1979 Red River Valley tornado outbreak

On Tuesday, April 10, 1979, a widespread and destructive outbreak of severe weather impacted areas near the Red River between Oklahoma and Texas. Thunderstorms developed over West and North Central Texas during the day within highly unstable atmospheric conditions following the northward surge of warm and moist air into the region, producing large hail, strong winds, and multiple tornadoes. At least 22 tornadoes were documented on April 10, of which two were assigned an F4 rating on the Fujita scale; four of the tornadoes caused fatalities.

Hardest hit were the communities of Vernon, Texas; Lawton, Oklahoma; and Wichita Falls, Texas, which were all impacted by strong tornadoes. The F4 Vernon tornado struck southern and eastern parts of the city at approximately 4:45 p.m. CST, destroying several residential blocks and killing 11 people. An F3 tornado spun up in Lawton at 5:05 p.m. CST, destroying 167 buildings and killing 3 people. The most significant tornado of the day was an F4 tornado that began east-northeast of Holliday, Texas, at around 5:50 p.m. CST and moved east-northeast into Wichita Falls, taking a 8 mi course through densely-populated areas of the city and destroying over two thousand homes across several neighborhoods. The tornado spanned as wide as 1.5 mi across during its passage through the city, with the most severe damage occurring within a 0.5 mi wide swath. At least 45 people were killed within the city and nearly 1,800 people were injured, ranking the tornado among the deadliest in Texas history. A majority of the fatalities occurred as the tornado mangled and tossed vehicles. The damage wrought by the Wichita Falls tornado was unprecedented, with the $400 million damage toll making it the costliest tornado on record at the time. The severe weather event was widely observed by scientific instruments due to its serendipitous occurrence during a NASA field campaign. Later studies referred to the tornado outbreak as the Red River Valley Tornado Outbreak, and in the areas affected the day came to be known as Terrible Tuesday.

The same weather system associated with the tornado outbreak produced additional thunderstorms and tornadoes on April 11, particularly in Arkansas. While tornadic activity was more widespread on April 11 compared to April 10 far fewer casualties occurred. The weather system also produced intense rainfall over Mississippi and Alabama that culminated in the 1979 Easter flood, the worst disaster to befall Jackson, Mississippi in over a century.

Preceding the event
On April 9, a strong upper-tropospheric trough was located over the western United States. At roughly 40000 ft above the surface, a powerful jet stream extended from the eastern Pacific to South Texas. Although a cold front moving south during the day brought substantially cooler conditions to Oklahoma, it quickly weakened over Texas. Warm and moist air remained over South Texas ahead of the disturbances. Meteorological computer models using data collected at 6 p.m. CST April 9 suggested that severe weather was possible the next day for parts of Oklahoma and Texas but did not as indicate as clearly the possibility of a tornado outbreak. The upper-tropospheric trough moved slower than modeling projected and by 6:00 a.m. CST April 10 had become oriented north-northeast to south-southwest, a characteristically negative tilt associated with many severe weather outbreaks. An occluded front stretched from a quickly intensifying low-pressure area in Colorado into the Texas Panhandle, representing one area conducive to storm development. A wide expanse of stratus clouds covered much of Central Texas during the morning hours as a result of a stable airmass over the area. However, strong southerly winds closer to the surface brought moisture northward from South Texas, concurrent with a rapid increase in atmospheric instability over North Texas. Due to the presence of directional wind shear, the winds over the Red River Valley brought moist air from the south closer to the surface, and drier air from the west further aloft, enhancing convective instability. The leading edge of the northward surge of moisture was marked by an ill-defined warm front. Dew points across the region increased by around 10 – to near 60 F.

A line of thunderstorms developed over North Texas on the morning of April 10 in response to the surge of warm air, with small hail up to 1 in diameter associated with some of the storms. Within the nearby jet stream, an area of locally intense winds – a jet streak – moved towards the northeast towards the expanding unstable airmass over north-central Texas. The passage of this core of intense winds aloft induced strong southwesterly winds closer to the surface over eastern New Mexico and West Texas, leading to widespread blowing dust. Wind gusts exceeded 60 mph across West Texas and reached as high as 93 mph at Guadalupe Mountains National Park. The dew point at Marfa, Texas, was 10 F compared to dew points near 70 F over central Texas. This surge of dry air, moving towards the east-northeast at roughly 30 knot, marked another potential region for storm development. A second area of thunderstorms began to quickly intensify along the leading edge of this dry line and along the occluded front between Amarillo, Texas, and Plainview, Texas. Hail up to 1.5 in in diameter was reported in association with these storms, as well as a tornado east of Plainview at around 1:38 p.m. CST that caused no damage. A third area of thunderstorms, albeit short-lived, developed west of Abilene, Texas, along the western edge of a small region of high pressure. While several of the morning's storms moved towards stable conditions and were poised to eventually weaken, particularly conducive conditions for intense thunderstorm development materialized near Lubbock, Texas, at the intersection of the surging dry line, the warm front, and the western edge of the high pressure region.

