The 1974 Super Outbreak was one of the most intense tornado outbreaks on record, occurring on April 3–4, 1974, across much of the United States. It was one of the deadliest tornado outbreaks in U.S. history. It was also the most violent tornado outbreak ever recorded, with 30 violent (F4 or F5 rated) tornadoes confirmed. From April 3–4, there were 149 tornadoes confirmed in 13 U.S. states and the Canadian province of Ontario. In the United States, the tornadoes struck Illinois, Indiana, Michigan, Ohio, Kentucky, Tennessee, Alabama, Mississippi, Georgia, North Carolina, Virginia, West Virginia, and New York. The outbreak caused roughly $600 million USD (equivalent to $ in ) in damage. The outbreak extensively damaged approximately along a total combined path length of . At one point, as many as 15 separate tornadoes were occurring simultaneously.
The 1974 Super Outbreak was the first tornado outbreak in recorded history to produce more than 100 tornadoes in under a 24-hour period, a feat that was not repeated globally until the 1981 United Kingdom tornado outbreak and in the United States until the 2011 Super Outbreak, the largest outbreak on record by number of tornadoes in a 24-hour period. In 2023, tornado expert Thomas P. Grazulis created the outbreak intensity score (OIS) as a way to rank various tornado outbreaks. The 1974 Super Outbreak received an OIS of 578, making it the most intense tornado outbreak in recorded history. __TOC__ Meteorological synopsis
A powerful springtime low pressure system developed across the North American Interior Plains on April 1. While moving into the Mississippi and Ohio Valley areas, a surge of unusually moist air intensified the storm further, while there were sharp temperature contrasts between both sides of the system. Officials at NOAA and in the National Weather Service forecast offices were expecting a severe weather outbreak on April 3, but not to the extent that ultimately occurred. Several F2 and F3 tornadoes had struck portions of the Ohio Valley and the South in a separate, earlier outbreak on April 1 and 2, which included three killer tornadoes in Kentucky, Alabama, and Tennessee. The town of Campbellsburg, northeast of Louisville, was hard-hit in this earlier outbreak, with a large portion of the town destroyed by an F3. Between the two outbreaks, an additional tornado was reported in Indiana in the early morning hours of April 3, several hours before the official start of the outbreak. On Wednesday, April 3, severe weather watches already were issued from the morning from south of the Great Lakes, while in portions of the Upper Midwest, snow was reported, with heavy rain falling across central Michigan and much of Ontario.
April 3 Morning setup and convection
By 12:00 UTC on April 3, a large-scale trough extended over most of the contiguous United States, with several modest shortwaves rotating around the broad base of the trough. The mid-latitude low-pressure center over Kansas continued to deepen to , and wind speeds at the 850-mb level increased to () over portions of Louisiana, Mississippi, and Alabama. Due to significant moisture advection, destabilization rapidly proceeded apace; the warm front near the Gulf Coast dissipated and then redeveloped northward over the Ohio River valley. Consequently, CAPE levels in the region rose to 1,000 J/kg. However, a warm temperature plume in the elevated mixed layer kept thunderstorms from initiating at the surface. Meanwhile, a large mesoscale convective system (MCS) that had developed overnight in Arkansas continued to strengthen due to strong environmental lapse rates. Later in the day, strong daytime heating caused instability to further rise. By 18:00 UTC, CAPE values in excess of 2,500 J/kg were present over the lower Ohio and the Mississippi Valley. As wind speeds in the troposphere increased, Large-scale lifting overspread the warm sector. At the same time, the forward-propagating MCS spread into the Tennessee and Ohio valleys, where it evolved into the first of three main convection bands that produced tornadoes. This first convective band moved rapidly northeast, at times reaching speeds of about (). However, thunderstorm activity, for the moment, remained mostly elevated in nature.
Afternoon supercells + Fatalities by state/province State/province FatalitiesAlabama 77Georgia 16Illinois 5Indiana 47Kentucky 71Michigan 2North Carolina 6Ohio 38Ontario 9Tennessee 45Virginia 1West Virginia 1 Total 319 By 16:30 UTC, the large MCS began to splinter into two sections: the southern part slowed, lagging into southeast Tennessee, while the northern part accelerated, reaching Pennsylvania by 19:30 UTC. The split was related to several factors, including a band of subsidence over eastern Kentucky and western West Virginia; local downslope winds over the Appalachians; and an inversion over the same area. These factors allowed the northern part of the MCS to accelerate due to efficient ducting, while the southern part slowed as the boundary layer warmed and moistened. Numerous surface-based supercells began to develop in the southern area, beginning with one that produced an F3 tornado at about 16:30 UTC near Cleveland, Tennessee. Meanwhile, a new band of scattered thunderstorms developed at 15:00 UTC over eastern Arkansas and Missouri; over the next four hours, this band became the focus for several intense supercells, starting in eastern Illinois and southern Indiana. In the wake of the MCS, backing low-level winds, rapid diurnal destabilization, and perhaps cool, mid-level advection had occurred over the warm sector, weakening the convective inhibition (CINH) layer, and favorable wind profiles bolstered helicity to over 230 m2/s²—a combination of factors conducive to tornadogenesis. Consequently, the storms increased in intensity and coverage as they moved into Illinois, Indiana, and northern Kentucky, producing several tornadoes, including the first F5 tornado of the day, at 19:20 UTC, near Depauw, Indiana. Several of the storms to form between 19:20 and 20:20 UTC became significant, long-lived supercells, producing many strong or violent tornadoes, including three F5s at Depauw; Xenia, Ohio; and Brandenburg, Kentucky. These storms formed the second of three convective bands to generate tornadoes.
