A significant and destructive tornado outbreak affected parts of the Midwestern United States and lower Great Plains in mid-May 2013. This event occurred just days after a deadly outbreak struck Texas and surrounding southern states on May 15. On May 16, a slow moving trough crossed the Rockies and traversed the western Great Plains. Initially, activity was limited to scattered severe storms; however, by May 18, the threat for organized severe thunderstorms and tornadoes greatly increased. A few tornadoes touched down that day in Kansas and Nebraska, including an EF4 tornado near Rozel, Kansas. Maintaining its slow eastward movement, the system produced another round of severe weather nearby. Activity significantly increased on May 19, with tornadoes confirmed in Oklahoma, Kansas, Iowa, Missouri, and Illinois. In Oklahoma, two strong tornadoes, one rated EF4, caused significant damage in rural areas of the eastern Oklahoma City metropolitan area; two people lost their lives near Shawnee. The worst event unfolded on May 20 as a large EF5 tornado devastated parts of Moore, Oklahoma, killing 24 people. Thousands of structures were destroyed, with many being completely flattened. Several other tornadoes occurred during the day in areas further eastward, though the majority were weak and caused little damage.
The severe threat shifted towards damaging straight-line winds on May 21 as a large squall line developed across the southern states. Further north in Ontario, three tornadoes, including an EF2, touched down. Over the following five days, the system responsible for the outbreak moved very little across the Eastern United States. By May 24, it virtually stalled off the coast of New England, resulting in several days of heavy rain across the region. It later moved northeastward and was last noted by the Weather Prediction Center late on May 26 over southeastern Canada. During the storm's eleven-day trek across the United States, it produced 75 tornadoes, 4 of which were rated EF3 or higher. Over 1,000 reports of damaging winds were received by the Storm Prediction Center as well. The system's slow movement also resulted in record-breaking rainfall in North Dakota, New York and Vermont. Additionally, during the overnight of May 25–26, unusually cold air behind the cyclone resulting in record-late snowfall across northern New England.
Overall, severe weather associated with the storm was responsible for 30 fatalities (26 from tornadoes) and over 200 injuries in the United States. Additionally, damage was estimated to amount to $2–5 billion, making it one of the costliest severe weather events on record.
Meteorological synopsis The cause of the outbreak was a large, slow-moving trough, an associated cold front, and several shortwave troughs that triggered severe weather from the Rockies eastward to New England and from Canada southward to the Gulf Coast of the United States. The system impacted the United States and Canada from May 16–26 and produced tornadoes on five of the eleven days. During this nearly two-week-long event, the Storm Prediction Center (SPC) received 1,967 reports of severe weather: 101 for tornadoes, 1,176 for wind, and 690 for hail. Some of these reports were associated with a separate system over the Southeastern United States on May 19–20.
The first indications of a possible tornado outbreak came on May 13 as weather forecast models began depicting a multi-day severe weather event across the Central United States. On May 16, a shortwave trough, along with a surface front emerged from the Rockies into the High Plains. Only scattered thunderstorms accompanied the system, with few reaching severe levels. Only 13 reports of large hail and damaging winds in Colorado, Kansas, and Nebraska, were received by the SPC. On May 17, a slow-moving mid- to upper-level trough in the same region brought a further risk of thunderstorms. Significant low-level moisture streamed northward from Texas into the Dakotas ahead of the system. This increased dew points in the area into the range; orographic lift and up-slope flow was expected to trigger the development of storms. The SPC issued a small slight risk for severe thunderstorms around the Black Hills into central Nebraska, with the main threat being large hail and the possibility of an isolated tornado. A second slight risk area was issued over central Texas as the northward flow collided with a dry line in the area. Though there was notable atmospheric capping, convective available potential energy (CAPE) values of 2,500–3,000 J/kg led to the potential of localized supercells capable of producing very large hail and downbursts. Additionally, a stationary outflow boundary over southern Minnesota and northern Iowa encountered moderate instability and produced scattered severe storms, some of which were capable of producing tornadoes. Two tornadoes were confirmed in Young County, Texas: one EF1 and one EF0 (these tornadoes were only partially associated with the main system). Throughout the day, the SPC received a total of 103 reports of severe weather in association with the system: 10 for tornadoes, 21 for wind, and 86 for hail. The tornadoes were not fully associated with the main system.
