During the afternoon of May 6, 1975, at least 12 tornadoes touched down in the Upper Midwest. The costliest of these tornadoes struck parts of western Omaha, Nebraska, causing at least $150 million in damage and killing three people. It was at the time the costliest tornado in U.S. history, damaging over a thousand homes across a nearly 2,000-block area on its roughly long path. The tornado's damage was later rated F4 on the Fujita scale. Another F4 tornado struck Magnet, Nebraska, destroying or damaging nearly every building in the town. The tornadoes were produced by thunderstorms moving across a narrow region of warm and moist air that had advanced northwards into the Upper Midwest as a result of a strong area of low pressure over South Dakota. Additional tornadoes on May 7 and May 8, including several in Louisiana, Texas, and Mississippi, were associated with the same storm system.

Meteorological synopsis

The tornado outbreak was associated with an intense area of low pressure that moved from Colorado into South Dakota beneath a strong upper-tropospheric trough. The combination of the low-pressure system and a ridge of high pressure over the Great Lakes moved unstable air in the lower levels of the troposphere towards eastern Nebraska. At 7 a.m. CDT on May 6, the area of low pressure was centered over southwestern South Dakota, with a central air pressure of 991 mbar (hPa; 29.26 inHg). A cold front extended from the low-pressure system southwestward to central Kansas, demarcating the boundary between a moist and warmer airmass to the east and a dry and cooler airmass to the west. Ahead of the cold front over eastern Kansas and Nebraska, dew points were near while they were below behind the cold front over western Kansas. An ongoing scientific field campaign run by NASAAtmospheric Variability Experimentssampled environmental conditions throughout the southwestern and southern United States during the eventual severe weather event. A weather balloon launched from Omaha, Nebraska, at 7 a.m. sampled atmospheric conditions moderately conducive to severe weather. During the morning hours, the broader wind pattern brought increasingly moist air in the lower troposphere into a narrow region encompassing eastern Nebraska, eastern Kansas, and northern Missouri. This corridor of moist air was bounded to the west by the cold front and to the east by a warm front, each slowly moving. Warmer and drier air persisted in the mid-levels of the troposphere above this moist air, resulting in conditions potentially favorable for the development of storms. Daytime heating and the increase in moisture with time within this narrow region, as well as a simultaneous divergence of air in the upper-troposphere, further increased atmospheric instability, producing increasingly favorable conditions for storm formation. Ahead of the warm front, thunderstorms and cloud cover over southern Iowa and northern and central Missouri caused cooler conditions ahead of the warm front, reinforcing a strong temperature gradient across the front. This further enhanced the favorability of atmospheric conditions for storm development.

The National Severe Storms Forecast Center (NSSFC) issued a tornado watch for much of the area in advance of the event at 12:37 p.m. May 6. The watch area was in effect from 2–8 p.m. and encompassed parts of eastern Nebraska, northeastern Kansas, northwestern Missouri, western Iowa, southeastern South Dakota, and southwestern Minnesota; Omaha was also included within the watch area. The first indications of storm development were apparent in South Dakota by 11 a.m. A squall line soon developed along much of the cold front, with the strongest thunderstorms occurring near the intersection between the cold and warm fronts ahead of the low-pressure area and along the warm front, including the storm that eventually produced a destructive tornado in Omaha. Within this area, vorticity and convergence of winds near the surface were higher than surrounding areas. The squall line was first apparent on weather radar at around 1 p.m., extending from central South Dakota to central Oklahoma. At around 1:15 p.m., the National Weather Service office in Omaha received a report of hail from Creighton, the office's first of the day. Between around 2–7 p.m., the squall line produced several damaging tornadoes in northeastern Nebraska. In the Omaha area, the development of thunderstorms was preceded by the movement of a comma-shaped area of cloudiness into the region, an indication of the movement of vorticity in the mid- to upper-troposphere into the region. Many of the thunderstorms that produced tornadoes showed rapid growth on satellite imagery around the time of tornado development, indicative of the rapid rise of air. The thunderstorms were supported by the channel of moist air, tracking north before eventually weakening after moving into cooler and drier air downwind. Twelve tornadoes ultimately occurred over the north-central Great Plains on May 6, with large and strong tornadoes affecting eastern Nebraska. All but one of the tornadoes had relatively short tracks, with their parent thunderstorms moving across the narrow corridor of moist air and the steep temperature gradient accompanying the warm front; the storm that produced the long-track tornado moved parallel to the temperature gradient. The same weather system led to at least 19 tornadoes on May 7, including 10 in South Dakota and Iowa.

Confirmed tornadoes

Note: One tornado is confirmed, but its rating is unknown

+ Color/symbol key Color / symbol Description † Data from Thomas Grazulis ※ Data from the 1975 Storm Data publication ‡ Data from the NCEI database

