1999 Bridge Creek–Moore tornado

The thunderstorm that eventually produced the F5 tornado began developing around 3:20 p.m. CDT that afternoon over northeastern Tillman County (southwest of Faxon). Despite the lack of overall lift prevalent in the region, the storm formed out of a contrail-like horizontal area of convective clouds that developed during peak surface heating over southwestern Oklahoma, located well ahead of the dry line that was still positioned farther to the west, which provided enhanced lift and speed shear necessary to develop the supercell.[15] Tracking northeast, the storm strengthened and entered Comanche County shortly after 4:00 p.m. CDT.[16] By 4:15 p.m. CDT, the NWS Norman office issued a severe thunderstorm warning for Comanche County, as the initial storm continued to rapidly intensify over the southern half of the county; there, hail up to 1.75 inches (4.4 cm) in diameter fell.[7][17]

As the supercell's mesocyclonic rotation began to rapidly strengthen at the cloud base, a tornado warning was issued for the counties of Comanche, Caddo, and Grady at 4:50 p.m. CDT;[18] one minute later, a small tornado roughly 25 yards (75 ft) in diameter − the first of fourteen associated with supercell "A" (NWS Norman designated lettered names for the three tornado-producing supercells in the outbreak in storm surveys) − touched down seven miles (11 km) east-northeast of Medicine Park along U.S. Route 62. Five more tornadoes developed as the storm continued northeast; a sixth one, which would be given an F3 rating, touched down a short time later and caused substantial damage in central Grady County, including some to the Chickasha Municipal Airport, where roofs were torn off of two hangars.[16]

At 6:23 p.m. CDT, a ninth tornado associated with supercell "A" touched down about two miles (3.2 km) south-southwest of Amber.[16] The tornado quickly intensified in both strength and size as it crossed Oklahoma State Highway 92, attaining F4 strength about 4 miles (6.4 km) east-northeast of Amber. It quickly became a wedge tornado, varying between one-quarter and one mile (0.40 and 1.61 km) in width at various points throughout the track. Damage consistent with this rating was sustained over the following 6.5 miles (10.5 km) of the path before striking Bridge Creek. There, it attained the highest-possible rating on the Fujita Scale, F5. A mobile Doppler weather radar recorded winds of 301 mph (484 km/h) within the tornado at Bridge Creek, the highest wind speed ever recorded on Earth.[19] Since the record for maximum winds are reported from only non-tornadic events, however, the 253 mph (407 km/h) wind gust from Cyclone Olivia in 1996 retained the title.[20][21]

Damage in Bridge Creek was extreme, as many homes were swept away completely, leaving only concrete slabs where the structures once stood. Damage surveyors noted that the remaining structural debris from some of the homes in this area was finely granulated into small fragments, and that trees and shrubs were completely debarked.[22] A few of these homes were bolted to their foundations. Approximately 200 houses and mobile homes were destroyed, and hundreds of other structures were damaged. The Ridgecrest Baptist Church in Bridge Creek was also destroyed in the process. Extensive ground scouring occurred, and vehicles were thrown hundreds of yards from where they originated, including a mangled pickup truck that was found wrapped around a telephone pole. About one inch (25 mm) of asphalt was scoured off one road.

Twelve people died in Bridge Creek, nine of whom were in mobile homes; all fatalities and the majority of injuries were concentrated in the Willow Lake and Southern Hills Additions and Bridge Creek Estates, consisting mostly of mobile homes. Over 39 people were injured in the area as well.[21][23] Continuing northeastward, the tornado briefly weakened to F4 status before re-strengthening to F5 intensity as it neared the Grady-McClain County line, where a car was thrown roughly 0.25 mi (0.40 km) in the air, and a well-built home with anchor bolts was reduced to a bare slab.[21] At this time, it had attained a width of one mile (1.6 km), having grown to its largest width after crossing the South Canadian River into the southern Oklahoma City limits. The tornado then became quickly rain-wrapped; Jim Gardner, then-helicopter pilot for KFOR-TV, reported during the station's live coverage of the storm that the tornado was at least one mile wide, and was embedded (or "rain-wrapped") in the precipitation core associated with the main circulation, making it difficult to see.[16]