By the afternoon of April 10, both the warm front and the region of strong winds associated with the jet stream had reached the Red River area, resulting in stronger wind shear. A low-pressure area had also developed in the vicinity of Childress, Texas at around 1–2 p.m. CST, and would serve as a focusing mechanism for converging moisture over the Red River Valley. The National Severe Storms Forecasting Center (NSSFC) issued a tornado watch at 1:55 p.m. CST on April 10 for a 30000 mi2 area encompassing parts of southwestern Oklahoma and north-central Texas. The watch statement highlighted the potential for tornadoes, large hail, and damaging winds within the watch area. Within the risk region, the air pressure fell by 6 – in three hours; such drastic pressure falls can precede tornado outbreaks. Strong convergence of moisture was also observed in the areas where storms eventually formed.

Tornado outbreak
The arrival of the jet streak contributed to the conversion of the atmospheric instability accumulating near the Red River into thunderstorm formation. Just before the onset of the tornado outbreak, a low level jet with winds of 20 m/s materialized over north-central Texas in response to the approach of the jet streak higher aloft, accelerating the transport of heat and moisture northward into the Red River Valley. Thunderstorms developed near Lubbock and moved east-northeastward into a region of high potential instability near the Red River, becoming the strongest thunderstorms of the day. In particular, three isolated supercell thunderstorms caused the majority of the severe weather during the afternoon and evening of April 10; all developed in the localized area of supportive conditions near the frontal intersection. The first thunderstorm formed around 1:00 p.m. CST spawned five tornadoes between 3:05 p.m. CST and 5:15 p.m. CST, including an F4-rated tornado that struck Vernon, Texas, and an F3-rated tornado that struck Lawton, Oklahoma. The second thunderstorm formed around 2:30 p.m. CST, while the third formed around 4:30 p.m. CST. The second supercell produced a tornado that began at 3:55 p.m. and took 64 mi long track – the longest track of any tornado during the event – through primarily rural areas near Harrold, Texas and Grandfield, Oklahoma. The most damaging tornado, associated with the third supercell, began at around 5:55 p.m. CST and struck Wichita Falls, Texas. These storms persisted into nighttime over central and eastern Oklahoma. The breakout of severe weather and tornadoes associated with the thunderstorms near the Red River during the afternoon and evening of April 10 was collectively termed the "Red River Valley outbreak" or the "Red River Valley tornado outbreak" by scientists studying the event. At least 12 tornadoes occurred within 75 km of Wichita Falls in a single two-hour period. The last tornado that occurred in the Red River Valley occurred around 8:00 p.m. CST in connection with the third supercell near Pruitt City, Oklahoma. The thunderstorms that produced the tornadoes in the Red River Valley continued into Arkansas and southern Missouri during the overnight hours but did not produce severe weather.

The arrival of a second and weaker region of strong winds within the nearby jet stream led to the development of severe thunderstorms, including some tornadoes, farther south over west-central and northern Texas during the overnight hours. These tornadoes caused extensive damage in rural areas and some injuries. Like the severe event earlier in the day along the Red River, the development of these storms was aided by a localized area of low pressure that developed near Abilene at around 7:20 p.m. CST. Additional storms also persisted over the Midwestern United States into the morning of April 11. The broader weather system that produced the severe weather on April 10 continued eastward towards the Mississippi River Valley the next day and intensified. April 11 ultimately proved to be a more active day for severe weather, with Arkansas experiencing most of the day's tornadoes; however, fewer casualties resulted from the tornadoes compared to the previous day. The 27 tornadoes that were ultimately documented by the National Oceanic and Atmospheric Administration in Arkansas during April 1979 set a new monthly record high for the state. Heavy rains over Mississippi and Alabama associated with the same weather system on April 11–13 led to extensive flooding along the banks of the Pearl River and Tombigbee River. Rainfall totals reached 10 –, with the flood near Jackson, Mississippi, being representative of a 1 in 500-year or 1 in 100-year flood event.

