A tornado outbreak sequence, or tornado outbreak day sequence, sometimes referred to as an extended tornado outbreak, is a period of continuous or nearly continuous high tornado activity consisting of a series of tornado outbreaks over multiple days with no or very few days lacking tornado outbreaks.
Major tornado outbreak sequences occurred in the United States in May 1917, 1930, 1949, 1965, and 2003. Another exceptional outbreak sequence apparently occurred during mid to late May 1896. Although some days lacked tornado outbreaks, the period from mid to late April 2011 also was a period of especially high tornado activity.
Tornado outbreak sequences tend to dominate the tornado statistics for a year and often cause a spike in tornado numbers for the entire year. Not all periods of active tornado occurrences are outbreak sequences, there must be no break in the activity to satisfy the definition. Active periods happen from every year to every several years whereas continuously active periods are less common and can be rare depending on the parameters applied to define a sequence.
The May 21–26, 2011 tornado outbreak was a deadly tornado outbreak across the Midwestern and Southern regions of the United States. It was one of the largest tornado outbreaks on record. Most of the tornadoes developed in a corridor from Lake Superior southwest to central Texas; isolated tornadoes occurred in other areas. An especially destructive tornado destroyed one-third of Joplin, Missouri, resulting in 158 deaths and 1000 injuries. The Joplin tornado is the deadliest in the US since April 9, 1947, when an intense tornado killed 181 in the Woodward, Oklahoma area. Tornado-related deaths also occurred in Arkansas, Kansas, Minnesota, and Oklahoma. Overall, the tornado outbreak resulted in 184 deaths, 6 of those non-tornadic, making it second only to the 2011 Super Outbreak as the deadliest since 1974, and the second costliest tornado outbreak in US history behind that same April 2011 outbreak, with insured damage estimated at $4–7 billion.