May 18
On May 17, the SPC issued a moderate risk of severe weather for northwestern Kansas and southwestern Nebraska for the following day. This was later extended southward into western Oklahoma. On May 18, the trough continued to slowly move from the Rockies into the Great Plains region. Extreme instability ahead of it, with CAPE values reaching 4,000–5,000 J/kg along with strong diabatic heating, was anticipated to spark widespread severe thunderstorms and some supercells. The main threat on May 18 was large hail and straight-line winds, with a lesser concern for tornadoes. The first tornado watch was issued at 2:45 p.m. CDT (1945 UTC) for much of western Nebraska, Kansas, and extreme northeast Colorado as moist northerly flow ahead of the trough met up with a dry line moving into the region. Tornadic activity was expected to be relatively limited at first, though a strengthening low-level jet along with increasing relative humidity along the boundary layer would provide more favorable conditions for development.
During the evening of May 18, tornadoes touched down in parts of Kansas and Nebraska. However, most remained in open country, causing little damage. Around 5:30 p.m. CDT (2230 UTC), a supercell thunderstorm developed to the northeast of Dodge City. At 7:18 p.m. CDT (0018 UTC), it spawned a large EF4 tornado near Rozel, Kansas; one home was largely destroyed and five farms were damaged. Around the time it reached EF4 intensity, the tornado took a sharp northward turn as it passed within of Rozel. Aaron Johnson of the National Weather Service in Dodge City stated that had it not made the turn, Rozel would have likely been destroyed. A second tornado briefly touched down around 7:50 p.m. CDT (0050 UTC). By 8:30 p.m. CDT (0130 UTC), a squall line began developing over south-central Nebraska and central Kansas. Several corridors of intense winds, possibly associated with brief tornadoes, were observed in Clay and Washington counties in Kansas. Several homes were damaged or destroyed by the winds. Near Milford Lake, a non-tornadic gust of was measured; two people were injured near the lake after their camper tipped over. Later on, strong southerly flow and isentropic lift intersected a stationary front over northern South Dakota, southern North Dakota, and western Minnesota, allowing for scattered severe storms to develop. One of these storms produced gusts up to to the east of Okreek, South Dakota. Throughout the day, the SPC received a total of 231 reports of severe weather: 24 for tornadoes, 92 for wind, and 115 for hail.
May 19
On May 19, the trough moved only slightly eastward, with many of the same areas impacted on May 18 being threatened again. During the day, two areas of low pressure developed along the trough: one over the north-central plains and the other over Oklahoma. A large plume of steep mid-level lapse rates accompanied by deep moisture (with dew points in some areas exceeding ) spread across much of the central and southern plains into the Mississippi River Valley. Thunderstorms in these areas were expected to develop early, though not be particularly widespread or damaging. The most intense storms were anticipated over the southern Missouri River Valley where strong daytime heating and "aggressive destabilization" would take place. Additionally, a diurnally strong jet and strong wind shear favored the development of long-lived supercells capable of producing large hail and tornadoes, some potentially violent. Activity was expected to become associated with a squall line by the evening as the cold front moved slowly eastward. In light of that, the SPC issued a moderate risk of severe thunderstorms for south-central Oklahoma, eastern Kansas, western Missouri, and extreme southeastern Nebraska.
By the afternoon hours, rapid destabilization of the boundary layer began across eastern Kansas, prompting a tornado watch for the area. Further south in Oklahoma, a rapidly mixing dry line accompanied by a deepening surface low moved eastward into an area with deep moisture. Though leftover outflow boundaries from the previous day's storms and capping limited initial convective activity, strong daytime heating would soon allow storms to break the cap. A tornado watch was issued for the central third of Oklahoma at 2:15 p.m. CDT (1915 UTC), with the SPC noting the risk for potentially long-lived supercells and a few strong tornadoes. Around 2:30 p.m. CDT (1930 UTC), a supercell developed over Harper County, Kansas and tracked northeastward. This cell produced three short-lived tornadoes, including an EF2 to the southwest of Wichita that prompted a tornado emergency for the city. Over the course of three hours, 10 other tornadoes touched down across the state, none of which exceeded EF1 intensity.
By 4:30 p.m. CDT (2130 UTC), the risk for significant tornadoes markedly increased over central Oklahoma, especially around the Oklahoma City metropolitan area. Maximized pressure falls ahead of the dry line allowed several storms to acquire intense updrafts and develop into rotating supercells within the state. Very strong low-level mesocyclones were expected to develop with these storms, with several tornadoes touching down. Around this time, tornadoes began touching down around the city. At 4:41 p.m. CDT (2141 UTC), a large EF3 tornado developed northwest of Luther and tracked for roughly . To the southeast, a violent EF4 tornado formed around 6:00 p.m. CDT (2300 UTC) over Cleveland County. That storm was the strongest of the day and resulted in significant damage, especially near the city of Shawnee where two people were killed. Two other tornadoes touched down during the evening in eastern Oklahoma: one EF2 and one EF1. In addition to the two fatalities, 39 people were injured across the state.