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
thumb|[[Surface weather analysis of the tornado outbreak region, with weather boundaries and weather observations at around 4 p.m. CDT May 6, including the eventual starting points of tornadoes|alt=Weather map of the Midwestern United States]]
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
{| class="wikitable"
|+ Color/symbol key
|-
! scope="col" | Color / symbol
! scope="col" | Description
|-
! scope="row" style="background-color:#9FCADF;"| †
| Data from Thomas Grazulis
|-
! scope="row" style="background-color:#E89483;"| ※
| Data from the 1975 Storm Data publication
|-
! scope="row" style="background-color:#F7D9D3;"| ‡
| Data from the NCEI database
|-
|}
May 6 event
{| class="wikitable sortable" style="width:100%;"
|+ List of confirmed tornadoes – Tuesday, May 6, 1975
|-
! scope="col" width="2%" align="center" |F#
! scope="col" align="center" class="unsortable" |Location
! scope="col" align="center" class="unsortable" |County / Parish
! scope="col" align="center" |State
! scope="col" align="center" |Start Coord.
! scope="col" align="center" |Time (UTC)
! scope="col" align="center" |Path length
! scope="col" align="center" |Max width
|-
! scope="row" rowspan="2" style="background-color:#; border-bottom: 1px solid black" | F2
|Near Ola to S of Chamberlain
|Brule
|SD
|
|bgcolor=#E89483|17:00–?※
|
|
|- class="expand-child"
| colspan="8" style=" border-bottom: 1px solid black;" |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.
|-
! scope="row" rowspan="2" style="background-color:#; border-bottom: 1px solid black" | F4
|E of Pierce to N of Magnet
|Pierce, Cedar
|NE
|
|19:05–?
|
|
|- class="expand-child"
| colspan="8" style=" border-bottom: 1px solid black;" | 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. estimated that the tornado that struck Magnet was stronger the Omaha tornado.
|-
! scope="row" rowspan="2" style="background-color:#; border-bottom: 1px solid black" | F2
|SW of Pierce, NE to SW of Yankton, SD
|Pierce, Knox, Cedar
|NE
|
|bgcolor=#9FCADF|19:15–?†
|bgcolor=#9FCADF|†
|
|- class="expand-child"
| colspan="8" style=" border-bottom: 1px solid black;" |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. This tornado is not listed in Thomas Grazulis' Significant Tornadoes book.
|-
! scope="row" rowspan="2" style="background-color:#; border-bottom: 1px solid black" | F2
|Nolanville
|Bell
|TX
|
|22:00–23:00
|
|
|- class="expand-child"
| colspan="8" style=" border-bottom: 1px solid black;" |Four manufactured homes were demolished, one was overturned, and eleven others were damaged. Two people were injured. Additional reports of funnel clouds and the appearance of a hook echo on weather radar prompted the Omaha office to issue a pre-prepared tornado warning for Sarpy and eastern Douglas counties in Nebraska and western Pottawattamie County in Iowa at 4:14 p.m.
The first confirmations of a tornado touching down were reported at around 4:29 p.m., leading to the activation of tornado sirens in Omaha. The Wentworth Apartments, the largest apartment complex in the Omaha area at the time, sustained severe damage with 70 percent of the complex's buildings destroyed. Damage at the hospital amounted to around $5 million. Three people were killed by the tornado and another 141 people were injured; of the injuries, 25 were considered serious injuries. The tornado's path was approximately long, with about in Sarpy County and in Douglas County. The Nebraska governor's office estimated a damage toll of $150–200 million (1975 USD; equivalent to $– billion in ), making the tornado the costliest tornado in U.S. history and the costliest natural disaster in Nebraska history at the time. Damage estimates ranged as high as $500 million (equivalent to $ billion in ).