A fallen tree rests atop a pickup truck in Holly Springs, Miss., after a storm struck the town on Wednesday. A storm system killed several people as it swept across the South. Phillip Lucas/AP hide caption

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Purdue University students are testing new software that may track and warn about tornadoes, such as this one which struck Rochelle, Ill., in early April. Walker Ashley/AP hide caption

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A still image from a video shows two tornadoes approaching Pilger, Neb., Monday. The National Weather Service said at least two people died in the large storm that hit northeast Nebraska. StormChasingVideo.com/AP hide caption

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After a tornado leveled Moore, Okla., last year, firefighter Shonn Neidel (left) developed an app that helps first responders locate storm shelters under the wreckage. Courtesy of Shonn Neidel hide caption

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A severe thunderstorm wall cloud is seen over the area of Canton, Miss., on Tuesday. At least 35 people in six states have been killed by tornadoes unleashed by a ferocious storm system this week. Gene Blevins/Reuters/Landov hide caption

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