As it was becoming clear that a particularly violent tornado was moving into some of the most densely populated areas of central Oklahoma, around 6:57 p.m. CDT, NWS Norman issued the first-ever tornado emergency for southern portions of the Oklahoma City metropolitan area. David Andra, a meteorologist at the NWS Norman office, said that, in drafting the enhanced warning, he wanted to "paint the picture [to residents] that a rare and deadly tornado was imminent in the metro area." For its initial usage, the enhanced wording was released as part of a standalone Severe Weather Statement (NWS code: SVS), which were (and still are) normally meant to update the public on an existing tornado warning or severe thunderstorm warning. For tornado warnings, the SVS provides updated information on the approximate location of the storm's base-level rotation or, if it occurred after the initial warning was issued, a tornado reported by the public, civil defense personnel or storm spotters, or with the later advent of dual-polarization radar in the early 2010s, verified by radar-detected debris signatures. (In future issuances, tornado emergencies were issued within either the initial tornado warning issuance or an SVS providing updated information on a tornado warning already in effect.)[24][25] Two minutes later, at 6:59 p.m. CDT, the SPC issued a Particularly Dangerous Situation tornado watch for much of the central third of Oklahoma, effective from 7:15 p.m. until midnight CDT on the early morning of May 4; the SPC watch product discussion noted that the extreme instability compensated for "somewhat marginal" wind shear to enhance the threat of strong to violent tornadoes.[14]

The foundation of a house in Bridge Creek that was completely destroyed, with severe debarking and ground scouring visible on the property.

Paralleling Interstate 44, the tornado moved into McClain County, where it crossed the highway twice at F4 intensity, killing a woman who was blown out from an underpass where she was attempting to seek shelter after being dragged down the embankment by the intense channeling winds; her 11-year-old son − with whom the woman vacated their stalled car nearby − survived, staying held tight onto the steel girders of the overpass. A man who helped the mother and son up the overpass suffered severe injuries to his leg, which was partially sliced by a highway sign thrown by the winds.[21][26] At 7:10 p.m. CDT, a satellite tornado touched down over an open field north of Newcastle; it was rated as an F0 due to lack of damage. In McClain County, 38 homes and two businesses were destroyed and 40 homes, some of which were leveled at F4 intensity, were flattened; seventeen people were injured.[21]

After crossing the Canadian River, the tornado entered Cleveland County and weakened to F2 intensity. By this time, it had entered the south side of Oklahoma City. Several minutes after entering the county, it re-attained F4 status, and then moved directly into the city of Moore, reaching F5 intensity for a third time. Some of the most severe damage took place in Cleveland County, mainly in Moore, where eleven people were killed and 293 others were injured. The tornado caused an estimated $450 million in damage across the county. The first area impacted in Moore was the Country Place Estates subdivision, where 50 homes were destroyed and one was swept cleanly from its foundation at F5 intensity. Several vehicles were picked up and tossed nearly 0.25 mi (0.40 km) away from their previous location. According to local police, an airplane wing, believed to have been from an airport in Grady County (possibly lofted into the storm's updraft when the supercell's sixth tornado hit Chickasha Municipal Airport), was found near Country Place Estates. Then, the powerful tornado struck the densely populated Greenbriar Eastlake Estates at F5 intensity, killing three people and reducing entire rows of homes to rubble. In one instance, four adjacent homes were completely destroyed, with only concrete slabs remaining, warranting an F5 rating at that location. Three other homes in this housing division also received F5 damage, with the remaining destruction rated high-end F4. Severe debarking of trees was noted in this area. At the Emerald Springs Apartments, three more people were killed and a two-story apartment building was mostly flattened.[7]

Approximate tracks of the supercells during the outbreak. The supercell that spawned the F5 tornado is marked "A" in green.

As the tornado entered Cleveland County, NWS Norman activated emergency procedures, preparing to evacuate staff and others present at the facility in the event that the supercell should turn right, placing areas surrounding the Norman campus in the tornado's path (under NOAA protocol in situations posing a danger to personnel at local Weather Forecast Offices and related guidance centers, responsibility over the issuance of warnings and statements on the unfolding outbreak would have been transferred to the nearest NWS Forecast Office, based in Tulsa, while the SPC's forecasting responsibilities would be turned over to the 557th Weather Wing at Offutt Air Force Base). The supercell, however, continued on a northeastward track, sparing the Norman area.[26]

