During the late-afternoon hours of May 24, 2011, a large, long-tracked and exceptionally violent EF5 tornado, commonly known as the El Reno–Piedmont tornado, impacted areas near or within the communities of El Reno, Piedmont, and Guthrie, killing nine people and injuring 181 others. After producing incredible damage in several locations along a path of more than , the tornado was given a rating of EF5 on the Enhanced Fujita scale, with peak wind speeds in excess of , although a mobile Doppler radar found that the tornado possessed wind speeds of up to . The tornado was the first F5/EF5 tornado to occur in Oklahoma since May 3, 1999, when an F5 tornado devastated areas in and around the Oklahoma City metropolitan area.
The tornado touched down in southwestern Canadian County and quickly became violent, debarking numerous trees as it passed through areas several miles southwest of Calumet. As it approached and crossed I-40 west of El Reno, it reached its maximum intensity. A nearby oil tanker truck that was parked at an oil production site near the interstate was thrown approximately into a wooded gully. A farmhouse was destroyed along I-40. At the nearby Cactus-117 oil rig site, a oil derrick was blown over and rolled three times. The tornado remained very violent as it passed north of El Reno and continued northeast, producing EF3 to EF4 damage in rural areas. The tornado then re-intensified and passed northwest of Piedmont at high-end EF4 intensity, leveling multiple homes and causing additional fatalities. Moving into Kingfisher County and Logan County south of Cashion, the tornado fluctuated several times between EF2 and EF3 intensity, causing varying degrees of damage. Afterwards, the tornado then rapidly weakened, causing EF0 to EF1 damage along the north side of Guthrie before dissipating.
2011 was a prolific year for tornadoes and tornado-associated fatalities, with multiple destructive outbreaks. The El Reno–Piedmont tornado occurred during an outbreak across Oklahoma and the Great Plains that produced multiple strong to violent tornadoes near the Oklahoma City metropolitan area on May 24, and was itself part of a tornado outbreak sequence spanning from May 21–26. The Oklahoma storms came just two days after a devastating EF5 tornado struck Joplin, Missouri, which killed 158 people and became the costliest tornado in U.S. history.
Additionally, the city of El Reno had and has infamously been the site of other intense tornadoes. On May 31, 2013, a tornado just south of the town became the largest ever recorded, with a width of and radar-indicated wind speeds in excess of . The massive multiple-vortex tornado killed eight people, including three storm chasers, and received a damage rating of EF3. In the late evening hours of May 25, 2019, a brief low-end EF3 tornado that spawned from an intense squall line struck just southeast of El Reno, killing two people and injuring dozens of others.
Meteorological synopsis
Setup Early on May 24, a strong upper-level trough (an elongated region of low atmospheric pressure aloft) advanced towards the Great Plains out of the southwestern United States and took on a negative tilt, becoming oriented northwest to southeast.
At the same time, southerly flow brought moisture north over Texas and the southern Great Plains, allowing dew points in Central Oklahoma to reach . This moisture, with temperatures in the mid range, allowed for ample convective available potential energy (or CAPE, a measure of atmospheric instability); values reached 2500–4000 J/kg. Mid-level lapse rates were nearly dry adiabatic.
In the late morning, a shortwave embedded within the main longwave trough advanced more rapidly, pushing the dryline into western Oklahoma, where it met the already-present moisture. The shortwave's advance also brought strong wind shear and "incredibly high" storm-relative helicity values of more than 500 m2s−2. The convergence of all these factors promised the development of intense convective thunderstorms.
Forecast
This risk was anticipated by the National Weather Service's Storm Prediction Center (SPC), and its local forecast office in Norman, Oklahoma. The Storm Prediction Center's outlooks for severe weather culminated in a high risk area being delineated over the Great Plains for May 24. Issued at 11:25 a.m. CDT, the Storm Prediction Center's convective outlook for the day highlighted the tornado risk, which included central Oklahoma inside a large region of a 45% chance of a tornado touchdown within of any given point, and a 10% or greater chance of a significant (EF2+) tornado within that same radius.
At 12:50 p.m., the Storm Prediction Center issued a particularly dangerous situation (PDS) tornado watch, to remain in effect until 10:00 p.m., for most of central Oklahoma extending from the state border with Kansas down through the Oklahoma City metropolitan area and into northern Texas. The text of the tornado watch again warned of the possible development of "destructive tornadoes... ...some of which could be long-tracked and strong to violent."
Initiation Thunderstorms began to develop before 2:00 p.m. in a north–south oriented line just east of the dryline where the capping inversion was weakest, including near Altus and Lawton in southwest Oklahoma. The tightly spaced storm cells rapidly developed classic supercell characteristics as they moved northeast. The storm that produced the El Reno–Piedmont tornado formed approximately west-southwest of Oklahoma City.
