Tornado Outbreak
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Tornado Outbreak

A tornado outbreak is the occurrence of multiple tornadoes spawned by the same synoptic scale weather system.[1] The number of tornadoes required to qualify as an outbreak typically are at least six to ten.[2][3]

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.[4] 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.[5]

Very large tornado outbreaks are known as super outbreaks. The largest tornado outbreak on record was the 2011 Super Outbreak, with 362 tornadoes and about $10 billion in direct damages.[6] 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 weaker tornadoes, but not stronger tornadoes, are reported in the US in recent decades than in previous ones due to improvements in tornado detection.[7]


  1. ^ Glickman, Todd S. (ed.) (2000). Glossary of Meteorology (2nd ed.). American Meteorological Society. ISBN 978-1-878220-34-9. Archived from the original on 2011-06-06.CS1 maint: extra text: authors list (link)
  2. ^ Grazulis, Thomas P. (1993). Significant Tornadoes 1680-1991: A Chronology and Analysis of Events. St. Johnsbury, VT: The Tornado Project of Environmental Films. ISBN 978-1-879362-03-1.
  3. ^ 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.
  4. ^ 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.
  5. ^ 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.
  6. ^ Smith A.B. and J. Matthews, 2015: Quantifying Uncertainty and Variable Sensitivity within the U.S. Billion-dollar Weather and Climate Disaster Cost Estimates. Natural Hazards, doi:10.1007/s11069-015-1678-x
  7. ^ 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.

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