Safety precautions were also enacted elsewhere in and near the storm's path; council members and citizens at Moore City Hall − where a council meeting was scheduled to be held that evening − sheltered in place in the building's first-floor restrooms, away from the multiple large-pane windows at its facade. In downtown Oklahoma City, spectators attending sporting events held that evening involving two of the city's minor league teams – a regular season Pacific Coast League baseball game between the Oklahoma RedHawks and the Memphis Redbirds (which was suspended during the second inning) and Game 2 of the Ray Miron President's Cup series between the Central Hockey League's Oklahoma City Blazers and Huntsville Channel Cats – were also evacuated to shelter in an underground storage area connected to the Southwestern Bell Bricktown Ballpark and Myriad Convention Center amid concerns that the storm would jog northward and place Oklahoma City itself in the tornado's path. Flights were grounded at Will Rogers World Airport as the northern edge of the supercell (containing hail up to 1.25 inches (3.2 cm), straight-line wind gusts up to 70 mph (110 km/h), and moderate to heavy rain) approached the area; the tornado turned right, away from southwestern parts of the city proper located within Oklahoma County, shortly before airport officials began evacuating employees and visitors at the terminals. Traffic on Interstate 35 in south Oklahoma City and north Moore became backed up for several miles, as drivers evacuated from their vehicles to seek shelter under an overpass overlooking South Shields Boulevard.[26]

Just outside the Eastlake Estates, an honors ceremony was being held at Westmoore High School at the time of the tornado. Adequate warning time allowed those at the school to seek shelter, however, and more than 400 adults and students attending the awards ceremony at the school's auditorium were moved to the main building, sheltering in reinforced hallways and bathrooms. Ultimately, Westmoore High sustained heavy damage and dozens of cars that were in the school's parking lot were tossed around, some of which were completely destroyed or thrown into nearby homes. No injuries took place at the school, though a horse was found dead between a couple of destroyed cars in this area. The tornado proceeded through additional densely populated areas of Moore shortly thereafter, where several large groups of homes were flattened in residential areas, with a mixture of high-end F4 and low-end F5 damage noted in the survey. Near Janeway Avenue, four people were killed in an area where multiple homes were completely destroyed. A woman, who took shelter with her husband and two children, was also killed when she was blown out from under the Shields overpass on Interstate 35. The tornado weakened somewhat as it moved through the Highland Park neighborhood of Moore, but still caused widespread F3 and F4 damage.[7][26][27]

An American flag blows in the wind next to the remains of a home destroyed by the tornado.

The tornado entered Oklahoma County and struck the southeast fringes of Oklahoma City, where it re-intensified to high-end F4 strength; two people were killed in this area as a building housing a trucking company was completely destroyed. Shortly before it tracked into the county, patrons and employees at Crossroads Mall were evacuated to storage areas in the basement of the building.[26] Numerous industrial buildings were leveled in this area of the city. A freight railroad car, weighing 36,000 lb (16,000 kg) was thrown 0.75 mi (1.21 km). The car bounced as it traveled, remaining airborne for 50 to 100 yd (46 to 91 m) at a time. Multiple homes were also completely destroyed in southeast Oklahoma City, and one woman was killed in that area. Crossing Southeast 44th Street into Del City, the tornado moved through the highly populated Del Aire housing addition, killing six people and damaging or destroying hundreds of homes, with many sustaining F3 to F4 damage.[7] Seven people were killed as a direct result of the tornado in Del City, and hundreds of homes were damaged or destroyed.[28]

The tornado then crossed Sooner Road, and subsequently damaged an entry gate and several buildings at Tinker Air Force Base; it then crossed 29th Street into Midwest City, destroying one building at the Boeing Complex and damaging two others. Widespread F3/F4 damage continued as the tornado moved across Interstate 40, affecting a large business district. Approximately 800 vehicles at Hudiburg Auto Group were damaged, located just south of Interstate 40. Hundreds of vehicles at the dealership were moved from their original location on the lot, and dozens of vehicles (including 30 awaiting tune-ups or repairs at Morris' Auto Machine and Supply, and an unoccupied Mid-Del School District bus) were picked up and tossed northward across the interstate into several motels, being carried a distance of approximately two-tenths of a mile. Numerous motels and other businesses, including Hampton Inn, Comfort Inn, Inn Suites, Clarion Inn, Cracker Barrel, and portions of Rose State College, were destroyed. While some of the damage through this area was rated high-end F4, low-end F5 was considered. The tornado then continued into another residential area located between Southeast 15th and Reno Avenue, where three fatalities occurred. Damage consistent with high-end F4 wind speeds was inflicted to four homes in this area. Two of these homes were located between Southeast 11th and 12th Streets, near Buena Vista, and the other two homes were located on Will Rogers Road, just south of Southeast 15th. Damage then diminished rapidly to F0/F1 strength as the tornado crossed Reno Avenue, before dissipating three blocks north of Reno, between Sooner Road and Air Depot Boulevard (south of the Midwest City–Oklahoma City line). Throughout Oklahoma County, twelve people were killed and 234 others were injured while losses amounted to $450 million.[7][26][27]

Aerial view of one of the overpasses under which one person was killed by the tornado.

