From April 26–27, 1991, a large and deadly tornado outbreak affected parts of the Great Plains region of the central United States. On April 26, commonly known as the Great Plains tornado outbreak, nearly five dozen tornadoes occurred primarily across the states of Iowa, Kansas, Nebraska, Oklahoma and Texas. The following day, April 27, featured six tornadoes across Iowa and Texas, but also Georgia and Missouri. This outbreak occurred within a transition period for the National Weather Service and proved the value of Next-Generation weather radars, which were utilized in Oklahoma to provide advanced warning to residents. Primarily in Kansas but also in Oklahoma, 21 people in total were killed by the tornadoes. More than 300 people were injured during the outbreak.
On Friday, April 26, several strong to violent tornadoes, rated from F2 up to F5 on the Fujita scale, swept through the southern and central regions of the Great Plains. The first tornadoes occurred from northern Oklahoma, to southeastern Nebraska, which included two F3 tornadoes that impacted farmsteads in Otoe County, Nebraska; and the small community of Lanham, Kansas and Nebraska. After these initial storms, one cyclic supercell thunderstorm produced a tornado family across southern Kansas, of which one infamous F5 tornado was a part of. This violent tornado tore across parts of Wichita, causing widespread destruction and killing more than a dozen people across the city's metropolitan area, notably the suburb of Andover. There were about 100 or more home videos taken on VHS cameras by residents across the Wichita area during the F5. A notable F2 tornado from the same storm, occurred along the Kansas Turnpike in Butler and Chase Counties, which was recorded by a news crew who sought shelter underneath an overpass, causing a misconception that overpasses can provide adequate shelter during a tornado. In northern Oklahoma, another supercell put down a tornado family, which included two intense F3 and F4 tornadoes that touched down in Garfield County. The latter was observed to have wind speeds of 286 miles per hour (460 km/h) by a University of Oklahoma research team, who were chasing the storm near Red Rock. At the same time the F4 tornado near Red Rock, Oklahoma took place, another violent but lethal F4 tornado developed and moved across Cowley County, Kansas; impacting several homes around Arkansas City and Winfield. Later after dusk, additional strong tornadoes developed in northern Oklahoma. One large F4 tornado struck the Cimarron Turnpike near Westport, flinging vehicles around and killing one person, before the storm moved into the town and caused major damage from there, and in areas in the vicinity of Skiatook. The parent storm that was responsible for the prior F4 tornado in Westport, cycled and put down yet another violent but shorter lived F4 tornado just outside of Oologah, which became the costliest and most destructive tornado in Oklahoma of the outbreak as it went through the city's northern side.
The second day, Saturday, April 27, only featured half a dozen tornadoes in Georgia, Iowa, Missouri and Texas. The strongest was an F2 tornado that impacted near Victor, Iowa; causing considerable damage to some farmsteads in the region. After these tornadoes, the outbreak came to an end.
On April 25, 1991, the National Weather Service issued a warning of an impending weather system, noting that computer models were "indicating this to be a very significant severe weather producer with tornadoes occurring across the Central/Southern Plains." On the morning of April 26, the organization delineated a High risk of severe weather across the Great Plains. A southeast-tilted trough existed across the southwestern United States that morning, and a distinct jet streak, or a region of enhanced winds at the base of the trough, on the order of 75–85 kn (85–100 mph; 140–155 km/h) was progressing northeast toward the Plains. Through the morning, an 850 mb or approximately 5,000 ft (1,525 m) low-level jet of up to 60 kn (70 mph; 110 km/h) overspread regions from south-central Kansas northward into eastern Nebraska. A surface low-pressure area existed over southwestern Nebraska, supporting a dry line southward into Texas and a warm front southeastward across eastern sections of Kansas and Oklahoma.
