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Tornado Alley is a colloquial term for the area of the United States where tornadoes are most frequent. The term was first used in 1952 as the title of a research project to study severe weather in parts of Texas and Oklahoma.
Although the boundaries of Tornado Alley are not clearly defined, its core is northern Texas, Oklahoma, Kansas, and Nebraska. Some research has indicated that tornadoes are becoming more frequent in the northern parts of Tornado Alley where it reaches the Canadian prairies.
There is also a secondary tornado alley in Ontario, Canada, which runs from the Great Lakes region to the upper St. Lawrence valley. It roughly follows the same route as Highway 401 from Windsor, Ontario to London, Ontario, with a secondary path from Lake Huron to Barrie, Ontario and across to Ottawa, Ontario. After the U.S., Canada gets the most tornadoes in the world, and a third of those tornadoes strike Southwestern Ontario.
Over the years, the location(s) of Tornado Alley have not been clearly defined. No official definition of 'tornado alley' or the geographical area of tornado alley has ever been designated by the National Weather Service. Thus, differences of location are the result of the different criteria used.
According to the National Severe Storms Laboratory FAQ, "Tornado Alley" is a term used by the media as a reference to areas that have higher numbers of tornadoes. A study of 1921-1995 tornadoes concluded almost one-fourth of all significant tornadoes occur in this area.
Though no state is entirely free of tornadoes, they occur more frequently in the Central United States, between the Rocky Mountains and Appalachian Mountains. Per square mile, Kansas and Oklahoma rank first and second respectively in the number of tornadoes. Florida also reports a high number and density of tornado occurrences, though tornadoes there rarely approach the strength of those that sometimes occur in the southern plains. Regionally, the frequency of tornadoes in the United States is closely tied with the progression of the warm season when warm and cold air masses often clash.
Another criteria for the location of Tornado Alley (or Tornado Alleys) can be where the strongest tornadoes occur more frequently.
Tornado Alley can also be defined as an area reaching from central Texas to the Canadian prairies and from eastern Colorado to western Pennsylvania.
It has also been asserted that there are numerous Tornado Alleys. In addition to the Texas/Oklahoma/Kansas core, such areas include the Upper Midwest, the lower Ohio Valley, the Tennessee Valley and the lower Mississippi valley. Some studies suggest that there are also smaller tornado alleys located across the United States.
The tornado alleys in the southeastern U.S., notably the lower Mississippi Valley and the upper Tennessee Valley, are sometimes called by the nickname "Dixie Alley", coined in 1971 by Allen Pearson, former director of the National Severe Storms Forecasting Center.
Another tornado alley has been identified in Ontario, Canada, running from the Great Lakes region to the upper St. Lawrence valley. It roughly follows the same route as Highway 401 from Windsor, Ontario to London, Ontario, with a secondary path from Lake Huron to Barrie, Ontario and across to Ottawa, Ontario.
The term "tornado alley" was first used in 1952 by U.S. Air Force meteorologists Major Ernest J. Fawbush (1915–1982) and Captain Robert C. Miller (1920–1998) as the title of a research project to study severe weather in parts of Texas and Oklahoma.
In the heart of tornado alley, building codes are often stricter than those for other parts of the U.S., requiring strengthened roofs and more secure connections between the building and its foundation. Other common precautionary measures include the construction of storm cellars, and the installation of tornado sirens. Tornado awareness and media weather coverage are also high.
The southeastern U.S. region is particularly prone to violent, long tracked tornadoes. Much of the housing in this region is less robust than in other parts of the USA and many people live in mobile homes. As a result, tornado related casualties in the southern U.S. are particularly high. One prime example was the April 25–28, 2011 tornado outbreak.
After the U.S., Canada gets the most tornadoes in the world. The average number of tornadoes per 10,000 kilometres is highest in the southern parts of Alberta, Saskatchewan, and Manitoba, as well as Southern Ontario. Northern Ontario between the Manitoba border and the Lake Superior lakehead is also prone to severe tornadoes, but tornadoes in this area are believed to be underestimated due to the extremely low population in this region.
Roughly half of all Canadian tornadoes strike the Canadian prairies and Northern Ontario as far east as Lake Superior. Together, these regions make up the northernmost border of the U.S. Tornado Alley. These tornadoes can be up to EF5 strength.
Another third of Canadian tornadoes strike Southern Ontario, especially in the region halfway between the Great Lakes from Lake St. Clair to Ottawa, Ontario. This happens because tornadoes in this region are triggered or augmented by lake breeze fronts from Lake Huron, Lake Erie, Lake Ontario, and Georgian Bay. Tornadoes do not often hit lake shadow regions, although they are not unknown, and some, such as the 2011 Goderich, Ontario tornado, have been violent. However, most Ontario tornadoes are concentrated in a narrow corridor from Windsor, Ontario and Sarnia, Ontario through London, Ontario, and then northeast to Barrie, Ontario and Ottawa, Ontario. This region can produce tornadoes of up to EF4 strength.
Southwestern Ontario weather is strongly influenced by its peninsular position between the Great Lakes. As a result, increases in temperature in this region are likely to increase the amount of precipitation in storms due to lake evaporation. Increased temperature contrasts may also increase the violence and possibly the number of tornadoes.