From a meteorological and safety standpoint, the tornado also brought into question the use of highway overpasses as shelters. Prior to the events on May 3, 1999, videos of people taking shelter in overpasses during tornadoes in the past – most notably one filmed near Wichita, Kansas, during the April 26, 1991 tornado outbreak involving a television news crew from Wichita NBC affiliate KSNW (channel 3), who decided to shelter under an overpass on the Kansas Turnpike alongside other bystanders who found themselves in the path of the tornado – gave the public misunderstanding that overpasses provided shelter from tornadoes. For nearly twenty years, meteorologists had questioned the safety of these structures; however, they lacked incidents involving loss of life. In the aforementioned 1991 footage, the subjects were able to safely ride out the tornado due to an unlikely combination of events: the storm in question was a weak tornado, the tornado did not directly strike the overpass, and the overpass itself was of a unique design.[54]

Indeed, the May 1999 tornado outbreak proved concerns that highway overpasses were dangerous places to seek shelter during a tornado, as three overpasses were directly struck by tornadoes, with a fatality taking place at each one. Two of these were from the F5 Bridge Creek–Moore tornado while the third was from a comparatively less intense F2, which struck a rural area in Payne County, north-northeast of Oklahoma City; many other serious to life-threatening injuries also occurred at these locations.[55] The casualties at the three overpasses are attributed to the Venturi effect, as tornadic winds were accelerated in the confined space of each of the overpasses that the tornadoes passed through, increasing the chances that those riding out the tornado would be blown out at high speeds even if they tried to anchor themselves to the girders. According to a study by the National Oceanic and Atmospheric Administration (NOAA), seeking shelter in an overpass "is to become a stationary target for flying debris."[56]

Preliminary damage surveys conducted by a group of structural engineers from Texas Tech University determined that many of the frame homes that were destroyed by the Bridge Creek−Moore tornado were constructed below minimal residential building code standards, discovering some structural deficiencies that violated codes, which were considered to be inadequate for regions prone to tornadic activity (under federal building code standards, frame homes that were properly strapped and bolted would have withstood winds between 152 and 157 miles per hour (245 and 253 kilometers per hour), equivalent to an F2 tornado). The team, led by meteorological researcher Charles Doswell and storm damage engineer/meteorologist Tim Marshall, determined that nails attached to a plywood roof deck in one damaged home were not properly anchored to the rafters; several homes in rural areas that were swept nearly 300 feet (91 m) from their original location did not have anchor bolts that secured the frame to their foundations, as was the case at Country Place Estates, where the homes − which left a trail of debris strewn 3,000 feet (910 m) away from their location − were attached to the concrete foundations by tapered cut nails that extended only a half-inch to the bases; many homes that were left at least partially standing also had their garage doors (mainly those made from aluminum material) collapse inward, allowing the tornado's destructive winds to enter the houses.[30]

Marshall discovered other building and vehicle remains that became debris missiles, including a twisted 36-inch (0.91 m) steel beam, a steel leg broken off of a lawn chair that was impaled into a 5-by-5-inch (13 cm × 13 cm) post by the violent winds and a six-foot (180 cm) section of a sewer pipe that was blown into the interior hallway of one house through the front door. The team's findings also revealed that several homes were obliterated before they experienced the full impact of the vortex's peak wind velocities, with some disintegrating as the external winds surrounding the parent tornado reached speeds of F2 intensity. Three months later, as homes were being built in the damage path, Marshall found their construction to be scarcely superior to that of the homes destroyed in the May 3 storm.[30]

The FEMA corroborated with Doswell and Marshall's findings in its Building Performance Assessment Team Report on the May 3 outbreak, noting that much of the structural damage resulted from strong winds generated by the tornado and associated windborne debris that often "produced forces on buildings not designed to withstand such forces" and in some cases, were due to improper construction techniques and "poor selection" of materials used in their construction. The report acknowledged that federal construction code requirements needed to be revised above the then-current minimum standards to allow newer buildings to better withstand higher wind speeds consistent with tornadoes of lesser intensity than the one that devastated Bridge Creek and Moore, thereby lessening the degree of damage, fatalities and injuries that are probable in buildings of typically less reinforced construction.[57][58]