A powerful tornado that tore through the town of Greenfield, Iowa last month produced some of the strongest winds ever observed on Earth, according to new data.

Iowa was pummelled by a flurry of severe thunderstorms and tornadoes during an outbreak of violent weather on May 21, 2024. The video below shows the line of severe storms passing over Iowa from west to east last month.

Video: GOES-16 visible satellite and lightning flash imagery showing a line of thunderstorms passing over Iowa on May 21, 2024. Source: CSU/CIRA & NOAA

According to post-event storm surveys carried out by the US National Weather Service (NWS), the strongest tornado from this outbreak, which impacted the town of Greenfield and surrounding areas, caused damage that was consistent with an EF-4 rating.

An EF-4 rating is the second highest category on the six-tiered Enhanced Fujita (EF) scale used for classifying tornadoes, based on the damage caused by the tornado. EF-4 tornadoes typically have peak wind speeds of 116 to 200 mph (187 to 322 km/h).

The NWS estimated a peak wind speed of 175 to 185 mph (282 to 298 km/h) from the Greenfield tornado on May 21. This was based on damage “located within the community of Greenfield and across rural portions of southern Adair County.”

However, the NWS event report for the Greenfield tornado also noted that “additional analysis of data will occur through the coming days and weeks, so further refinement of the tornado statistics are (sic) possible.”

STRONGEST #TORNADO IN HISTORY? 300+ mph wind confirmed in Greenfield, Iowa tornado. Check out this new drone footage from this prior to this measurement! WATCH: https://t.co/hIgQ4RVSws pic.twitter.com/g4m5QmBFec

— Reed Timmer, PhD (@ReedTimmerUSA) June 23, 2024

Video: Drone footage of the Greenfield tornado, captured by storm chaser Reed Timmer on May 21, 2024.

One additional source of data that has emerged since the event suggests that wind speeds inside the Greenfield tornado may have been much stronger than the NWS estimate, at times exceeding 300 mph (483 km/h). This was likely due to the subvorticies that can be seen clearly in the video above.

A doppler on wheels (DOW) mobile radar operated by the University of Illinois collected data suggesting wind speeds near the ground reached as high as 309 to 318 mph (497 to 512 km/h) inside the Greenfield tornado on May 21.

Doppler radar measurements have only resulted in peak wind speeds above 300 mph (483 km/h) on two other occasions: the Bridge Creek, Oklahoma tornado in 1999 and the El Reno, Oklahoma tornado in 2013.

Taking tornadic wind measurement uncertainty into account, the wind speeds calculated from the Greenfield tornado are similar to the DOW-derived winds from the Bridge Creek and El Reno events, making it one of the strongest winds ever detected on Earth.