The National Weather Service has upgraded the rating of the tornado that hit Greenfield, Iowa, on Tuesday to an EF-4. This is one of the strongest tornadoes of 2024 so far, but the ongoing EF-5 drought continues.

It's been an active week for severe weather, with multiple tornadoes hitting Iowa, including a violent tornado that devastated Greenfield. This tornado was part of a line of storms that produced nearly two dozen tornado reports and damaging reports from Oklahoma northeastward into the western Great Lakes and Michigan’s Upper Peninsula.

National Weather Service survey teams identified damage in Greenfield indicating that Tuesday’s tornado was an EF4, with a maximum width of 1000 yards and a path length of 44 miles. It's estimated to have reached peak internal wind speeds of between 175 and 185 miles an hour.

There have been two other EF-4 tornadoes this year, both in Oklahoma. A low-end EF-4 hit Marietta, Okla., on April 27, destroying a warehouse. An EF-4 with peak winds estimated at 180 mph, devastated Barnsdall, Okla., on May 6. While there have been numerous tornado outbreaks since April, a tornado at the highest end of the Enhanced Fujita scale, an EF-5 remains elusive.

The last EF-5 tornado recorded in the U.S. was in Moore, Okla., on May 20, 2013. Our current EF-5 drought is the longest on record and has lasted just over 11 years. A total of 50 tornadoes have been rated F5/EF5 since records began in the United States in 1950.

So why are we in an EF-5 drought? One reason is that the Enhanced Fujita Scale, or EF Scale, which became operational on February 1, 2007, assigns a tornado a rating based on estimated wind speeds and related damage. When tornado-related damage is surveyed, it is compared to a list of Damage Indicators (DIs) and Degrees of Damage (DoD), which help estimate the wind speeds the tornado likely produced.

Damage to a "well-constructed building" is the most common factor that helps the NWS confirm an EF5, yet many homes in the U.S. do not meet that criteria. So while we have seen violent tornadoes in the last 11 years, none have damaged "well-constructed buildings" enough to garner an EF-5 rating.