A tornado outbreak is the occurrence of multiple tornadoes spawned by the same synoptic scale weather system. The number of tornadoes required to qualify as an outbreak typically are at least six to ten.
The tornadoes usually occur within the same day, or continue into the early morning hours of the succeeding day, and within the same region. Most definitions allow for a break in tornado activity (time elapsed from the end of last tornado to the beginning of next tornado) of six hours. If tornado activity indeed resumes after such a lull, many definitions consider the event to be a new outbreak. A series of continuous or nearly continuous tornado outbreak days is a tornado outbreak sequence. Tornado outbreaks usually occur from March through June in the Great Plains of the United States and Canada, the Midwestern United States, and the Southeastern United States in an area colloquially referred to as Tornado Alley. Tornado outbreaks do occur during other times of the year and in other parts of the world, however. A secondary less active and annually inconsistent tornado "season" in the U.S. occurs in late autumn.
The largest tornado outbreak on record, depending on the definition applied and time elapsed between breaks in tornadic activity, was the April 25–28, 2011 tornado outbreak, with 355 tornadoes. It surpasses the 1974 Super Outbreak, in which 148 tornadoes were counted. Both occurred within the United States and Canada. The total number of tornadoes is a problematic method of comparing outbreaks from different periods, however, as many more smaller tornadoes, but not stronger tornadoes, are reported in the US in recent decades than in previous ones. The Super Outbreak retains the distinction for the intensity of that outbreak with 7 F5 and 23 F4 tornadoes.
- Glickman, Todd S. (ed.) (2000). Glossary of Meteorology (2nd ed.). American Meteorological Society. ISBN 978-1-878220-34-9.
- Grazulis, Thomas P. (1993). Significant Tornadoes 1680-1991: A Chronology and Analysis of Events. St. Johnsbury, VT: The Tornado Project of Environmental Films. ISBN 1-879362-03-1.
- Galway, Joseph G. (1977). "Some Climatological Aspects of Tornado Outbreaks". Mon. Weather Rev. 105 (4): 477–84. Bibcode:1977MWRv..105..477G. doi:10.1175/1520-0493(1977)105<0477:SCAOTO>2.0.CO;2.
- Schneider, Russell; H. E. Brooks; J. T. Schaefer (2004). "Tornado Outbreak Day Sequences: historic events and climatology (1875-2003)". 22nd Conference on Severe Local Storms. Hyannis, MA: American Meteorological Society.
- Schneider, Russell; H. E. Brooks; J. T. Schaefer (2004). "Tornado Outbreak Days: an updated and expanded climatology (1875-2003)". 22nd Conference on Severe Local Storms. Hyannis, MA: American Meteorological Society.
- Doswell, Charles A., III (2007). "Small Sample Size and Data Quality Issues Illustrated Using Tornado Occurrence Data". Electronic J. Severe Storms Meteor. 2 (5): 1–16.
- Corfidi, Stephen (2013). "Noteworthy North American Tornado Outbreaks". NOAA-NWS-NCEP Storm Prediction Center (poster).
- Doswell, Charles A., III; R. Edwards, R.L. Thompson, J.A. Hart, K.C. Crosbie (2006). "A Simple and Flexible Method for Ranking Severe Weather Events". Wea. Forecasting 21 (6): 939–51. Bibcode:2006WtFor..21..939D. doi:10.1175/WAF959.1.
- Edwards, Roger; R. L. Thompson; C. Crosbie; J. A. Hart; C. A. Doswell (2004). "Proposals for modernizing definitions of tornado and severe thunderstorm outbreaks". 22nd Conference on Severe Local Storms. Hyannis, MA: American Meteorological Society.
- Forbes, Gregory S. (2006). "Meteorological aspects of high-impact tornado outbreaks". Symposium on the Challenges of Severe Convective Storms. Atlanta, GA: American Meteorological Society.
- Mercer, Andrew E.; C.M. Shafer, C.A. Doswell III, L.M. Leslie, M.B. Richman (Dec 2009). "Objective Classification of Tornadic and Nontornadic Severe Weather Outbreaks". Mon. Wea. Rev. 137 (12): 4355–68. Bibcode:2009MWRv..137.4355M. doi:10.1175/2009MWR2897.1.
- Moller, Alan R. (1979). "The Climatology and Synoptic Meteorology of Southern Plains' Tornado Outbreaks". M.S. Thesis (Norman, OK: University of Oklahoma).
- Shafer, Chad; C.A. Doswell (2010). "A Multivariate Index for Ranking and Classifying Severe Weather Outbreaks". Electronic J. Severe Storms Meteor. 5 (1).
- Shafer, Chad; C. Doswell; L. Leslie; M. Richman (2010). "On the use of areal coverage of parameters favorable for severe weather to discriminate major outbreaks". Electronic J. Severe Storms Meteor. 5 (7).
- Shafer, Chad; C. Doswell (2011). "Using kernel density estimation to identify, rank, and classify severe weather outbreak events". Electronic J. Severe Storms Meteor. 6 (2).
- Thompson, Richard L.; M.D. Vescio (1998). "The Destruction Potential Index – a method for comparing tornado days". 19th Conference on Severe Local Storms. Minneapolis, MN: American Meteorological Society.
- Verbout, Stephanie M.; H.E. Brooks, L.M. Leslie, D.M. Schultz (2006). "Evolution of the U.S. Tornado Database: 1954-2003". Wea. Forecasting 21 (1): 86–93. Bibcode:2006WtFor..21...86V. doi:10.1175/WAF910.1.