Tornado Alley is a colloquial and popular media term that most often refers to the area of the United States where tornadoes are most frequent. Although an official location is not defined, the area between the Rocky Mountains and Appalachian Mountains is usually associated with it.
Although no U.S. state is entirely free of tornadoes, they are most frequent in the plains states between the Rocky and Appalachian Mountains. According to the storm events database of the National Climatic Data Center, Texas reports more tornadoes than any other state, though this state's very large land area should be taken into account. Kansas and Oklahoma are second and third, respectively, for sheer number of tornadoes reported but report more of them, per unit of land area, than Texas. However, the density of tornado occurrences in northern Texas is comparable to that of Oklahoma and Kansas. Florida also reports a high number and density of tornado occurrences, though only rarely do tornadoes there approach the strength of those that sometimes strike the southern plains.
Although Tornado Alley is considered to be in the areas of the Central United States, the National Weather Service has no official definition of the term. According to the National Severe Storms Laboratory FAQ, "Tornado Alley" is a term created by the media to refer to areas that have greater numbers of tornadoes. There are several ideas of what Tornado Alley is, but those ideas are the result of the different criteria used to refer to it. 90% of tornadoes hit this region of the U.S because cold, dry air from Canada and the Rocky Mountains meets warm, moist air from the Gulf of Mexico and hot, dry air from the Sonoran Desert, which combines with atmospheric instability to produce intense thunderstorms.
The most common definition of Tornado Alley is the location where the strongest tornadoes occur most frequently and was first coined by Jennifer L. Wiley in 1904. The core of Tornado Alley consists of the Texas Panhandle, Oklahoma, Kansas, Nebraska, eastern South Dakota, and the Colorado Eastern Plains. However, Tornado Alley can also be defined as an area stretching from central Texas to the Canadian prairies and from eastern Colorado to western Pennsylvania. It can also be argued that there are numerous Tornado Alleys. In addition to the Texas/Oklahoma/Kansas core, such areas include the Ohio Valley, the southern Great Lakes, the Tennessee Valley and the lower Mississippi Valley.
The nickname, "Dixie Alley" is sometimes used for the areas in the southeastern U.S., notably the lower Mississippi Valley and the upper Tennessee Valley. This region is particularly vulnerable to violent, long tracked tornadoes. Also, an area stretching from southern Michigan to southern Indiana, and from eastern Illinois to western Ohio is sometimes referred to as Hoosier Alley. Tornadoes are common in the late spring and early summer months in this region, although they are rarely as violent as tornadoes in Tornado and Dixie Alleys.
Some studies suggest that there are also smaller tornado alleys located across the United States.
There were more than 500 tornado-related deaths in 2011, the most in more than 50 years.
Source: The New York Times; FEMA, NOAA