U.S. may have up to four ‘tornado alley’ locations

AccuWeather.com reports Moore, Okla., is in the heart of an area known as Tornado Alley. The people who call Moore home are still rebuilding from last year's devastating tornado that struck the area on May 20, 2013. In fact, HelpMooreRebuild.org is just one website where requests for help can be made as well as an area for people to sign up, serve and donate.

According to NOAA, about 1,300 tornadoes hit the United States per year and about 60 people die from a tornado-related death such as falling debris, per year. With severe weather season already underway, it is important to be able to identify where tornadoes touch down most frequently. Arkansas has averaged 39 tornadoes per year between 1991 and 2010.

The most popular answer, Tornado Alley, is a stretch of land famous for its frequent tornado watches and warnings during the spring. Tornadoes, however, can occur almost anywhere in the United States, including west of the Rockies and east of the Appalachians. In fact, some weather experts suggest there is more than one tornado alley and up to as many as four different domestic locations.

Senior Vice President of AccuWeather Enterprise Solutions and Tornado Expert Mike Smith said that he believes in only two domestic tornado alleys: the classic stretch from Dallas to Des Moines, Iowa and Dixie Alley.

The National Weather Service (NWS) Storm Prediction Center (SPC) routinely collects reports of severe weather and compiles them with a Graphic Information System (GIS). This file contains track information regarding known tornados during the period 1950 to 2006.

"Dixie consists of northeastern Arkansas, western Tennessee, northern Mississippi and Alabama. This area can estimate seeing about eight to 13 tornado watches per year and it's because of the low pressure systems that come through the area that mixes with moisture from the Gulf," Smith said.

The collision of cool air and warm, moist air is also what makes the Plains such a prime location for frequent tornadoes.

"The Great Plains is so susceptible to tornadoes because of the collision between moisture from the Gulf of Mexico, cool air from Canada, and most uniquely, very dry air from New Mexico and California that collides with the moisture," Smith said.

Those same ingredients can be found just about anywhere on the map of the United States, thus sporting the theory that there may be a Hoosier Alley (Kentucky, Illinois, Indiana and Ohio) and Carolina Alley (North Carolina and South Carolina) as well.

Since it's not rare for the atmosphere to bring these conditions together, some say it's nearly impossible to define one tornado alley, let alone multiple.

"I don't think there are any [tornado alleys]. It isn't a well-defined concept and it's pretty clear if you ask different meteorologists that they will draw different maps. If you ask the public, I think they would draw an area that would be something that would be based on occurrence," said Harold Brooks, senior scientist of Forecast Research and Development Division for NOAA, said.

There are several different ways that a tornado alley could be defined. It could be based on how often tornadoes touch down in an area, how often you include path lengths, if there is a definable severe weather season, or you could talk about impacts and where the most damage occurs, Brooks said. Dixie Alley is characterized by the presence of a defined severe weather season in the springtime as well as a strong fall signal which is slightly different from the definitive severe weather season of the Plains.

"There has been a high fatality rate from the 1950s through the present with F2 storms or higher in Dixie. These are the kind of storms that can take out a person's home," Smith said.

Defining an alley based on frequency or damage is not as easily agreed upon in areas referred to as Carolina Alley and Hoosier Alley.

"Carolina Alley is something that I don't buy. An interesting theory, as is Hoosier Alley, but they are terms for broad regions between the Rockies and the Appalachians where there is enhanced tornado occurrence, but the strength of that seasonal cycle weakens as you go east from the Plains," Brooks said.

Although Brooks does not buy into Carolina or Hoosier Alley's there is research that disagrees with him. Despite the location, it's important to pass on and understand the difference between tornado watches and warnings.

"Tornado Alley receives 10-15 watches per year and this is when the forecast conditions are just right for a severe weather event to occur, but a warning is when a specific area is advised to take cover," Smith said.

"Essentially, the Plains are an ideal place to make storms very frequently because it's easy for the atmosphere to do it there. You need something to initiate it and get the storm started and that the conditions to support the storms in April and May are perfect. But again, this can happen just about anywhere if the conditions are just right," Brooks said.