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Tornadoes struck the U.S. south and Midwest, raising questions about the new normal

AYESHA RASCOE, HOST:

A series of storms and tornadoes hit parts of the South and Midwest this weekend, causing destruction and sadly dozens of deaths. This just a week after tornadoes devastated communities in Mississippi and Alabama. So what's driving these storms? Let's turn now to someone who's been tracking them, Matthew Cappucci. He's a meteorologist at The Washington Post. Good morning.

MATTHEW CAPPUCCI: Hey. Good morning. Thanks so much for having me.

RASCOE: So two big storm systems, two weekends in a row - like, what is driving all of this?

CAPPUCCI: Yeah, definitely. So unfortunately, it's kind of the time of year where we see these things like clockwork across the Deep South, the Midwest and the Plains. Every year, right around March, April, May, June, we start seeing the winter's cold in the upper atmosphere retreat over the lower 48. At the same time, warmth is trying to build from the Gulf of Mexico and push northwards with warmth and moisture. And so you get a seasonal clash of the air masses. That brews strong thunderstorms that can grow really tall in the atmosphere.

Now, at the same time, the jet stream, or that river of winds in the upper atmosphere, is still in its wintertime position over the country. And so these storm clouds that grow tall - they start feeling that jet stream, and they start to spin. That causes episode after episode of tornado. And, you know, during April, May, June, the peak of tornado season, we tend to see about 660 tornadoes on average across the lower 48. That said, this year has been above average already. January saw about 120, 130 of them, which is the second or third most on record. In February, we saw 55 of them, which is twice as many as normal, and March will be a busy month, as well. So unfortunately, there really is no end in sight anytime soon.

RASCOE: So you're saying that this storm season is worse than usual so far. So is climate change a factor in that? Or what may be making it worse than usual?

CAPPUCCI: Yeah. So there are a couple of contributing factors. I don't think climate change is one of them. Like we talked about, you know, it's sort of a typical time of year to see these storm systems. They typically go through with tornadoes. It's an unfortunate thing that every year that we see these, but it is rather predictable. Now, this season might be a little bit worse due to something called a La Nina, which is basically a large, overarching weather pattern that begins with a cooling of water temperatures in the eastern tropical Pacific. That essentially changes the position of key weather systems, including the jet stream, which makes severe weather a little bit more likely.

Now, La Nina kind of had its final burst, kind of its sort of final episode recently and now is switching to a neutral state and pushing eventually towards the opposite of La Nina, which is an El Nino. There is some research to suggest that during the transition of La Nina to El Nino, we get a flare-up of activity, which may be playing a role. So that could be potentially a contributing factor. That said, climate change does not have a firm link to tornado activity. We note that there isn't any evidence to suggest an uptick in the number or the intensity of tornadoes, but there may be shifts in where and when they occur.

For example, we're noticing more in, like, December, January, February, the traditional cool season, because temperatures are becoming a little bit warmer, enough to sustain supercell or rotating thunderstorms at a time of year when we wouldn't ordinarily have as many. We're also noticing a little bit drier weather over the High Plains in parts of April and May, which is reducing the number of tornadoes in what was traditionally once considered Tornado Alley. So shifting patterns but not necessarily shifting numbers.

RASCOE: In the 30 seconds we have left, I gather there are reports of more storms coming this week.

CAPPUCCI: Most definitely. The Dallas-Fort Worth metroplex could see more tornado activity today, including the risk of a strong tornado. Then Tuesday, another large-scale synoptic outbreak could stretch from areas of Iowa all the way down to parts of the Mississippi Valley towards Tennessee.

RASCOE: That's Matthew Cappucci, meteorologist at The Washington Post. Thank you so much for being with us.

CAPPUCCI: Thank you. Transcript provided by NPR, Copyright NPR.

Ayesha Rascoe is a White House correspondent for NPR. She is currently covering her third presidential administration. Rascoe's White House coverage has included a number of high profile foreign trips, including President Trump's 2019 summit with North Korean leader Kim Jong Un in Hanoi, Vietnam, and President Obama's final NATO summit in Warsaw, Poland in 2016. As a part of the White House team, she's also a regular on the NPR Politics Podcast.