Oklahoma's Deadliest Tornadoes

On May 20, 2013, a ferocious EF5 tornado over a mile wide tore through Moore, Oklahoma, inflicting 24 deaths and obliterating entire neighborhoods. It was the third time an exceptionally violent tornado had struck the city in 14 years. Yet predicting when and where these killer storms will hit still poses a huge challenge. Why was 2011—the worst ever recorded tornado season?