U.S. on Pace for Record Tornado Season

This year is shaping up to be one of the most active tornado seasons on record. More than 100 people have died so far, and the number of storms is far above the 10-year average. Scientists say the season got off to a strong start especially early this year, perhaps because of unusually cool ocean temperatures in the western Pacific.

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A vigil was held last night for four teenage Boy Scouts killed by a tornado. At least a dozen other victims are hospitalized in Iowa. Fifty-two tornadoes in all swept throughout the Midwest in the same 24-hour stretch. And for residents of that region, the only surprise, really, was the sheer number because there are more tornadoes in the United States than anywhere else in the world.

This year could set a record. Weather officials don't have an exact count right now, but they estimate that more than 1,000 tornadoes have already touched down, and the season is by no means over. NPR's Richard Harris has more.

RICHARD HARRIS: Tornado season usually starts in earnest in the late spring, but not so this year. The official count started climbing steeply in January. By the end of March, the tally was already up to 350 tornadoes, but it still wasn't obvious that we were on a record pace.

Mr. MORRIS WEISMAN (National Center for Atmospheric Research): For me, it unfolded gradually.

HARRIS: Morris Weisman, at the National Center for Atmospheric Research, noticed that conditions for tornadoes started appearing especially early. That is, there was a lot of heat and humidity near the ground, contrasting with cooler air higher up in the atmosphere. And there was also a big contrast with wind speeds at the ground and aloft.

Each week, Weisman would look at the weather data and see this pattern.

Mr. WEISMAN: And you'd say okay, well, this pattern looks like it's going to be very productive for the next week or so, and then a week goes by, and you see that the pattern hasn't changed, and you say okay, it's going to be productive for another week.

HARRIS: Normally in the winter, big storms blow through and disrupt those weather patterns, but Weisman says that just didn't happen this year. The result: lots and lots of tornadoes.

Greg Carbin at the Noah Storm Prediction Center in Norman, Oklahoma, says now we're in the middle of regular tornado season.

Mr. GREG CARBIN (Noah Storm Prediction Center): And it continues now that we're in the peak of severe weather season with not much of a break between systems moving across the country, producing widespread, dangerous weather.

HARRIS: It's hard to know just how unusual this year is compared with past decades. We've gotten so much better at spotting tornadoes that it's hard to tell whether high tallies are due to the number of storms or the number of spotters.

Carbin says this is the worst year, from the standpoint of fatalities, in a decade, but the 116 deaths so far in 2008 is nowhere near a record.

Mr. CARBIN: In fact, we've had many years in the past, especially the '50s and 1960s, where annual fatality numbers were in the hundreds. We're about double the recent average, but hopefully that starts to drop off, we start to see a general decline in the violent weather as we go into the summer months.

HARRIS: As the summer progresses, the two ingredients necessary for tornadoes gradually diminish, and violent storms are more likely to consist of thunder and lightning. These days, all weather trends quickly evoke the question: Is this global warming?

Well, in fact, Carbin says, the conditions for all those wintertime tornadoes could be partly the result of unusually cool waters in the Western Pacific Ocean, a "La Nina."

Carbin says it's debatable whether global warming will affect tornado activity in the long run.

Mr. CARBIN: There are some indications that a warmer world will give us more fuel for thunderstorm development, but at the same time, it may result in westerly winds - the stronger winds you need for tornadoes to form - to actually retreat northward into Canada. And so you'll have some ingredients that would support more severe weather but other ingredients that may actually suppress it.

HARRIS: Whatever the trend may be, Carbin says at least we're getting better at forecasting tornado conditions, which means there's often more chance for people to get out of the way. Richard Harris, NPR News.

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