Confirmed tornadoes
NWS offices in Texas and Oklahoma reported 56 fatalities from the tornadoes on April 10, with 53 in Texas and 3 in Oklahoma across a combined 12 counties. Another 1,916 people were injured, of which 256 were hospitalized. The American Red Cross reported 2,934 homes destroyed with another 3,263 damaged. April 10 featured the most damaging tornado outbreak since the 1974 Super Outbreak. Four tornadoes caused fatalities that day. The NWS characterized the tornado outbreak as "one of the most significant tornado outbreaks that ever occurred in western north Texas and southern Oklahoma." The atmospheric environment surrounding the April 10 severe weather outbreak and the thunderstorms that eventually formed were closely observed as part of the Severe Environmental Storms and Mesoscale Experiment (SESAME), a scientific field campaign aimed at studying severe weather. As part of SESAME, an extensive survey of affected areas was undertaken following the event, including interviews of civil defense directors and ground and aerial surveys of tornado paths. Damage along the paths of 12 of the tornadoes on April 10 were rated with the Fujita scale by Ted Fujita and Roger Wakimoto based on observations from a low-flying Cessna aircraft on April 12–13, with a presumed ±1 margin of error.

Thalia–Vernon, Texas/Davidson, Oklahoma
The first of three intense and tornadic thunderstorms produced a tornado in southern Foard County, Texas, at 2:05 p.m. CST. While this first tornado was still on the ground, the same thunderstorm produced a second tornado in Foard County 2 mi so north of Thalia, at around 3:20 p.m. CST; the two tornadoes were separated by 1.5 mi and moved in parallel for 5 mi. The second tornado continued into Wilbarger County, after the first dissipated. It tracked towards the Lockett, area along US 70, damaging farms and homes. Fujita and Wakimoto assessed up to F2-rated tornado damage between Thalia and Lockett. The tornado crossed the highway 1 mi north of Lockett. One person was killed by the tornado after their vehicle was blown 200 yd off US-70 into a pasture.

The tornado then moved northeast towards the Vernon, area. Its large size made identification of its tornadic nature by storm spotters difficult. Tornado sirens were activated shortly before the tornado struck Vernon. The tornado entered Vernon at around 3:45 p.m. CST and moved over southern and eastern parts of the city. Several residential blocks in southern Vernon were destroyed by the tornado, leading to at least three deaths. The damage was especially severe along Gordon and Atlanta streets. In eastern Vernon, the tornado razed several buildings, including the Wilbarger Exhibit Building and a Texas Highway Department warehouse. The tornado then crossed US 287, blowing vehicles off the highway and killing seven people. A total of ten fatalities occurred in Vernon. Fujita and Wakimoto assessed up to low-end F4-rated tornado damage in the Vernon area. The tornado crossed the Pease River outside of Vernon, killing cows along the riverbank, and crossed into Oklahoma near the confluence of the Pease River and Red River with approximately F1 intensity. The tornado passed east-northeast of Davidson, destroying at least three homes and inflicting major damage on another five. One person was injured in Oklahoma after their car was blown by the tornado. Numerous cattle were also killed or injured by the tornado. Fujita and Wakimoto asssessed up to F2-rated tornado damage in the Davidson area. The tornado lifted at around 4:00 p.m. CST.

The tornado's path spanned 37 –, with about 26 mi in Texas and 11 mi in Oklahoma. The tornado's width averaged about 880 yd. The Institute for Disaster Research conducted aerial surveys of the tornado, rating it an F4 on the Fujita scale based on the resulting damage. The institute estimated that the tornado's maximum winds were 150 – based on the degree to which residences were damaged, the damage to the Texas Highway Department, the destruction of a motel and restaurant along US-287, and the distance over which impacted vehicles were blown by the tornado. The same thunderstorm that produced the Vernon tornado later produced another three tornadoes in Oklahoma.

