Tornado outbreak of May 18–21, 2013

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 50 °F (10 °C) 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 1 mile (1.6 km) 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 100 mph (160 km/h) 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 85 mph (137 km/h) 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 70 °F (21 °C)) 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 3 mi (4.8 km) northwest of Luther and tracked for roughly 20 mi (32 km). 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.

Storm coverage over eastern Kansas soon increased as well, with the main threat being damaging winds. Further north over parts of Iowa and Minnesota, a mid-level vortex over the region accompanied by scattered thunderstorms presented a risk of severe storms. Increasing wind shear and an influx of moist air from the south was expected to allow convection to persist over the region for much of the evening, with the main threat being damaging winds and a few tornadoes. By the evening hours, several squall lines developed across the region. Unstable air over much of Missouri allowed for these storms to maintain themselves as they moved slowly eastward. Around 7:00 p.m. CDT (0000 UTC), a rapidly moving vorticity maximum over Iowa resulted in a significant increase in thunderstorms over the area, some of which were tornadic. The most intense cell tracked from Dallas County to Story County, producing three brief tornadoes and straight-line winds along its path. The touchdowns in Iowa marked the end of a record 358-day span with no tornadoes in the state.

Severe activity began decreasing by 10:00 p.m. CDT (0300 UTC) as the main squall line over Missouri entered an area of increased capping. A secondary line, extending northward into Wisconsin, behind it with bowing segments, indicative of locally intense winds, produced additional damaging winds, though it gradually weakened during the overnight of May 19–20. Throughout the day, the SPC received a total of 558 reports of severe weather: 34 for tornadoes, 325 for wind, and 199 for hail; however, some of the wind and hail reports were associated with a separate system over the Southeastern United States.

May 20

On May 20, a prominent central upper trough moved eastward with a lead upper low pivoting over the Dakotas and Upper Midwest. A Southern stream shortwave trough/moderately strong polar jet moved east-northeastward over the southern Rockies to the southern Great Plains and Ozarks area, with severe thunderstorms likely with peak heating. The Storm Prediction Center issued a moderate risk of severe thunderstorms during the early morning hours of May 20 from southeastern Missouri to north-central Texas, for the possibility of isolated strong tornadoes across central and eastern Oklahoma, in addition to the threat of large hail and damaging straight-line wind gusts.

The most catastrophic event of the day occurred when a violent EF5 tornado struck Moore, Oklahoma, damaging or destroying more than 13,000 homes and killing 25 people including seven children who died while taking shelter inside Plaza Towers Elementary School, which was flattened by the tornado. The tornado shared some similarities to the F5 tornado in the same area on May 3, 1999.

In Michigan, gusts from the squall line reached 80 mph (130 km/h), downing numerous trees and power lines. Some trees were 1.5 feet (0.46 m) in diameter. A few trucks were blown onto their sides as well. Roughly 30,000 customers of Consumers Energy lost power across the state. Further south, considerable damage took place across Missouri and Arkansas due to a squall line. Gusts reached 88 mph (142 km/h) in Hannibal, Missouri. One person was killed in Springdale, Arkansas after his vehicle was struck by wind-driven debris. Several tornadoes touched down within this squall line in the two states, none of which exceeded EF1 intensity. The squall line continued to produce damaging winds as it progressed eastward into Illinois and Indiana. Three additional tornadoes touched down into the early hours of May 21, including a brief EF2 in Mount Olive, Illinois around 10:14 p.m. CDT (0314 UTC).

Throughout the day, the SPC received a total of 447 reports of severe weather: 37 for tornadoes, 281 for wind, and 129 for hail; however, some of the wind and hail reports were associated with a separate system over the Southeastern United States.

May 21

Similar to the preceding days, the slow moving trough and associated cold front moved slowly eastward on May 21. Strong instability ahead of the system prompted the SPC to issue a moderate risk of severe thunderstorms for extreme southeast Oklahoma, southwest Arkansas, northwest Louisiana, and northeast Texas; however, the main threat was damaging winds rather than tornadoes. With CAPE values of 2,500–4,000 J/kg and 50 to 65 mph (80 to 105 km/h) shear across the region, conditions were once again favorable for the development of supercells capable of producing large hail. Activity was expected to be scattered at first, with some isolated supercells capable of producing tornadoes, but a squall line was anticipated to develop quickly and become the dominant feature. In the Northeast and Great Lakes region, southwesterly flow ahead of the trough brought warm, moist air into the area. Moderate instability, with CAPE values ranging from 1,000 to 2,000 J/kg and steep lapse rates were supporting of damaging straight-line winds, especially across Michigan, New York and Massachusetts. Later on, instability increased in the Northeast along a quasi-stationary front, leading to the risk of isolated supercells and a mesoscale convective system.

During the morning of May 21, a strong shortwave trough over the Texas Panhandle moved eastward into Oklahoma. Moderate instability in the region led to the formation of thunderstorms across the state, with strong thunderstorms developing near the Oklahoma City metropolitan area, including the city of Moore.

Ahead of the main line of storms, a loosely organized squall line formed over central Tennessee by 11:15 a.m. CDT (1615 UTC). Initiating along the edge of an old outflow boundary, the system moved into an area with CAPE values of 2,000–3,000 J/kg and strong daytime heating. As it moved east-northeastward, it became increasingly intense and organized. Widespread wind damage occurred from central Tennessee into eastern Kentucky. The highest winds associated with this line reached 77 mph (124 km/h). Numerous trees and power lines were downed across both states. One person was injured in Wilson County, Tennessee after a construction crane was blown over onto a car by high winds.

Severe storms extended into Ontario, Canada, where a squall line produced some damage. Strong winds downed many trees and power lines, leaving roughly 3,000 people without power. Heavy rains also caused some flooding, leaving cars stranded. Three tornadoes touched down across the province: one EF2 near Glenarm and two EF0s. In New York, two microbursts, both with winds up to 70 mph (110 km/h), caused minor damage in Cayuga and Seneca counties. Further south, a tornado warning was issued for Columbia and Dutchess Counties after a trained spotter mistakenly reported a tornado on the ground near the town Copake; it was later discovered that the spotter was looking at scud clouds rather than a funnel or a tornado. The outbreak ended at that point.

Throughout May 21, the SPC received a total of 413 reports of severe weather: none for tornadoes, 307 for wind, and 106 for hail.