Tornado summary Background The supercell that eventually generated the El Reno–Piedmont tornado first produced a tornado in Caddo County, Oklahoma, which tracked from west of the town of Lookeba to just northeast of it. The Lookeba tornado developed at 3:31 p.m. and persisted for approximately 16 minutes, traveling and destroying multiple structures. The tornado earned a damage rating of EF3. As the Lookeba tornado moved northeast towards the Caddo/Canadian county border, between 3:40 and 3:42 p.m. the supercell's mesocyclone broadened and weakened slightly; this was followed by the development of a second mesocyclone from the original to the east-southeast between 3:42 and 3:44 p.m. This marked the start of the decay of the Lookeba tornado, which fully dissipated by 3:47 p.m. Over the next several minutes the original western mesocyclone dwindled as the newer mesocyclone coalesced, and the radar detection of a bounded weak echo region within the storm indicated that its updraft intensified during this period.
Formation, peak intensity and Interstate 40
The El Reno–Piedmont tornado formed in western Canadian County at 3:51 p.m., as determined by mobile radar data. Over the next four minutes, the tornado's condensation funnel extending from a low-hanging wall cloud broadened, causing the tornado to take on a large 'wedge' appearance. It intensified quickly as it moved northeast, debarking and destroying many trees at EF5 intensity. During this period, the storm was being monitored by a truck-mounted Rapid-Scan X-band Polarimetric (RaXPol) mobile Doppler weather radar, operated by the University of Oklahoma's Advanced Radar Research Center (ARRC) led by Howard Bluestein. That radar, stationed near the intersection of Smith Road and Walbaum Road less than south of I-40, captured the "first polarimetric, rapid-scan, mobile Doppler weather radar dataset of an EF-5 tornado."
Radar analysis
As the tornado moved towards I-40 to the southeast of the RaXPol radar and was peaking in intensity, it detected some of the fastest wind speeds ever measured on the planet. Interpretations slightly differ: the maximum instantaneous radial velocity sampled by the radar was originally reported as having been , measured above the ground at 4:00:26 p.m.; however, the maximum velocity was later reported as having been measured ~ "above radar level" at 4:00:39 p.m. in a 2014 paper by Bluestein et al. on the use of radar data for tornado ratings. Maximum radial velocities were also reported to have remained "greater than for several minutes." Additionally, multiple consecutive radar scans were averaged to yield an estimated 2-second average radial velocity of and an estimated 4-second average velocity of . This was reported as "likely to be an underestimate of the true 2- and 4-s average wind speeds."
The instantaneous velocity readings taken are not directly equivalent to the three-second gust at that the Enhanced Fujita scale attempts to estimate, but they mark the second-highest wind speed ever recorded in a tornado, after wind speeds of approximately were recorded in both the 1999 Bridge Creek–Moore tornado and a sub-vortex within the 2013 El Reno tornado. Where the most intense winds are generally present in a tornado is an unresolved question, but the limited existing research suggests that wind speeds are likely to be highest closer to the ground. After the detection of the wind speeds, the quality of the data degraded until collection ceased altogether at 4:16 p.m., as the tornado turned to the east-northeast and towards I-40 where it would produce its most intense damage.
El Reno outskirts When the tornado crossed I-40, the RaXPol radar away was still recording maximum radial velocities over , above the ground. There, the tornado struck multiple people in their cars at EF3 strength. Three people—Terry Peoples, 50; Don Wesley Krug, 71; and Joan Krug, 67—were killed in two separate full-size pickup trucks, which were hurled thousands of feet from the road. Their bodies were found more than north of the interstate, outside their vehicles, stripped of clothing, and rendered "unrecognizable", according to responding state troopers. Several others were injured here as their vehicles were battered and overturned, including a truck driver whose semi truck was flipped. The interstate was left littered with pieces of cars. Two more fatalities in cars occurred just northeast of the interstate. According to the National Weather Service, the tornado is believed to have caused its maximum amount of damage just after crossing I-40.
Just after crossing US 270, the tornado struck a complex of buildings (including a scrap yard, auto repair shop, garage, and grain storage facility) at EF3 intensity. The repair shop, the garage, and a farmhouse were destroyed, and the grain facility was "damaged beyond repair". Flying debris from the salvage yard impacted a new natural gas processing plant operated by Devon Energy, but all employees present avoided injury by sheltering on-site in time. The damage caused a major gas leak at the plant, which was not secured until nightfall. The tornado then quickly reached EF5 intensity as it struck the Cactus 117 oil drilling rig site at the corner of the Old 66 and North Courtney Road intersection, completely destroying it. When it hit, the rig's pipes and drill head were inserted deep in the well's borehole, which provided the drilling pipe with of downforce. Despite this, and despite the fact that the drilling rig weighed 862 metric tons—or almost two million pounds—the rig was toppled onto its side and rolled several times. The well's blowout preventer was left bent at a 30-degree angle to the north. Elsewhere on the site, vehicles and cargo containers were lofted into the air and tossed.