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Why there have been so many tornado threats this year is harder to say.

Viewing pictures of the tornado aftermath it’s hard to overestimate the power of such storms, and records bear out how strong they can be.

“You see pictures of World War II, the devastation and all that with the bombing. That’s really what it looked like,” said Kerry Sachetta, the principal of a flattened Joplin High School. “I couldn’t even make out the side of the building. It was total devastation in my view. I just couldn’t believe what I saw.”

And that movie image a few years ago was no joke: A cow was transported 10 miles by a twister in Iowa in 1878 and a tornado in Minnesota moved a headstone three miles in 1886.

One Joplin resident said a picture that was sucked off his house’s wall was found in Springfield, 70 miles away. An insurance policy was found more than 40 miles from its original residence in Oklahoma in 1957 and a 210-mile trip was taken by a canceled check in Nebraska in 1915, according to a study several years ago by researchers at the University of Oklahoma and St. Louis University.

Typically, tornadoes spawn in the clash between warm, moist air from the Gulf of Mexico and cooler, dry air from the north and west _ conditions that mark Tornado Alley in the Midwest and South, the most common breeding grounds for twisters.

Factors in this year’s excess may include La Nina, a periodic cooling of the tropical Pacific Ocean which can affect weather worldwide. In a La Nina year there tend to be more tornadoes than average. If that is a factor, the good news is that La Nina is weakening and is expected to end in a month or so.

The meandering jet stream high in the atmosphere that directs the movements of weather also has been in a pattern that encourages warm Gulf air to move in and clash with drier air masses.

While studies of global warming have suggested it could cause more and stronger storms, National Weather Service Director Jack Hayes isn’t ready to blame climate change _ at least not yet _ saying it’s too soon to link individual events with the ongoing warming.

Tornado researcher Howard B. Bluestein of the University of Oklahoma says his best guess is this unusual outburst of twisters is due to natural variability of the weather.

“Sometimes you get a weather pattern in which the ingredients for a tornado are there over a wide area and persist for a long time. That’s what we’re having this year,” he said.

“If we see this happen next year and the following year and the following year,” then maybe climate change could be to blame, he said.

Whatever the reasons it’s an extraordinary year for tornadoes and the worst may not be over. May is usually the peak month, but June traditionally gets lots of twisters, and they can occur in any month.

“You can never completely breathe easy,” concluded Russell Schneider, director of the government’s Storm Prediction Center in Norman, Okla.

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