Tornado outbreak sequence in the United States
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A multi-day period of significant tornado activity along with significant derechos occurred across the Midwestern United States and the Mississippi Valley as well as an additional tornado in the Canadian province of Quebec. From May 19–27, 2024, two derechos occurred and tornadoes were reported across large portions of the Central United States, with multiple Particularly Dangerous Situation (PDS) watches issued across the sequence. On May 19, strong tornadoes occurred with isolated supercells in Colorado and Oklahoma while a derecho produced widespread wind damage and weak tornadoes across Kansas into the early morning hours of May 20. Limited tornadic activity took place on May 20, but another outbreak along with widespread damage struck mainly Iowa and Wisconsin on May 21. Five fatalities were confirmed with a large, violent, long-tracked EF4 tornado that went through Greenfield, Iowa. Scattered to widespread severe weather and tornadoes occurred over the next two days, including an EF2 tornado that injured 30 people on the west side of Temple, Texas. Another derecho formed in southwestern Nebraska late on May 23 and moved eastward, producing widespread wind damage and weak tornadoes through Nebraska and Iowa and northwestern Illinois before withering away in the northern part of the state during the morning hours of May 24.
A nocturnal outbreak occurred during the overnight hours of May 25 into May 26. An isolated supercell in northern Texas produced multiple tornadoes, including a low-end EF3 tornado that passed near Valley View, Texas, killing seven people. Another longer-lived supercell moved through northeastern Oklahoma and across northern Arkansas, producing several tornadoes along with straight-line winds of . Two fatalities were confirmed from an EF3 tornado that struck Claremore, Oklahoma along with areas near Pryor. Later, it produced a very large EF3 tornado near Decatur, Arkansas, which became the largest tornado ever recorded in Arkansas. Another EF3 tornado killed four people near Olvey and Pyatt while an additional tornadic death occurred with yet another EF3 tornado that passed near Yellville and through Briarcliff. Another supercell in southern Missouri produced a low-end EF3 tornado that passed near Morehouse and through Sikeston, killing two people indirectly. May 26 would be the most active day of severe weather; several rounds of squall lines and tornadic supercells moved through the Mid-Mississippi and the Ohio Valleys, producing widespread wind damage, large hail, and tornadoes. This included a very destructive, intense high-end EF3 tornado that prompted the issuance of four tornado emergencies across areas that had been previously impacted by the 2021 Western Kentucky tornado. One person was killed by this tornado. Severe weather activity became more isolated and scattered on May 27, marking the end of the outbreak sequence.
In all, 248 tornadoes occurred during the outbreak sequence; 20 (+1 indirect) people were killed by tornadoes while 10 other people died due to non-tornadic events as well. Over 240 people were injured.
Meteorological synopsis.
May 19.
On May 19, the Storm Prediction Center (SPC) outlined a level 4/Moderate risk of severe weather across southwest to central Kansas, encompassed by a level 3/Enhanced risk that extended into northwestern Oklahoma as well. Across the highest risk area, forecasters warned of the potential for a developing derecho capable of producing damaging winds upwards of . By the early afternoon hours, a northwest-to-southeast-oriented dryline extended from eastern Colorado into the Texas Panhandle, while a cold front laid across northern and central Kansas. Between these boundaries, dewpoints rose into the lower 60s °F and effective wind shear reached , supportive of supercell thunderstorms that would likely evolve into a line of storms as a shortwave trough approached from the west. Given the environment, the SPC issued a particularly dangerous situation severe thunderstorm watch across much of Kansas, northwestern Oklahoma, and the far northeastern Texas Panhandle. An intense supercell in Custer County, Oklahoma, formed an intense multi-vortex tornado near Custer City, resulting in a tornado emergency. This tornado was accompanied by an intense rear-flank downdraft surge, with the total damage path width being recorded at . A Ka-POL radar from Texas Tech University had been recording the tornado throughout its life cycle, and recorded a wind gust of near the surface, while confirming the tornado's actual width of . An intense supercell developed in north-central Kansas, producing large hail in excess of , tornadoes, and a wind gust to near Russell, eventually growing upscale as additional convective clusters formed to the southwest. In Oklahoma, an isolated intense supercell developed and tracked eastward toward the Oklahoma City metropolitan area as low-level wind shear began to increase. Meanwhile, dual mesoscale convective vortices tracked across eastern Kansas, contributing to widespread damaging wind reports—including multiple high wind gusts in excess of —as well as several tornadoes. As the complexes continued into Missouri, they encountered more stable air, causing the severe threat to gradually diminish.
May 20–21.
The severe threat shifted northward on May 20 as the SPC issued a level 3/Enhanced risk across northeastern Colorado and southwestern Nebraska. Here, forecasters expected a mixture of supercells and clusters of storms to originate near the Palmer Divide. Although rich moisture waned with westward extent, dewpoints in the upper 50s to lower 60s °F were expected to spread northwestward from Kansas and into Colorado. Thunderstorms developed in this area by the late afternoon hours, though their growth was stunted by marginal instability. Several discrete supercells eventually evolved across northeastern Colorado and began to grow upscale while encountering a more moist and unstable environment to the east across western Nebraska. The bowing line of storms traversed Nebraska and eventually Iowa through the pre-dawn and early morning hours of May 21, resulting in continued damaging wind gusts.
