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The scene plays out whenever waves of twisters sweep through a populated area: Residents scramble for shelter with just minutes of warning — and sometimes there's no warning at all. If we can predict where hurricanes are heading days in advance, why can't we do the same for tornadoes?

"With a hurricane, we get a precise warning, and you know pretty well where it is. We're just forecasting its evolution," explained Joshua Wurman, president of the Center for Severe Weather Research in Boulder, Colo. "But with tornadoes, we want to forecast them before they're there. We're trying to forecast their birth, in addition to their evolution, and that's a much harder problem."

It's a problem that experts on severe weather have been struggling with for decades. The stakes are high: Tornadoes — rotating columns of air that form over land — are the most violent storms on the planet, killing scores of people and averaging a billion dollars in property damage annually. But the difficulty is high, too: Twisters are so localized and so brief in duration (generally lasting less than 10 minutes) that it's hard to say exactly where and when they'll touch down.

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The average lead time between a tornado warning and a tornado touchdown is 13 minutes, according to the National Weather Service. A quarter of the time, there's no advance warning at all.

Does that sound grim? Maybe it shouldn't.

"We're a whole lot better than we used to be," said Harold Brooks, a research meteorologist at the National Oceanic and Atmospheric Administration's National Severe Storms Laboratory in Norman, Okla. "Thirty years ago, 75 percent of all tornadoes didn't have a warning in advance."

Devil is in the details

Brooks and Wurman both say forecasters have gotten good at forecasting the broad outlines of tornado-worthy weather. They knew, for instance, that Arkansas was in for a string of strong storms this weekend. "But we have almost no skill predicting which of those storms will make the big tornadoes," Wurman said. "We also have very little skill predicting when in a storm's lifetime it will become a big tornado."

As a result, Wurman said forecasters tend to "overwarn" about tornadoes. The false-alarm rate for tornado warnings is about 75 percent. But Brooks said it's better to sound a false alarm than to risk missing a killer tornado. "That 75 percent number is a result of the fact that deciding whether this is a tornado-making storm is a fundamentally hard problem," he said.

Researchers are trying to make the problem less hard, particularly by extending the lead time for a tornado warning. With a half-hour's warning, it would be more reasonable to evacuate homes and send people to a neighborhood shelter.