heats up, it makes sense that tornado conditions could show up more. There have been some studies that suggest that there might be more tornadoes in the fall and winter as the Earth heats up, as opposed to now, when most tornadoes happen in the spring and summer. But it's really an area of active research.

INSKEEP: Rebecca, thanks so much.

HERSHER: Thanks.

INSKEEP: That's NPR's Rebecca Hersher.

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