1975 Omaha tornado outbreak

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 15 mi 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 65 F while they were below 20 F behind the cold front over western Kansas. An ongoing scientific field campaign run by NASA – Atmospheric Variability Experiments – sampled 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 Creigton, 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

Omaha, Nebraska
A destructive tornado – at the time the costliest in U.S. history – impacted residential areas of west-central Omaha on the afternoon of May 6, tracking primarily south to north along 72nd Street. During the afternoon and evening, a tornado watch was in effect for parts of eastern Nebraska. Tornadoes were reported near several Nebraska communities during the early afternoon. The National Weather Service forecast office in Omaha issued two severe thunderstorm warnings for Douglas County, Nebraska, including the Omaha area, during the afternoon of May 6. Teams of storm spotters associated with the National Weather Service and Offutt Air Force Base were deployed following the watch and warning issuances between around 1–2 p.m. At 4:09 p.m., a volunteer storm spotter reported an emerging funnel cloud northwest of Springfield, Nebraska. Tornado sirens were activated for Sarpy County, Nebraska, following the report. 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 Storm Data publication for May 1975 indicates that the tornado touched down slightly later at around 4:33 p.m. This gave a nearly four hour leadtime between issuance of the tornado watch and tornado occurrence, and over an hour leadtime for the tornado warning. The tornado's path began in a largely rural area near the intersection of Nebraska Highway 370 and 132nd Street in northern Sarpy County. The first homes destroyed by the tornado were hit at around 4:35 p.m. Following initial reports of damage near the intersection of 96th and Q streets, Nebraska governor J. James Exon activated the Nebraska Army National Guard. Moving northeast, the tornado struck parts of northwestern Ralston, damaging neighborhoods and businesses near 84th and L streets. Large airborne debris was observed near 72nd and L streets. 2×4s were reported falling from the sky over Omaha ahead of the tornado. 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. Parts of the roof of Ralston High School were peeled away by the twister.

The twister's path then curved north towards and over west-central sections of the city of Omaha, crossing Interstate 80 near 84th Street, inflicting injuries on the highway and prompting the highway's closure. Beginning to track along and near 72nd Street, the tornado cut across the Westgate subdivision, causing extensive damage and destroying Westgate Elementary School. Well-constructed homes were destroyed in the neighborhood. At 4:40 p.m., the tornado struck Bergan Mercy Hospital, blowing out windows and disheveling furniture; all 825 employees and patients of the hospital survived. Vehicles in the hospital parking lot were tossed and destroyed. Damage at the hospital amounted to around $5 million. The tornado narrowly avoided the Ak-Sar-Ben racetrack, forcing most of the 8,700 attendees at the racetrack that day into shelter. One person was killed while taking shelter at a restaurant at 78th and Pacific streets. Numerous buildings sustained significant damage near the intersection of 72nd and Dodge streets, including the Nebraska Furniture Mart. Most of the mail carrier fleet at a nearby U.S. post office was lost, with 80 of the office's 90 cars destroyed. Some of the most severe damage associated with the tornado occurred after it crossed Dodge Street at around 4:45 p.m., with buildings reduced to rubble. The Omaha Community Playhouse lost most of its roof and the Downtowner Motor Inn suffered major damage. Extensive damage occurred along 72nd Street, with numerous homes and apartments severely damaged, along with Creighton Prep School, Lewis and Clark Junior High school, the First United Methodist Church, and the Temple Israel Jewish Synagogue. Two people were killed near the intersection of 69th and Maple streets. A police officer who had been tasked with following the tornado and communicating live updates reported that the tornado lifted near Benson Park at around 4:58 p.m. Storm Data listed the tornado as dissipating after around 4:50 p.m.

Approximately 30,000 people may have been in the path of the tornado. 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 15 mi long, with about 6 mi in Sarpy County and 9 mi in Douglas County. A contiguous stretch of property damage occurred along a 8 mi stretch. The swath of damage ranged from about 0.25 – wide, spanning two to six city blocks at a time. Across the roughly 2,000-block area impacted by the tornado, 287 homes were destroyed, 650 were heavily damaged, and 787 were moderately damaged; another 53 apartment buildings sustained damage. The damage rendered thousands of people homeless. 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 ). Researcher Thomas P. Grazulis estimated that the tornado caused F4-rated damage on the Fujita scale.