While violent tornado activity increased over the warm sector, a third band of convection developed at about 16:00 UTC and extended from near St. Louis into west-central Illinois. Based upon real-time satellite imagery and model data, differential positive vorticity advection coincided with the left exit region of an upper-level jet streak which reached wind speeds of up to (), thereby enhancing thunderstorm growth. Storms grew rapidly in height and extent, producing baseball-sized hail by 17:20 UTC in Illinois and, shortly thereafter, in St. Louis, Missouri, which reported a very severe thunderstorm early in the afternoon that, while not producing a tornado, was the costliest storm to hit the city up to that time. By 19:50 UTC, supercells producing F3 tornadoes hit the Decatur and Normal areas in Illinois. As thunderstorms moved into the warmer, moister air mass over eastern Illinois and Indiana, they produced longer-lived tornadoes—one of which began near Otterbein and ended near Valentine in Indiana, a distance of .
Continued activity Meanwhile, by 00:00 UTC the southern half of the first convective band became indistinguishable from new convection that had formed farther south over Alabama and Tennessee in connection with convective band two. In this area, increasing west-southwesterly wind shear at all levels of the troposphere, juxtaposed over near-parallel outflow boundaries, allowed successive supercells, all producing strong, long-tracked tornadoes, to develop unconstrained by their outflow in a broad region from eastern Mississippi to southern Tennessee. These storms, forming after 23:00 UTC, produced some of the most powerful tornadoes of the outbreak, including a large and long-tracked F4 that struck the western and central portions of Alabama, tracking for just over , two F5s that both slammed into Tanner, causing extensive fatalities, an extremely potent F5 that devastated Guin in Alabama, and multiple violent, deadly tornadoes that affected and caused fatalities in Tennessee.
April 4
Michigan was not hit as hard as neighboring states or Windsor, although one deadly tornado hit near Coldwater and Hillsdale, killing people in mobile homes; however, thunderstorm downpours caused flash floods, and north of the warm front in the Upper Peninsula, heavy snowfall was reported. Activity in the south moved towards the Appalachians during the overnight hours and produced the final tornadoes across the southeast during the morning of April 4. A series of studies by Dr. Tetsuya T. Fujita in 1974–75—which were later cited in a 2004 survey by Risk Management Solutions—found that three-quarters of all tornadoes in the 1974 Super Outbreak were produced by 30 'families' of tornadoes—multiple tornadoes spawned in succession by a single thunderstorm cell. The majority of these were long-lived and long-tracked individual supercells.
Confirmed tornadoes
+ Daily statistics during the 1974 Super Outbreak DateTotalFujita scale ratingDeathsInjuries F0 F1 F2 F3 F4 F5 April 3130 12 24 33 31 23 7 10–11211–254 April 419 6 8 2 3 0 0 Total149 F018 F132 F235 F334* F423 F57310–3355,454–6,142
Note: An F3 tornado was confirmed in Ontario.
This tornado outbreak produced the most violent (F4 and F5) tornadoes ever observed in a single tornado outbreak. There were seven F5 tornadoes and 23 F4 tornadoes. More than 100 tornadoes associated with 33 tornado families. The first tornado of the outbreak is disputed, with some sources indicating an isolated F2 in Indiana at 13:30 UTC while Fujita marked the outbreak's onset at 18:10 UTC with an F0 in Illinois. As the storm system moved east where daytime heating had made the air more unstable, the tornadoes grew more intense. A tornado that struck near Monticello, Indiana was an F4 and had a path length of , the longest path length of any tornado for this outbreak. A total of 19 people were killed in this tornado.Data from the Storm Prediction Center archives, which are accessible through free software created and maintained by John Hart, lead forecaster for the SPC. The first F5 tornado of the day struck the city of Depauw, Indiana, at 3:20 pm EDT. It killed 6 and injured 86 others along its 65-mile path, leveling and sweeping away homes in Depauw and Daisy Hill.
Seven F5 tornadoes were observed—one each in Indiana, Ohio and Kentucky, three in Alabama and the final one which crossed through parts of Indiana, Ohio and Kentucky. Thirty-one people were killed in Brandenburg, Kentucky, and 28 died in Guin, Alabama. An F3 tornado also occurred in Windsor, Ontario, Canada, killing nine and injuring 30 others there, all of them at the former Windsor Curling Club.
There were 18 hours of nearly continuous tornado activity that ended in Caldwell County, North Carolina, at about 7:00 am on April 4. A total of 319 were killed in 148 tornadoes from April 3 through April 4 and 5,484 were injured.
The 1974 Super Outbreak occurred at the end of a very strong, nearly record-setting La Niña event. The 1973–74 La Niña was just as strong as the 1998–99 La Niña. Despite the apparent connection between La Niña and two of the largest tornado outbreaks in United States history, no definitive linkage exists between La Niña and this outbreak or tornado activity in general. Some tornado myths were soundly debunked (not necessarily for the first time) by tornado activity during the outbreak.