While there is no single agreed upon definition, generally at least 6-10
tornadoA tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es produced by the same
synoptic scaleThe synoptic scale in meteorology is a horizontal length scale of the order of 1000 kilometres or more. This corresponds to a horizontal scale typical of mid-latitude depressions...
weather system is considered a
tornado outbreak. 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 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 sequenceA tornado outbreak sequence 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....
. Tornado outbreaks usually occur from March through June in the
Great PlainsThe Great Plains are a broad expanse of flat land, much of it covered in prairie, steppe and grassland, which lies west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. This area covers parts of the U.S...
of the United States and Canada,
Midwestern United StatesThe Midwestern United States is one of the four U.S. geographic regions defined by the United States Census Bureau, providing an official definition of the American Midwest....
, and
Southeastern United StatesThe Southeastern United States, colloquially referred to as the Southeast, is the eastern portion of the Southern United States. It is one of the most populous regions in the United States of America....
. Tornado outbreaks do occur during other times of the year and in other parts of the world, however.
The largest tornado outbreak on record may be, depending on the definition applied and time elapsed between breaks in tornadic activity, the
April 25–28, 2011 tornado outbreakAn extremely large and violent tornado outbreak, the largest tornado outbreak ever recorded, and popularly known as the 2011 Super Outbreak, occurred from April 25 to 28, 2011. The outbreak affected the Southern, Midwestern, and Northeastern United States, leaving catastrophic destruction in...
, with as many 327 tornadoes. It probably surpasses the
Super OutbreakThe Super Outbreak is the second largest tornado outbreak on record for a single 24-hour period, just behind the tornado outbreak of April 25–28, 2011...
of April 3–4, 1974, in which there were 148 tornadoes. Both occurred within the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
and
CanadaCanada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. 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 distinction for the intensity of that outbreak with 6 F5 and 24 F4 tornadoes.