In the unstable atmosphere between those two boundaries, surface dewpoints rose above 60 °F (15 °C). Abundant sunshine contributed to destabilization as lifted indices topped -12 from central Oklahoma into central Kansas and convective available potential energy reached 4,000 J/kg. A minimal capping inversion existed across Oklahoma even during the morning, and tornado-producing storms first developed across western Oklahoma around sunrise. These storms weakened as they moved northeast into Kansas. Back to the west, the dryline progressed rapidly eastward but began to slow precipitously during the afternoon. Attempts at thunderstorm development along this feature initially failed. At 17:10 UTC (12:20 p.m. CDT), the National Weather Service issued a particularly dangerous situation tornado watch, warning of the potential for multiple strong to violent tornadoes. This would be one of 24 convective watches issued during the day. Despite early failure at convective initiation, supercell thunderstorms rapidly erupted along the dryline during the afternoon as the jet streak propagated into the Great Plains, resulting in a regional outbreak of tornadoes stretching from Texas to Iowa. Violent tornadoes were concentrated in southern Kansas and northern Oklahoma, although intense tornadoes were also observed in Iowa, Texas, and Nebraska.
Clearwater–Haysville–Andover–El Dorado, Kansas
At 5:49 p.m. CDT (22:49 UTC), this deadly and violent tornado began east of Clearwater, in Sedgwick County, Kansas. At 6:05 p.m. CDT (23:05 UTC), the National Weather Service issued a statement urging residents in Haysville, Derby, and Mulvane to seek shelter. This was succeeded by a tornado warning four minutes later. Around 6:16 p.m. CDT (23:16 UTC), the intensifying tornado began to affect southeastern sections of Wichita and directly struck Haysville. It produced strong F2 to F3 damage in Haysville while growing to a width of about 220 yards (200 m) and acquiring multiple-vortex characteristics. The tornado crossed the Kansas Turnpike about 0.5 miles (0.80 km) south of the South Wichita Interchange. In eastern Wichita, some well-built houses in the Greenwich Heights Subdivision were completely leveled, indicative of strong F3 to F4 damage. Four people were killed at this location. At 6:24 p.m. CDT (23:24 UTC), the violent tornado struck the McConnell Air Force Base, where it narrowly missed a lineup of 10 B-1B bombers each worth $280 million and two of which were equipped with nuclear warheads. Nine major facilities on the base were destroyed, including the officer's club, base hospital, library, and elementary school. In addition, 102 housing units were demolished. No fatalities were recorded there, though 16 people were injured and total losses reached $62 million. As the tornado continued to move toward US 54 in the direction of Andover, it prompted forecasters to issue a heightened tornado warning alerting residents in Augusta and Andover that a damaging tornado was approaching. Despite this warning, the tornado sirens in Andover failed.
At 6:31 p.m. CDT (23:31 UTC), with the sirens not functional, the police drove through the Golden Spur Mobile Home Park and through the town warning residents to seek shelter. 10 minutes later, the now large wedge-stovepipe hybrid tornado entered southern Andover and began to impact the mobile home park, which ultimately sustained a direct hit. Of the 244 manufactured homes, 205, or about 84 percent of them, were destroyed. Post-storm interviews by health officials found that 339 residents were home during the tornado, of which 146 evacuated, 149 sought refuge in the community shelter, and 38 remained in their homes. No casualties occurred among individuals who fled or utilized the shelter. However, 13 people were killed, another 17 were hospitalized, and nine sustained minor injuries among the group who remained in their structures. Additional homes were swept from their foundations to the west of this park, where the Andover tornado earned its F5 rating. Throughout the city, over 1,500 residences were devastated. The tornado continued northeast, affecting the outskirts of Towanda. Twenty minutes later, the violent tornado dissipated north-northwest of El Dorado and north of the Kansas Turnpike, though the parent supercell later produced additional tornadoes further across Butler County. Along the tornado's path, 84 frame houses and 14 businesses were leveled. A total of 225 people were injured. It was the final Kansas tornado to be given an F5 rating prior to the implementation of the Enhanced Fujita Scale, which was principally used on the Greensburg EF5 tornado on May 4, 2007.