May 6 event + List of confirmed tornadoes – Tuesday, May 6, 1975F#LocationCounty / ParishStateStart Coord.Time (UTC)Path lengthMax width F2Near Ola to S of ChamberlainBruleSD17:00–?※Numerous farm buildings were destroyed across four farms, and five cattle were killed. The NCEI lists this tornado at F0 intensity, but it is assessed at F2 strength by Grazulis. F4E of Pierce to N of MagnetPierce, CedarNE19:05–? A violent tornado moved through Magnet on a southeasterly to northwesterly heading at around 2:15 p.m. CDT, damaging or destroying nearly every structure in town. The Magnet city hall was destroyed. Some homes were completely leveled. Cattle were killed, cars were thrown over , and power lines were toppled. One person was injured. Meteorologist Ted Fujita, who later photographed cycloidal scar patterns left by the tornado in cornfields in Cedar and Pierce counties, estimated that the tornado that struck Magnet was stronger the Omaha tornado. F2SW of Pierce, NE to SW of Yankton, SDPierce, Knox, CedarNE19:15–?††A strong tornado caused damage intermittently along its path, especially in western Pierce where homes were damaged, barns were destroyed, cars were tossed, power lines were toppled, and livestock were killed. The NCEI lists this tornado at F3 intensity, but it is assessed at F2 strength by Grazulis. F0W of BloomfieldKnoxNE19:30–?‡Farm buildings, irrigation equipment, and trees were sporadically damaged. F3SE of Stanton to NE of CarrollStanton, WayneNE19:45–?††An intense tornado caused extensive damage to farms and power lines. Multiple barns were leveled, and one home was reduced to two standing walls. Five hundred chickens were killed. One person was injured. F2SE of Tyndall to NW of Scotland†Bon Homme, Hutchinson†SD19:46–?††Small farm buildings were damaged. The Storm Data publication and the NCEI consider this event as two separate tornadoes focused in Bon Homme County. However, they are considered one event by Grazulis, and the path was extended into Hutchinson County. Additionally, the NCEI lists this tornado at F1 intensity, but it is assessed at F2 strength by Grazulis. F0W of ScotlandBon HommeSD20:10–?Small farm buildings were damaged. F0E of ColonSaundersNE21:00–?A brief tornado caused damage to trees and farm buildings on two farms. F0SW of WaterburyDixonNE21:30–?Trees, power lines, and farm buildings were damaged. F4OmahaSarpy, DouglasNE21:33–21:50※3 deaths – See the section on this tornado F2NE of Crescent to E of BeebeetownPottawattamie, HarrisonIA22:00–22:30※Numerous farm houses and outbuildings were damaged or destroyed. An aluminum silo was lofted away from a farmstead southwest of Beebeetown, Iowa. This tornado is not listed in Thomas Grazulis' Significant Tornadoes book. F2N of Honey Creek to S of LoganPottawattamie, HarrisonIA22:20–22:45※Numerous farm houses and outbuildings were damaged or destroyed over intermittent stretches of a tornado's path. This tornado is not listed in Thomas Grazulis' Significant Tornadoes book.

May 7 event + List of confirmed tornadoes – Wednesday, May 7, 1975F#LocationCounty / ParishStateStart Coord.Time (UTC)Path lengthMax width F2S of PioneerWest CarrollLA06:05–?A manufactured home was destroyed, and the occupant was severely injured after being tossed across the road. Portions of the home were scattered for . Another house lost its roof and had its furniture tossed. The occupants were trapped and injured by a fallen tree. F1N of NatchezAdamsMS11:45–?A farm co-op building had one of its sides knocked down and its roof ripped off and subsequently wrapped around a utility pole. A concession stand was destroyed, a warehouse was damaged, and several tree tops were twisted. F1E of MidlandJonesSD16:55–?A brief tornado caused some property damage. F1HattiesburgForrestMS17:10–?Several pavilion shelters in Kemper Park were blown down. Playground equipment was damaged, power lines were toppled, and trees were uprooted. Some tree limbs fell onto a car. F1S of RichtonPerryMS17:50–?Trees were uprooted, some of which fell on a manufactured home and a car, injuring the occupant. F1E of WendteStanleySD18:00–?A brief tornado caused some property damage. F2WSW of Eastabuchie to ProvidenceForrest, JonesMS19:30–?Two houses lost their roofs while a third was heavily damaged. Two chicken houses were destroyed and trees were uprooted, some of which fell on homes or blocked roads. F0N of PierreSullySD19:55–?A brief tornado was reported to the sheriff's office. F0NE of PierreSullySD21:30–?A brief tornado was reported to the sheriff's office. F1LeanderWilliamsonTX22:00–?An F1 tornado caused damage in Leander. F2NolanvilleBellTX22:00–23:00Four manufactured homes were demolished, one was overturned, and eleven others were damaged. Two people were injured. This tornado is not listed in Thomas Grazulis' Significant Tornadoes book. F0NW of PierreSullySD22:20–?A brief tornado was reported to the sheriff's office. F?BurnetBurnetTX22:30–?Small buildings and sheds were blown down, and trees were uprooted. This tornado is listed in the Storm Data publication but not by the NCEI. F2S of Dow CityCrawfordIA23:30–?Grain bins, a brick silo, and a machine shed were destroyed. The roof was ripped off a feeding shed. A garage was overturned. A home lost shingles and windows. This tornado is not listed in Thomas Grazulis' Significant Tornadoes book. F2E of Harlan to IrwinShelbyIA00:00–?※Some farm buildings, a barn, and a corn crib were destroyed. The roof of a home was damaged. This tornado is not listed in Thomas Grazulis' Significant Tornadoes book. F0CrestonUnionIA00:15–?A tornado touched down in an open field. F2E of Grand RiverDecaturIA01:00–?※Two hog houses and a barn were destroyed. Two hogs were killed. Trees were uprooted as well. F2OsceolaClarkeIA01:30–?※A strong tornado caused damage at six farms. A barn and a shed were destroyed. The roof and doors were ripped off a house, windows were blown out of a car, and two cows were killed.

May 8 event