A prolonged and widespread tornado outbreak affected a large portion of the United States in the final days of May 2013. The outbreak was the result of a slow-moving but powerful storm system that produced several strong tornadoes across the Great Plains states, especially in Kansas and Oklahoma. Other strong tornadoes caused severe damage in Nebraska, Missouri, Illinois, and Michigan. The outbreak extended as far east as Upstate New York. 27 fatalities were reported in total, with nine resulting from tornadoes (eight in Oklahoma and one in Arkansas).
By far the most significant tornado of the outbreak was an extremely large EF3 tornado that struck areas near the town of El Reno, Oklahoma on May 31. With a maximum width of , it was the largest tornado on record. The second-highest wind speeds recorded by a Doppler radar, up to , were also observed in this tornado. The twister was responsible for eight deaths, including famous storm chaser Tim Samaras, his son Paul, and Carl Young as a result of being caught off-guard by the tornado's unprecedented width. In addition, the tornado caused 151 injures.
Meteorological synopsis On May 22, an upper-level low moved eastward over the Western United States, with forward progression limited by a blocking high around the Northern Plains and Great Lakes. With ample low-level moisture streaming north ahead of the low into an area of moderate instability, scattered severe weather was anticipated over parts of Colorado, Kansas, and Nebraska starting on May 24.
May 26–28 The first tornado associated with the system was a small landspout tornado that touched down in rural Idaho on May 26. The tornadic activity became more intense the following day. On May 27, while only scattered tornadoes touched down, four of them were strong and caused considerable damage in parts of Kansas and Nebraska. One tornado near Lebanon, Kansas reached EF3 intensity, and an EF2 near Marysville caused major damage to several structures. In Nebraska, the town of Edgar took a direct hit from an EF2.
Vigorous tornado activity continued on May 28. While the Storm Prediction Center only issued a slight risk that day, widespread tornadoes touched down in several states, some strong to violent. Several powerful supercell thunderstorms developed in central Kansas that afternoon, and large tornadoes were reported. Near the town of Corning, a strong EF3 carved an unusual V-shaped path through rural areas west of town, and completely destroyed two homes. An even larger tornado, an EF3 wedge that was over a quarter mile wide, touched down just west of Bennington, and remained nearly stationary in a field for over an hour. This tornado resulted in relatively moderate damage, but killed over 100 head of livestock. The tornado was originally rated EF4 based on mobile radar readings, but ground surveys did not reveal damage indicative of EF4 wind speeds, and the tornado was downgraded to an EF3 as a result. Later that evening, large tornadoes were sighted in southern Michigan, prompting several tornado emergencies. A total of six tornadoes were confirmed to have touched down in areas near Flint, two of which reached EF2 intensity and caused significant damage. Numerous weak tornadoes were documented in other states.
May 29–30 On May 29, the Storm Prediction Center issued a moderate risk for parts of Texas, Oklahoma, and Kansas. The outlook included a 10% hatched risk for tornadoes. A separate slight risk was issued for parts of New England. In the main risk area, only scattered weak tornadoes touched down, though a mile-wide EF2 wedge tornado touched down in upstate New York near Schenectady. On May 30, for the second day in a row, the Storm Prediction Center issued a moderate risk with a 10% hatched risk of tornadoes, this time for eastern Oklahoma and a small part of Kansas. Numerous tornadoes touched down in Oklahoma and Arkansas, including a few that were strong. An EF2 ripped through the Tulsa suburb of Broken Arrow, resulting in severe damage to homes and businesses. A brief EF1 near Tull, Arkansas unfortunately caused a fatality due to a falling tree.
May 31
On the morning of May 31, a strong cap was in place over much of central Oklahoma into eastern Oklahoma. With dewpoints creeping into the low 70s, the Storm Prediction Center early on considered upgrading part of the moderate risk forecast zone to a high risk forecast zone. As the day went on, there was no upgrade, but it became evident the atmosphere was ripe with CAPE values in excess of 4000 J/kg. With the presence of an extremely unstable air mass over central Oklahoma, the Storm Prediction Center issued a Particularly Dangerous Situation Tornado Watch for much of the state during the afternoon of May 31 and lasting into the night. Later that evening, severe storms rapidly developed, and an extremely large and violent multiple-vortex tornado devastated areas near El Reno, Oklahoma, and killed eight people including three men from the storm chasing crew, TWISTEX. The tornado had been rated EF5 based on mobile radar readings, Other weak tornadoes and major flooding devastated parts of the Oklahoma City metro that evening. Further north, a powerful squall line of severe storms with several embedded strong tornadoes developed in Missouri. One of the tornadoes, a large and rain-wrapped EF3, caused major damage in several St. Louis suburbs. Weak tornadoes touched down in several other states.
