On May 20, 2013, an EF-5 tornado touched down in Moore, Okla., devastating a path up to 1.3 miles (2.1 kilometers) wide along a 17-mile-long (27 km) stretch of central Oklahoma, the National Weather Service said. EF-5 tornadoes are the strongest on the Enhanced Fujita scale, with wind speeds exceeding 200 miles per hour (322 kph). Since 1950, there have been 59 confirmed F-5/EF-5 tornadoes in the United States and one in Canada.
The process of tornado formation, known as tornadogenesis, begins when a strong thunderstorm develops a rotating mesocyclone several kilometers above ground. The axis of rotation of such a mesocyclone begins roughly parallel to the ground. If sufficiently cold air is present in the upper levels of a thunderstorm, such a rotation can carry rain droplets up and down repeatedly, allowing them to freeze and grow into hailstones. As this rotation can form a tornado, hailstones, especially those of large size, are an important warning sign that a tornado may appear.
If the amount of rainfall produced by a thunderstorm increases, the rainfall can pull down an area of rapidly descending air known as a rear flank downdraft. Continuous acceleration due to gravity means that the downdraft will accelerate towards the ground, which can drag a mesocyclone from a rotational axis parallel to the ground to a rotational axis perpendicular to the ground. During this process, the mesocyclone will begin to absorb cool, moist air from the rear flank downdraft as the mesocyclone lowers past the base level of the thunderstorm cloud. The convergence of warm air moving upward and cool air moving downward will begin forming a condensation funnel. The rear flank downdraft has a focusing effect on the base of a mesocyclone, which causes it to pull air from an ever smaller area. The updraft will create a low pressure area on the ground, which pulls the mesocyclone toward the ground. These effects are manifested by a descending funnel cloud. When the rear flank downdraft reaches the ground, a gust front is produced that can cause straight-line wind damage a significant distance from the mesocyclone. Within a few minutes, the funnel cloud will begin destroying property on the ground, becoming a tornado.
The formation of violent EF-4 and EF-5 tornadoes occurs by the same process as less powerful tornadoes, but certain conditions contribute to making some tornadoes exceptionally strong. The amounts of cool air and humidity must be just right, or the temperature and dewpoint differences will not be large enough to produce a violent tornado. The wind currents also play a role in making some places more likely to suffer violent tornadoes, which is why the central United States is the most common area for violent tornadoes to form. The Rocky Mountains and Appalachian Mountains act as a giant air funnel, allowing warm, moist air from the Gulf of Mexico to meet cold air from the Arctic. The resulting wind shear means that violent tornadoes form more easily from thunderstorms in the central United States than anywhere else in the world.