Why Was 2011 Such a Deadly Tornado Year

1925 was the deadliest year in the U.S., with 794 killed, according to National Oceanic and Atmospheric Administration (NOAA). That year is infamous for the Tri-State tornado, the longest-tracking, deadliest tornado on record. The tornado's path went through portions of Missouri, Illinois and Indiana, over 219 miles long. The twister killed 695 people along its path.

There were 552 deaths in 1936 and 551 deaths in 1917, ranking as the second and third most deaths caused by tornadoes in a year. According to the Storm Prediction Center (SPC), the yearly average for tornado deaths is around 60.

2011 had an unusually high number of large, destructive tornado outbreaks; 1,709 tornadoes touched down, a close second to the record 1,817 tornadoes set in 2004. In comparison, the average number of tornadoes per year over the past decade is around 1,300.

April 2011 was the most active tornado month ever in the U.S. with 753 twisters touching down. In April alone, 364 people were killed by tornadoes.

Why So Many Tornadoes in 2011?
A key ingredient for the violent severe weather in 2011 was a very strong jet stream. La Niña, a phenomenon where the sea surface temperature in the central and eastern Pacific around the equator are below normal, helped to cause the strong northern jet stream, which frequently plunged into the South. This set the stage for powerful supercell thunderstorms, which are the type of storms that spawn tornadoes.

People typically think of "Tornado Alley" as the corridor from Texas to Kansas that is frequently hit by tornadoes in the spring. Warm, humid air from the Gulf of Mexico clashes with drier air from the Rockies. During 2011, many tornadoes touched down east of the typical "Tornado Alley," which is often the case in La Niña years. More densely populated areas sit in the path of severe storms capable of spawning tornadoes.