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

itoggle caption Walker Ashley/AP

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

itoggle caption StormChasingVideo.com/AP

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

itoggle caption Courtesy of Shonn Neidel

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

itoggle caption Gene Blevins/Reuters/Landov

One of the homes destroyed in Washington, Ill., by Sunday's storms. Tasos Katopodis/Getty Images hide caption

itoggle caption Tasos Katopodis/Getty Images