On May 21, the SPC issued another Moderate risk for severe weather in the states of Iowa, Missouri, Minnesota, Wisconsin, and Illinois, including a 15% risk for significant tornadoes. Upper-air soundings observed steep lapse rates in the middle troposphere over parts of Kansas and Missouri on the morning of May 21. The SPC predicted that these conditions would spread into the primary severe weather risk area, overlapping with a moist airmass with dew points approaching and leading to strong potential atmospheric instability. That morning, an organized cluster of thunderstorms moved into Iowa after having produced severe wind gusts in central and eastern Nebraska. This line of storms continued across central and eastern Iowa just north of a warm front, presenting a continued threat for damaging wind gusts. Additional storms began to develop along the border between Kansas and Nebraska ahead of an approaching trough. The SPC anticipated that the atmospheric environment over eastern Nebraska and western Iowa would become increasingly conducive for further organization of these storms into supercells. A tornado watch was issued by the SPC for areas near and along the Nebraska–Iowa border region at 11:10 a.m. CDT in anticipation of these changes. At 1:10 p.m. CDT, the SPC issued a tornado watch flagged as a particularly dangerous situation for much of Iowa and parts of surrounding states, including an 80% likelihood of a significant tornado occurring within the watch area. Supercells were active over southwestern Iowa by the mid-afternoon, approaching an environment favorable for tornadogenesis leading into the Des Moines area.
Multiple tornadoes were reported in Iowa, including a damaging tornado near Corning and in Greenfield, where multiple fatalities were reported. Tornadoes were also reported in Minnesota and Wisconsin.
May 22.
On May 22, the SPC outlined an Enhanced risk extending from central Texas across southeastern Oklahoma, extreme northwestern Louisiana, and southwestern Arkansas. In Texas, a cold front stretched from the Red River southwestward into the Permian Basin, with a surface trough just ahead of that front. Supercells were expected to evolve within a sheared and deeply unstable environment given effective wind shear of and mixed-layer CAPE of 3,000 J/kg. As such, very large hail was anticipated to be the main threat, although isolated tornadoes were possible too before storms grew upscale into one or more convective lines. To the northeast, lines of storms tracked across Oklahoma, Arkansas, and Tennessee, with a primary risk of damaging winds and large hail. Meanwhile, intense supercells developed across central Texas in a very unstable and deeply sheared environment, riding along outflow boundaries that enhanced the tornado threat. A particularly intense EF3 tornado developed southwest of Sterling City where rocks, grass, and topsoil were stripped off of the ground. A wide path of trees were debarked and sandblasted, with only trunks remaining in some cases. Caliche roads were also stripped. Ongoing storms across Texas eventually evolved into an intense mesoscale convective system moving across eastern portions of the state and into Louisiana through the late evening.
May 23–24.
On May 23, the SPC issued an Enhanced risk over central Nebraska to Western Iowa for the threat of damaging winds. Conditions weren't quite as favorable this day compared to others with dew points only expected to reach the upper 50s °F, creating conditions for high-base supercells, lowering the tornado potential. However, CAPE values were still able to reach 1000-3000 J/kg. Combined with shear reaching 45-55 kts, conditions were favorable for severe weather. Storms initiated along a front over northwest Nebraska around 4pm CDT, though the line quickly dissipated with only a supercell able to maintain itself as it went around the corner of Colorado as it produced several tornadoes. Eventually the line reformed over Southern Nebraska and began to produce damaging winds, growing to the height of the state at 1am CDT. Despite nighttime cooling, the line maintained itself and swept across all of Iowa bringing damaging winds up to 85 mph to the entire state and producing brief, weak tornadoes.
Meanwhile, in Texas and Oklahoma, a Slight Risk for all hazards was issued, though the tornado threat was seen as secondary to the more likely hail threat, with dew points in the low 70s °F and CAPE values around 3000 J/kg, but a supercellular storm mode. Supercells initially developed over western Texas, though failed to sustain themselves, except for a cluster that sustained themselves on the TX/OK border.
For May 24, the SPC initially issued a large Slight Risk from Southern Wisconsin to Central Texas, but in the 1300z Outlook upgraded northeast Illinois (for damaging winds) and a thin area of western/southern Oklahoma and northern Texas (for large hail) to an Enhanced Risk. The line MCS from the previous night in Iowa continued into the late morning and produced a couple tornadoes over that area of Illinois. Following this, little tornado activity occurred during the day. However, due to its long-lived hazard of damaging winds and long track length, the MCS was officially declared as a derecho by the Storm Prediction Center.
May 25–27.
On May 25, the SPC warned of an outbreak of severe thunderstorms across a level 4/Moderate risk area that encompassed much of Oklahoma and Kansas, as well as southwestern Missouri. In this region, forecasters expected the development of a few discrete supercells that would be capable of producing giant hail and strong to violent tornadoes. Across the northern portion of the risk, these supercells were forecast to evolve into a mesoscale convective system with swaths of damaging winds into the overnight hours. The potential for a level 5/High risk was discussed by forecasters in the preceding 24 hours given "a rare combination of instability and shear" that was depicted by model guidance. However, multiple uncertainties precluded a categorical upgrade, particularly questions about the influence of storms in Texas on the risk area farther north. A broad upper-level trough existed over the Western United States, with several embedded shortwaves, one of which was expected to translate across the risk area during the afternoon. A stationary boundary lifted northward as a warm front while a dryline sharpened from western Kansas into western Texas. In the warm sector between these boundaries, dewpoints rose into the upper 60s to even mid-70s °F, aiding in the development of extreme mixed-layer instability of 4,000–5,000 J/kg. A particularly dangerous situation tornado watch was subsequently issued for portions of extreme northern Texas, much of Oklahoma, and south-central Kansas.
Shortly thereafter, multiple supercells evolved over time across Texas and Oklahoma, but the convective evolution quickly became messy as splitting storms developed in close proximity. Additional supercells evolved across northern Oklahoma and southern Kansas, but those too underwent negative interaction with left-split storms and their accompanying outflow approaching from the south. As a result, central Oklahoma was completely void of storms for the entire day. Just before midnight, however two distinct areas of severe