On April 3-4, 1974, an outbreak of 148 tornadoes devastated portions of eight American States (Illinois, Indiana, Michigan, Ohio, Alabama, Tennessee, Georgia and North Carolina) and one Canadian Province (Ontario). Before the storm blitz was over, this super-outbreak had killed 315 people and injured 4,584 others. Damage was estimated as over half a billion dollars. The outbreak was the greatest ever observed in North America. Of the 148 twisters, 95 were classified as significant (F2 category or higher -- see the Fujita Scale); 30 were classified as violent (F4 or F5); 48 were killer storms.
What is a tornado outbreak? A tornado outbreak occurs when a large number (six or more) of tornadoes are formed in groups or individual storms within a 24-48-hour period over a specific geographical area and spawned from the same general weather system. Storm-researcher T. Galway classified tornado outbreaks into three categories:
Local outbreak -- lasts 3 to 7 hours and is confined to a relatively small regional area (approximately 10,000 square miles / 26,000 square kilometres).
Line outbreak -- tornadoes occur at about the same time along a line running approximately North/South axis that may be 1000 miles (1,600 km) long.
Progressive outbreak -- tornadoes form over a 6 to 15 hour period across an area 350 miles (560 km) wide; tornado activity progresses toward the NE, E or SE.
Major outbreaks happen, on average, every few years in the United States. Between 1880 and 1995, 40 outbreaks have occurred that contained more than 20 significant tornadoes (F2 or higher). Seventeen of those came between 1916 and 1949. Thirty outbreaks, during the 1880-1995 period, have resulted in more than 50 deaths. Forty-seven outbreaks contained four or more violent tornadoes (F4 or F5); three have had 10 or more violent tornadoes associated with them.