Tornadoes are among the most violent atmospheric phenomena on Earth. Their intensity is measured using a standardized classification system that categorizes the destructive power of these storms based on the damage they leave behind. The highest classification, the EF5 rating, represents extreme power, signifying the rarest and most violent storms. These events are capable of catastrophic destruction.
Identifying the Last EF5 Event
The last officially confirmed EF5 tornado to strike the United States occurred on May 20, 2013, in Moore, Oklahoma. This powerful storm carved a 14-mile path of destruction through the Oklahoma City suburb, remaining on the ground for approximately 40 minutes. The tornado’s winds were estimated to be in excess of 200 miles per hour, meeting the threshold for the highest rating on the scale.
The Moore tornado resulted in the loss of 24 lives and injured more than 200 people. The immense force of the storm leveled hundreds of homes, completely destroying a local elementary school and a hospital. The EF5 rating was assigned because the damage demonstrated the obliteration of well-built, frame homes with little debris left on the foundation.
The previous EF5 event was the devastating Joplin, Missouri, tornado on May 22, 2011, one of the deadliest single tornadoes in modern U.S. history. The 2013 Moore event initiated an extended period without another confirmed EF5 rating. This long gap highlights the exceptional rarity of these most intense storms.
Understanding the Enhanced Fujita Scale
The intensity of a tornado is officially rated using the Enhanced Fujita Scale, or EF-Scale, which was implemented in the United States in 2007. This system replaced the original Fujita Scale, which was developed in 1971 by Dr. T. Theodore Fujita. The revision was necessary because engineering studies found the wind speeds assigned to the damage on the original F-Scale were often too high.
The EF-Scale categorizes tornadoes from EF0 to EF5, with the rating determined by surveying the damage after the storm passes. Investigators compare the destruction to a list of 28 different Damage Indicators (DIs) and their corresponding Degrees of Damage (DoD). These indicators cover various structures, from small barns and mobile homes to high-rise buildings, providing a more accurate correlation between damage and wind speed.
An EF5 rating is assigned when estimated three-second wind gusts are above 200 miles per hour, based on catastrophic damage. The EF-Scale does not measure wind speed directly but estimates it based on the severity of the destruction. The damage must suggest winds capable of completely removing a well-constructed house from its foundation.
Frequency and Impact of EF5 Tornadoes
EF5 tornadoes are exceptionally rare events, accounting for only a tiny fraction of all tornadoes that occur annually in the United States. Since 1950, only about 59 tornadoes have been officially rated F5 or EF5, averaging less than one per year. In comparison, the U.S. experiences an average of over 1,200 tornadoes of all strengths each year.
Despite their rarity, these violent storms are disproportionately responsible for fatalities. Tornadoes rated EF4 or EF5 account for approximately 70% of all tornado-related deaths in the country. The extreme wind speeds and resulting debris make survival extremely difficult for those without substantial underground shelter.
The formation of an EF5 tornado requires a perfect alignment of specific atmospheric ingredients, including significant vertical wind shear and a highly unstable atmosphere. This combination involves warm, moist air rising rapidly into a region where wind speed and direction change sharply with altitude. These conditions often occur within Tornado Alley, though EF5 storms can strike in other areas.