U.S. could see record twisters this season

A short-term answer is that the nation's heartland is stuck in a tornado rut with usually temporary weather conditions that can lead to tornadoes parked over the Plains, said Adam Houston, a professor of meteorology at the University of Nebraska-Lincoln. Cooler air at high altitudes and warmer moist air coming from the Gulf of Mexico are combining and settling over the region.

"You get day after day of severe weather and day after day of tornadoes until the pattern changes," Houston said.

But why that happens, Houston doesn't know. While scientists can forecast hurricane seasons, predicting their land-bound cousins is much harder, Brooks said. While tornadoes, like hurricanes, rely on large-scale weather phenomena, the crucial triggers are extremely local weather conditions.

On top of that, tornadoes have a "Goldilocks" issue. To make a tornado, the conditions have to be just right. Too much or too little of one ingredient and there is no tornado. For example, wind shear — when upper and lower winds are at different speeds or coming from different directions — is crucial to create a funnel cloud. Too little and there is no spin. Too much and the tornado falls apart.

And tornadoes form most often in late afternoon, between 5 and 9 p.m., so if a thunderstorm starts up early in the morning, it's far less likely to throw off a tornado, Brooks said.

As for why so many people are getting killed, Brooks suggests thinking of the landscape as a dartboard: "We're throwing more darts and throwing bigger darts than normal."

More people are living in mobile homes in the past few decades, and that has shown up in tornado fatality statistics. In 1970, about one-quarter of all tornado deaths were in mobile homes; now it's about half, Brooks said. In 1970, Census data showed that 3 percent of the U.S. population lived in mobile homes; now it is 7.6 percent, with a higher rate in the Southeast and other parts of Tornado Alley, such as Oklahoma, Brooks said.

But as deadly as this year has been, it used to be far worse in the United States. In 1925, tornadoes killed 794 people. From 1916 to 1936, tornadoes killed an average of nearly 280 Americans a year. That's because tornado warnings were not as good, people couldn't hear them and housing was not as sturdy, Brooks said.

Even with a busy tornado year, meteorologists are getting the word out. Of the 110 deaths so far this year, 101 came while there was a tornado watch in effect, according to the National Weather Service.

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