Holliday–Wichita Falls, Texas/Waurika, Oklahoma
Parts of Wichita Falls, were struck by a violent tornado on the evening of April 10. The tornado was up to 1.5 mi wide as it tore through the city, with the swath of particularly intense damage spanning 0.25 – wide. The tornado swept east-northeastward through a 8 mi stretch of residential areas covering 11 mi2, directly causing 42 fatalities according to the National Weather Service (NWS); another three people later died of heart attacks. A survey by the Center for Disease Control enumerated 44 fatalities and 171 injuries requiring hospitalization, while a later study published in Science in February 1980 enumerated 45 deaths from traumatic injuries inflicted by the tornado, 1 from sepsis, and 1 from tetanus. Of the fatalities, 25 were vehicle-related deaths; 11 people died while attempting to evade the tornado in their vehicles after fleeing their otherwise undamaged homes. Only five deaths occurred indoors. Around 1,700 people sustained injuries in Wichita Falls, including 59 with serious injuries. Numerous residences were destroyed in the city, including 2,095 homes, 1,062 apartment units and 93 mobile homes. Over 3,000 homes left uninhabitable by the tornado. The widespread devastation left approximately 20,000 people from 5,000 families homeless, accounting for about 10–20% of the city's population. Vehicles were thrown as far as 200 yd away by the tornado. The destruction amounted to $400 million in damage, making the tornado the costliest tornado on record at the time. When normalized for wealth and inflation, the tornado caused approximately $1.14 billion in damage in 1997 United States dollars. Advance notice of the approaching tornado and the awareness of prior tornadoes earlier in the day in nearby Rocky Point and Vernon may have contributed to lowering the ultimate death toll.

The thunderstorm that eventually produced the Wichita Falls tornado formed north of Abilene and generally moved towards the northeast. Weather radar observed reflectivity values exceeded 50 dBZ throughout the storm's lifetime, with the radar signal routinely exceeding 50000 ft above the surface; satellite observations suggested cloud top heights reaching as high as 15.6 km. The thunderstorm produced an F2 tornado earlier in the day in Baylor Countyy, and hail up to 3 in in diameter in Archer, Baylor, Wichita, and Wilbarger counties. Another thunderstorm produced a tornado in Wichita County about 12 mi west-southwest of Iowa Park. The Wichita Falls weather service office issued a tornado warning for Wichita County at 5:08 p.m. in response to sightings of this tornado, and tornado sirens were sounded in Wichita Falls at 5:25 p.m. The first thunderstorm eventually approached Wichita County, prompting a severe thunderstorm warning for the county at 5:45 p.m. CST. Two mesoscyclones within the thunderstorm were apparent on radar, including one associated with the earlier tornado in Baylor County.

At around 5:50 p.m. CST, a tornado developed in connection with the second mesocyclone on the southwestern flanks of the thunderstorm 3 mi east-northeast of Holliday, Texas, in Archer County. Witnesses observed smaller vortices embedded in the broader tornadic circulation as it developed, highlighting the multiple-vortex nature of the tornado; the individual vorticies coalesced as the tornado approached Wichita Falls. Evidence of a tornado first became apparent on weather radar at 5:58 p.m. CST, while the tornado was about 10 mi southwest of the center of Wichita Falls. The initial reports of the tornado by amateur storm spotters provided residents of Wichita Falls with about 5–10 minutes of notice before the tornado's eventual arrival. The Wichita Falls weather service office issued a tornado warning for Wichita County at 6:00 p.m. CST, just as the first tornado warning expired, warning of the approach of this new tornado. The tornado moved along the Fort Worth and Denver Railway into Wichita County, blowing away two oil storage tanks and unroofing several homes near FM 2560; the swept oil tanks produced an oil spill. Six high-tension steel electrical transmission towers east of the road were damaged, with three blown prostrate. The tornado darkened and widened to a width of roughly 0.5 mi across as it moved into Wichita Falls at around 6:15 p.m. CST, impacting Memorial Stadium. The stadium field house was heavily damaged and stadium lighting was snapped. As the tornado passed just south of the stadium, it rapidly strengthened to high-end F4 intensity and maintained this strength for the next 11 mi of its path. The tornado then caused the collapse of the western half of McNiel Junior High School; the school was largely unoccupied when the tornado struck. After striking the stadium, the tornado began to move through heavily populated residential areas. The Western Hills neighborhood south of Southwest Parkway was struck next, resulting in the first fatalities caused by the tornado. Numerous homes and several apartment complexes were destroyed in the neighborhood.

