1925 was the deadliest year in the U.S., with 794 killed, according to the 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.
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.
"My friend, Jenna Blum, coined the term 'Metronado,' which is what we had last year," Mike Smith, senior vice president of AccuWeather Enterprise Solutions said.
While two of the most deadly tornadoes touched down in Joplin, Missouri on May 22, 2011, and Tuscaloosa, Alabama on April 27, 2011, tornadoes hit many cities outside of "Tornado Alley," like Minneapolis, Minnesota; Springfield (the first tornado to hit Massachusetts since 1953); Raleigh, North Carolina; St. Louis, Missouri; Birmingham, Alabama; New York City, New York; and Philadelphia, Pennsylvania.
"No matter how good the warnings are, if you take a densely populated area and put a F-4 or F-5 tornado in there, tragically, people are going to lose their lives," Smith added.
Inadequate Shelters Led to More Deaths in 2011
Another factor in how deadly tornadoes were in 2011 was inadequate shelters both in solid structure homes and mobile home parks. The safest place to take shelter during a tornado is in the lowest interior room of a house or building, preferably in a basement, but there many who live in "Tornado Alley" do not have basements, like in Joplin. Many mobile homes are devastated by tornadoes, leaving many to debate whether mobile home parks should be required to have community shelters.
Commentary by Meghan Evans, Meteorologist Writer for AccuWeather.com