From April 26 to 27, 1991, multiple supercells across Oklahoma and Kansas led to a regional tornado outbreak. Forced by a potent trough and focused along a dryline, these distinct thunderstorms moved northeast through a moist and highly unstable environment. A total of 55 tornadoes were confirmed, many of which were strong, F2 or greater on the Fujita scale. A widely documented F5 tornado tore through Andover, Kansas, killing 17 people. Additional fatalities occurred from significant tornadoes in other portions of Kansas and Oklahoma, with 21 deaths recorded in total. An F4 tornado was detected by a mobile doppler weather radar team which observed winds up to at the top of the funnel, the first time winds of F5 intensity were measured by radar, and the highest winds recorded by radar at the time. A news team filming an F2 tornado sought shelter under a Kansas Turnpike overpass, causing a misconception that overpasses can provide adequate shelter during a tornado. This outbreak occurred within a transition period for the National Weather Service and proved the value of NEXRAD radars, which were utilized in Oklahoma to provide advanced warning to residents.
Meteorological synopsis
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 was progressing northeast toward the Plains. Through the morning hours, an 850 mb or approximately low-level jet of up to 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 . 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 hours, 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 hours. 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 hours 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 Oklahoma, although intense tornadoes were also observed in Iowa, Texas, and Nebraska.