On the afternoon of May 21, 2024, a violent and destructive EF4 tornado tracked across southwestern Iowa, United States, devastating the city of Greenfield. The tornado, known most commonly as the Greenfield tornado, destroyed many buildings and wind turbines across its path that stretched through Page, Taylor, Adams, and Adair counties, while also causing more than $31 million in property damage, killing five people and injuring 35 more. The tornado reached peak intensity within Greenfield, where National Weather Service surveyors denoted maximum wind speeds estimated at 185 mph (298 km/h), or EF4 on the Enhanced Fujita scale. However, estimated winds of 309–318 mph (497–512 km/h) were briefly determined from inside the tornado by a Doppler on Wheels portable radar unit, one of only three times that wind speeds exceeding 300 miles per hour (480 km/h) have been determined in a tornado from radar observations.
The tornado formed amidst a significant tornado outbreak in an area expected to be strongly conducive for the development of long-tracked and fast-moving tornadoes. Over the next 48 minutes, the tornado was observed by multiple storm chasers and research teams, who would determine the extreme wind figure from a direct radar measurement above the city of Greenfield. In addition to one fatality on a highway in Adams County, four fatalities occurred in Greenfield, with the damage in the city described as "horrific", as surveyors revealed damage consistent with a violent EF4 tornado, contradicting analysis of mobile radar data. The tornado has been noted as an important milestone in the practical efficacy of the Warn-on-Forecast system, which predicted tornadic activity in the vicinity of Greenfield about 75 minutes before the tornado reached the community. State legislation, introduced in the aftermath of the Greenfield and Minden tornadoes and passed in both that summer and the following year, would fund housing projects and disaster recovery programs throughout Iowa.
On May 21, forecasters at the Storm Prediction Center issued a moderate (4/5) risk for severe weather over the states of Iowa, Missouri, Minnesota, Wisconsin, and Illinois, highlighting an elevated regional risk for tornadoes and strong wind gusts. Early on that morning, convective storms along a bow echo, a formation of storms associated with high winds, had formed along a boundary consisting of outflow from storm activity over western Nebraska. These storms were expected to move eastward through the southern third of Iowa, bringing a small risk of surface-based severe weather. To the north, a roughly west-to-east band of non-severe storms existed along a composite frontal and outflow boundary. North of this boundary existed a large area of rain showers that covered an area of dew points near 60 Fahrenheit and surface temperature in the lower 60s Fahrenheit. Dew points above 55 °F (13 °C) are typically considered more conducive to severe weather. These showers led to atmospheric stabilization, curtailing any severe risk north of the boundary. As the convection moved eastward out of the area, its severe risk was expected to lessen.
Later on that morning, a second severe weather threat developed as a large surface low-pressure trough had appeared over the Midwest, primarily centered over north-central Kansas, with a secondary and weaker low center near the tri-point of South Dakota, Iowa, and Minnesota. A warm front extended from the southern low center to the northeast through southeastern Nebraska and over southern Iowa, creating a large warm sector. A cold front anchored by the same low center extended southeast through central Kansas into the Oklahoma panhandle. The primary surface low was expected to move quickly to the northeast, moving the warm sector bounded by the warm front further north into east-central to southeastern Nebraska and west-central to southwestern Iowa, where atmospheric conditions conducive to severe weather would develop. Strong vertical ascent brought by the shortwave trough led to probable elevated thunderstorms over the central Nebraska/Kansas border, which would contribute to a surface-based storm system as they approached the Nebraska/Iowa border region. These surface-based storms were expected to develop into fast-moving supercells capable of producing severe weather hazards.
Early in the afternoon, the southern low center had moved to southeastern Nebraska. The warm front was expected to rapidly move northward ahead of the shortwave trough, which would bring higher dew points in the upper 60s Fahrenheit into much of Iowa. In conjunction with this, a large change in temperatures with altitude hallmarked by steep mid-level lapse rates, and mid-level CAPE values of 2500–3500 j/kg, a measure of atmospheric instability approximating the power thunderstorm updrafts have to rise, in addition to powerful wind fields producing elongated hodographs, a measure indicative of a shift in wind speed with height that allows the development of rotating thunderstorms, were expected to be present in the region throughout the early afternoon. These factors would result in an environment very conducive to significant and fast-moving severe thunderstorms. The storm mode was initially expected to be discrete, with fast-moving individual storms staying ahead of the cold front in the severe-conducive warm sector. These storms were expected to quickly develop the characteristics of severe supercell thunderstorms, with strong wind gusts up to 75 mph (121 km/h), very large hail up to 3 in (7.6 cm), and strong and long-lasting tornadoes all being possible hazards.
A particularly dangerous situation level of tornado watch was issued at 1:10 p.m. CDT over much of Iowa and parts of surrounding states. All severe weather hazards were expected as the risk of "[s]everal tornadoes and a few intense tornadoes" would exist over a large region, including Adair County. The watch outlined a 90% chance of two or more tornadoes, with an 80% chance of one or more tornadoes significant on the Enhanced Fujita scale (EF2–EF5) in the watch area.
Formation through Adams County
The tornado first touched down at the intersection of 110th Street and Vine Avenue in rural Page County, Iowa at 2:57 p.m. Central Daylight Time (UTC–5). Across Page County, damage up to EF1 intensity occurred, restricted primarily to tree branches, while the roof of a farm outbuilding was torn off. It then briefly tracked through the far northwestern tip of Taylor County, causing no damage, before moving into Adams County, where damage occurred to trees, power poles and farm buildings past the county border. The tornado intensified as it passed through rural areas to the southeast of Nodaway, before crossing US 34 north of Brooks, where numerous videos displayed the tornado's large funnel with an area of swirling debris below. The tornado then crossed Iowa 148, where 41-year-old Monica Irma Zamarron was killed after she was ejected from her vehicle after being caught in the tornado's circulation. The tornado began producing more EF2 damage past this point, before producing EF3 damage at the intersection of 150th Street and Notchwood Avenue, where an unanchored home lost its walls and slid off its foundation, while all nearby outbuildings were destroyed. Throughout the rest of Adams County, the tornado caused damage to multiple wind turbines, many of which collapsed, while reaching a maximum width in the county of 1,300 yd (1,200 m) and exhibiting multiple suction vortices each producing damage.
The tornado then crossed into Adair County, still with multiple-vortex characteristics, while downing a wind turbine and a metal truss tower. The tornado then approached Fontanelle Road, where it destroyed a well-anchored home at EF4 intensity with the debris thrown into a nearby field. An outbuilding on the property was completely obliterated, although the farm equipment was mostly unmoved and not severely damaged, and trees were shredded. Northeast of there on 310th Street, a nailed down cinderblock foundation home and an outbuilding were obliterated and swept away. Vehicles inside the garage of the home were damaged, but not tossed, while debris from the structures and vehicles within it were left straddling along the sides of the foundation. The tornado then moved back over open terrain, inflicting damage to an outbuilding, which was rated EF1, before downing more wind turbines. Low-end EF4 damage was observed as the tornado crossed 290th Street, where another home was leveled.