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This satellite image, taken on April 28, 2014, at 13:01 UTC, shows that the storm system that generated the recent severe weather has moved to the east.


Recent Tornadoes Highlight Vast Strike Zone

Last week's storms are typical for this time of year in the United States.

When Americans imagine tornadoes, many tend to think of them churning through plains states like Kansas or Oklahoma. But destructive twisters strike hard in a very broad area of the eastern United States.

The tornadoes that have left at least 31 dead over the past two days—wreaking havoc in Arkansas, Kansas, Mississippi, Alabama, Tennessee, and elsewhere—are not extraordinary in terms of their strength, location, or numbers.

"Anywhere east of the Rockies can get tornadoes," says Harold Brooks, a senior research scientist with the National Oceanic and Atmospheric Administration's (NOAA) National Severe Storm Laboratory in Norman, Oklahoma.

He said the recent days were "nothing outlandish" in weather terms. "They have been ordinary big days."

Animation of a tornado forming.

Animation: How a Tornado Forms

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Animation: How a Tornado Forms

Brooks estimates that the tornadoes that hit Sunday are on par with what would be expected on about 10 to 15 days in any given year. The storms Monday are typically seen three to four days out of a year. (Related: "How Scientists Hope to Improve Tornado Forecasting.")

Severe weather warnings were announced again Tuesday for much of the eastern United States.

Storm Strength and Direction

The fact that the storms have been trending from west to east is also typical, Brooks adds. "Almost all tornadoes go from west to east because the winds aloft in the atmosphere are typically blowing from somewhere out of the west. That's the driving factor."

The National Weather Service should be issuing a preliminary estimate of the tornadoes' ratings within the next few days. A final designation won't be made until two months after the end of the calendar month, or the end of June.

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In Brooks's estimation, the twister that hit Faulkner County in central Arkansas, leaving 11 dead in the towns of Mayflower and Vilonia, might qualify as an EF4 (Enhanced Fujita 4), but not as an EF5, the highest rating. (See "Tornadoes: The Science Behind the Destruction.")

Tornadoes have been observed on every continent except Antarctica. About 1,200 of the storms strike the United States each year, killing an average of 60 people, reports NOAA. The U.S. sees about 75 percent of known tornadoes globally, although some experts have cautioned that tracking in many countries is limited.

The most notoriously affected region in the United States, called "Tornado Alley," includes the Great Plains states of Oklahoma, Kansas, Nebraska, and the Dakotas, as well as parts of Texas. Moist air from the Gulf of Mexico tends to collide there with dry air from the Southwest, making powerful tornadoes especially likely.

"There aren't a huge number of tornadoes in the U.S. west of the Rockies, except some in the Los Angeles Basin," says Brooks.

But strong tornadoes can happen in the majority of American states.

The state that has the highest number of tornadoes per square mile is Florida, according to the American Meteorological Society, followed by Indiana, Illinois, Iowa, and Louisiana. Many of the tornadoes in Florida and Louisiana are weaker storms that form as waterspouts over bodies of water, however, and do less damage.

Tornado Season?

Tornadoes can happen at any hour of the day and any time of the year, though they are most common in the spring, especially during May and June. They are least common in the winter because cold air has less energy.

This year had seen particularly few tornadoes before this past weekend because the relatively cold air in much of the east and the warmer air in the west made conditions unfavorable for thunderstorm formation, says Brooks.

He adds that scientists aren't able to predict how bad a tornado season will be far in advance. And when it comes to individual tornadoes, "we don't have the ability to forecast out more than a few days."

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