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U.S. Experiences Wild Weather

LINDA WERTHEIMER, host:

The tornadoes that struck St. Louis are just the latest in a record-breaking number of twisters that have swept across the country this month, not to mention historic droughts and fires in Texas, record cold in Seattle and snow and flooding in the Middle West. So the perennial question of spring: What is with this weather?

We asked John Christy. He is director of the Earth System Science Center at the University of Alabama in Huntsville. The culprit, he says, high pressure in the middle of the country.

Mr. JOHN CHRISTY (Director, Earth System Science Center): The main storm track in the last few months has been through the Ohio Valley and trailing down through the Southeastern United States. The normally random pattern of storminess has sort of been focused more along the Ohio Valley.

WERTHEIMER: And this is also a La Nina year, and I understand that La Nina is causing lots of things to happen. I mean, who is La Nina and why is she doing this to us?

Mr. CHRISTY: La Nina is a pattern of temperatures in the Central Pacific Ocean. When they are colder than average, that's a La Nina. That is actually part of a mechanism that helps to steer the jet stream, which also steers the storms, and therefore you get these repeated storms coming in a particular area and then repeated misses of storms in places like Texas into Mexico.

WERTHEIMER: Now, the story that I'm the most interested in, because it's, you know, something that I remember from my childhood, is tornadoes. There have been more than 240 tornadoes reported since last Thursday. The National Weather Service says this is an increase over last year. Is this part of some kind of trend? Are tornadoes on the rise?

Mr. CHRISTY: Well, it turns out when you look at the type of tornadoes that we have been able to count for the past 60 years, there really is no trend at all in those numbers. There are many tornadoes now that we detect with Doppler radars. You know, my favorite is the video device that, you know, millions of people now have little video cameras. And the demand for that use on Facebook and YouTube and hundreds of cable channels for this reality TV kind of disaster video is so great that today a tornado has a tough time not being observed. And plus, the stuff that's in the way that we build on the ground is being destroyed at a greater rate simply because there's more stuff out there for them to hit.

WERTHEIMER: Now, there were some elements of this recent storm system that seemed to me to be quite remarkable. For example, we saw a video of three tornadoes which appeared to be rotating around each other.

Mr. CHRISTY: Yes. That's a multiple vortices tornado. Now, that's not a typical storm that you're describing, but it has...

WERTHEIMER: Thank God.

Mr. CHRISTY: ...been noted in the past.

WERTHEIMER: Then there was a tornado in Southern Alabama that plowed up the ground for 35 miles. That seems extraordinary.

Mr. CHRISTY: Thirty-five miles is a long-track tornado. It means that the parent thunderstorm from which the tornado came had enough energy to stay active, in that case, with the Gulf moisture continuing to flow from the South and the active front that was moving through was able to keep that storm alive for a long time.

WERTHEIMER: So, John Christy, we're looking at the wild weather continuing?

Mr. CHRISTY: The wild weather is part of what happens in the United States in spring. I would certainly say somewhere in the United States, you will see some more wild weather coming this year. However, I think the tornado outbreak that we experienced last weekend, you know, is a fairly unusual event. You just don't see those happening more than once a year.

WERTHEIMER: John Christy is a man of many titles. He is director of the Earth System Science Center at the University of Alabama in Huntsville, and he's also a climatologist for the state of Alabama. He joined us from WLRH in Huntsville.

Thank you very much.

Mr. CHRISTY: My pleasure. Transcript provided by NPR, Copyright NPR.