The tornado widened to a width of about 1 mi as it crossed Southwest Parkway into the Faith Village neighborhood, levelling several businesses and tossing vehicles about. The Southwest National Bank Building was levelled with the exception of its vault. The tornado destroyed most homes in Faith Village. Small interior rooms in most of the destroyed homes remained intact, resulting in relatively few fatalities despite the severity of destruction. The cafeteria auditorium and exterior classrooms at Ben Milam Elementary School sustained heavy damage. The tornado proceeded to raze businesses along Kemp Boulevard, including a restaurant where three people were killed. The northern periphery of the tornado struck the Sikes Senter shopping mall, causing the partial collapse of a JCPenney store and inflicting light to moderate damage to other stores in the mall. Several of the roughly 1,000 shoppers in the mall suffered major injuries. The tornadic winds heaped cars in the shopping mall's parking lot atop one another or blew them away. Several people were killed or injured in the parking lot, including some who had fled from the mall to their cars. A church 0.5 mi south of the mall was flattened, resulting in one fatalitity. The tornado entered the Colonial Park neighborhood after traversing a greenbelt and clipping the southern side of Midwestern State University, destroying many homes and apartment complexes in Colonial Park. The second stories of apartment buildings saw particularly severe damage. Additional homes and a shopping center were razed in the Southmoor subdivision. The tornado then crossed US 281 and moved into the Sun Valley neighborhood along the southern side of US 287, destroying apartments, houses, and businesses. Cars were wrecked on US 287, resulting in a few fatalities. Among the fatalities were some who sought shelter underneath a highway overpass. The Sunnyside Heights mobile home park north of the highway was destroyed, though no fatalities resulted as residents had evacuated.

Industrial plants were subsequently destroyed, including a Levi Strauss & Co. manufacturing plant, as the tornado moved into Clay County south of SH 79. The Wichita Falls weather service office issued a tornado warning for Clay County at 6:11 p.m. CST shortly before the office lost power. Homes were destroyed by the tornado south of TX-79 along a path from Wichita Falls to Dean, and Petrolia, contributing to the $15 million damage toll wrought by the twister in Clay County. Forty people were injured in the county. Fujita and Wakimoto assessed up to F4-rated tornado damage between Wichita Falls and Dean, with F0-rated damage south of Petrolia. The tornado uprooted 200 trees along the Red River as it crossed into Oklahoma around 4 mi east of Byers at 6:30 p.m. CST. The tornado's path gradually curved towards the left after entering Oklahoma, bringing it to areas southwest of Waurika. The most severe damage inflicted by the tornado in the state occurred alone Noble Wray Road, where 20 homes were destroyed or damaged. The frame of one mobile home was greatly contorted and blown 0.5 mi away. Fujita and Wakimoto assessed up to F2-level damage southwest of Waurika. The tornado dissipated just before 7:00 p.m. CST north-northeast of Waurika. Widespread F0-intensity damage continued for 30 mi past Waurika, though this damage was likely caused by a large downburst rather than a continuation of the tornado.

In total, the tornado was on the ground for approximately 70 minutes, with of path 47 mi long, including 36 mi in Texas and 11 mi in Oklahoma. Based on aerial surveys of the damage, the Institute for Disaster Research assessed the tornado as reaching F4 intensity and estimated that the tornado's maximum wind speeds reached 175 –. The most extreme damage wrought by the tornado occurred at McNiel Junior High School and Southwest National Bank. Later engineering analysis of the higher-end damage suggested winds consistent with that of a strong F4 tornado, though there was ample preceding scientific discussion concerning whether or not the extreme damage could be graded as F5-intensity damage. Unlike most tornadoes, the corridor of strongest winds associated with the Wichita Falls tornado was unusually wide; the swath of F4-severity damage was nearly 0.5 mi across in vicinity of Ben Milam Elementary School and the Faith Village neighborhood. The thunderstorm that produced the Wichita Falls tornado also dropped hail up to 2 in diameter in Wichita Falls north of the tornado path. Survivors of the tornado also observed hail the size of golf balls immediately preceding and following passage of the tornado. The broader winds associated with the mesocyclone that spawned the Wichita Falls tornado caused light damage throughout much of the city.

Aftermath
Within Wichita Falls $63 million in losses were eligible for federal disaster aid. The apparent survivability of small interior rooms despite the widespread destruction of homes and businesses encouraged the development of reinforced safe rooms. The viability of safe rooms and the high number of fatalities among those who fled their homes led to increased emphasis on seeking indoor shelter in tornado preparedness, as opposed to the older idea of opening windows to reduce tornado damage.

April 10, 1979, came to be known as "Terrible Tuesday" in the areas affected by the tornadoes along the Oklahoma–Texas border. In 1984, the NWS produced a documentary covering the Wichita Falls tornado, entitlted Terrible Tuesday, in collaboration with the American Red Cross and FEMA. One Wichita Falls park features a plaque with the names of the 45 people who died during and after the Wichita Falls tornado, with a tree dedicated to each of the victims. Another park near the downtown area bears crape myrtle sculptures symbolizing the city's recovery.