Confirmed tornadoes
Bennington, Kansas
This large, very slow-moving, and erratic tornado remained on the ground for just over an hour as it executed a cyclonic loop in Ottawa County south of the town of Minneapolis and west of Bennington. It initially touched down several miles west of Bennington on May 28 at 5:45 p.m. CDT (2239 UTC). Upon formation, it snapped power poles and moved southeast before turning north as it approached U.S. Route 81. As the tornado reached the northern end of the track (its strongest point), it swept away a machinery shed, with one combine harvester stored inside being thrown and torn apart. Trees and shrubs were completely debarked. Ground scouring occurred and a large semi-like truck was moved several feet and tipped over. About 1000 cattle were killed in this area as well. The tornado then moved to the southwest and destroyed the roof of a well constructed shed and throwing the heavy metal shed door into a field, blew a trailer , snapped many more power poles, and downed numerous trees. A home that was under construction lost all four exterior walls and a fifth-wheel camper was blown . A second home suffered minor shingle damage and impact dents in the siding from flying debris, a few more sheds/outbuildings were either damaged or destroyed, and a third home had siding and chimney damage (mostly from debris impact) and was slightly moved off of the foundation. The tornado lifted around 6:45 p.m. CDT (2345 UTC), just to the south-southwest of where it touched down.
Initially, the tornado was rated as an EF4, with the rating based on Doppler on Wheels surface wind measurements, which indicated a far larger and stronger tornado. Gusts were measured far into the EF5 intensity range, with a peak value of ; however, those winds were measured above the surface. However, DOW windspeed measurements were found to be almost 264 mph near the core (47m above ground level). Based on damage indicators and the surface wind measurements, the highest winds that translated to the ground were around . Because mobile doppler radar data is not a method by which tornadoes are rated, it was later officially downgraded to an EF3. Despite the downgrade, the National Climatic Data Center maintains that the Bennington tornado was violent, and likely reached EF4 intensity at some point in its life. The wind gusts above the surface were among the highest ever measured/estimated on record, comparable to the highest non-tornadic gust of measured during Cyclone Olivia in 1996. Some damage was observed from areas that were not directly in the path of the tornado, suggesting a very large wind field as well. Winds of were also measured over an area approximately wide while winds covered an area in diameter. Due to the tornado's slow motion, several areas in its path were likely exposed to winds well in excess of for at least 45 minutes.
El Reno, Oklahoma
This erratic, powerful and enormous tornado touched down from a large and low-based wall cloud about west-southwest of El Reno at 6:03 p.m. CDT (2303 UTC). The tornado rapidly grew and became violent. Remaining over mostly open terrain, it did not impact many structures; however, measurements from mobile Doppler radars revealed extreme winds in excess of within the tornado. As it crossed US 81, it had grown to a record-breaking width of , larger than any other tornado in recorded history. After crossing Interstate 40, the tornado dissipated around 6:43 p.m. CDT (2343 UTC) after tracking for . An EF2 anticyclonic satellite tornado also occurred and such companion tornadoes tend to be observed with especially large and intense tornadoes.
Although the tornado remained over mostly open terrain, dozens of storm chasers unaware of its immense size were caught off-guard. Along US 81, renowned chaser and researcher Tim Samaras, along with his son Paul and research partner Carl Young, were killed when their vehicle was tossed by the tornado or a sub-vortex associated with it. Another amateur storm chaser was killed in the area. Other storm chasers, including The Weather Channel's Mike Bettes as well as Reed Timmer, were either injured or had their vehicles damaged. Overall, the tornado was responsible for eight fatalities and an unknown number of injuries.
Weldon Spring–Northern St. Louis County, Missouri
This large rain-wrapped tornado touched down at 7:50 p.m. CDT (0050 UTC) on May 31, southwest of Weldon Spring in St. Charles County, Missouri. It first produced EF1 damage as it downed trees, blew over empty trailers, and caused minor damage to construction equipment. It moved generally east-northeast into Weldon Spring Heights (on the south side of Weldon Spring) and caused roof, window, and solar panel damage to a few structures as well as downing more trees before crossing Interstate 64. The tornado continued moving east-northeast through densely populated residential areas of southern St. Charles County. Roofing was torn off of a middle school and blown , many houses suffered varying degrees of roof (from minor damage to removal of entire roof), wall (from minor damage to total collapse), and overall structural damage, and hundreds of trees were downed. Damage in this area ranged from EF1 to EF2. A shed was destroyed, a garage door was compromised and the roof was removed and blown away, another garage had a wall collapsed, and two more garages were completely destroyed. The tornado reached its maximum intensity to the east-northeast of Weldon Spring, west-southwest of St. Charles and Harvester, and south of St. Peters. Three adjacent frame homes at this location sustained total loss of their roofs and exterior walls, and this small pocket of damage was rated EF3. The tornado continued causing damage in residential areas further to the east and northeast, snapping many trees and causing roof and wall damage to many homes. A few homes had their roofs entirely ripped off, and damage along this portion of the path was rated EF1 to EF2. Crossing the Missouri River, the tornado moved into St. Louis County and Earth City, Bridgeton, and the northern side of Maryland Heights as it moved along Interstate 70 near its intersection with Interstate 270.