Alabama is located in the heart of "Dixie Alley," a region in the southern United States that is susceptible to severe thunderstorms and tornadoes, particularly in the spring and late fall. The term "Dixie Alley" was coined by Allen Pearson, director of the National Severe Storms Forecast Center (now known as the Storm Prediction Center), in 1971 after a tornado outbreak across the Mississippi Delta. Alabama averages 42 tornadoes a year, based on data since 1985, when Doppler radar installations were constructed across the United States. Since then, Alabama's annual average death toll of 41 persons has been the highest in the nation. There is still much to learn about tornado formation. Research projects, such as the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX), hopefully will provide more information in coming years and decades.
The American Meteorological Society defines a tornado as "a rotating column of air, in contact with the surface, pendant from a cumuliform (predominantly vertical) cloud, and often visible as a funnel cloud or circulating debris or dust at the ground." Tornadoes are relatively rare and form on the right rear flank of severe thunderstorms. A severe thunderstorm is defined as one with 58-mile-per-hour (mph) winds or higher or producing hail one inch in diameter or larger. The primary elements needed for the creation of a tornado, known as tornadogenesis, include instability in the atmosphere (the ability of pockets of air to rise freely), speed and directional shear (changes in the speed and direction of winds as they move from the surface through Earth's troposphere), and some type of trigger, such as an upper-air trough (an elongated area of low atmospheric pressure) or a surface boundary like a cold front. Sharp changes in wind direction parallel to the ground, known as shear, can lead to rotation in the rising warm air of a thunderstorm, called updraft, and create a rotating column of air called a "mesocyclone." This event can often be identified on weather radar by the distinctive shape it creates: the "hook echo" signature. This pattern occurs where air and precipitation flow into the mesocyclone and produce a curved feature shaped like a hook. Occasionally, a mesocyclone can produce a tornado when the storm's rotation tightens and extends toward the ground, and radar can actually show debris being lofted into the air by a tornado, as well.
Tornadoes are rated on the Enhanced Fujita (EF) scale, which runs from zero to five. Data since 1950 show that 71 percent of tornadoes in Alabama have been rated "weak" and short-lived, EF-0 and EF-1. Only 2 percent of tornadoes since 1950 were "strong/violent," or EF-4 and EF-5. But these larger tornadoes have been responsible for most of the deaths.
In Alabama, tornadoes differ from those in the traditional "tornado alley" across the U.S. Great Plains because they usually arise during strong storms known as "high-precipitation supercells." In these storms, rain often wraps around the tornado, making it impossible to see. Tornadoes in Alabama can also be difficult to see because they frequently happen at night, and hills, trees, and other topographical features obscure the view. Tornadoes may occur at any time of the year, but in Alabama the core months are March, April, and May in the spring, and November and December in late autumn. These are the months when conditions are most likely to come together for severe thunderstorms. Most tornadoes that touch down in Alabama during the summer months are associated with tropical storms or hurricanes. The typical thunderstorms that occur on hot summer afternoons in Alabama rarely have tornado potential, but they can produce "wet microbursts," or local areas of strong straight-line winds, strong enough to damage structures.
Historic Tornado Events
One of the earliest tornado events on record in the state occurred on February 19-20 in 1884. A tornado moved through Jefferson and St. Clair Counties, killing more than 13 individuals, based on old newspaper accounts. Another notable tornado outbreak came on March 21, 1932. Chilton County in central Alabama was hit extremely hard, with 58 persons killed. The Union Grove community near Jemison was destroyed. Doctors and nurses from Montgomery and Birmingham worked all night by lantern and flashlight to relieve the widespread suffering. In Clay County, one of the tornadoes remained on the ground for 30 miles, cutting a path 400 yards wide and sending an automobile airborne and carrying it through the air for approximately 400 yards. This outbreak killed 12 people in Clay County, injured 200, and forced people who lost their homes to live in the Clay County Courthouse for a time. Another tornado on this day moved through the western part of the city of Tuscaloosa, across the Black Warrior River, and into downtown Northport. The exact death toll March 21, 1932, will probably never be known due to incomplete records (causing it to become known as the "Enigma Tornado Outbreak" for this reason), but at least 300 individuals died statewide.
The longest track for a single tornado recorded in Alabama is 139.1 miles. It occurred on May 27, 1973, when an EF-4 tornado moved from just north of Demopolis, Marengo County, in west-central Alabama to the slope of Mount Cheaha in Cleburne County in northeast Alabama. There was severe damage in Greensboro, Hale County, and Brent, Bibb County, and five people were killed. One year later, Alabama was part of the "Super Outbreak" of tornadoes on April 3-4 in 1974. During that event, as many as 86 individuals died when the most violent type of tornado, an EF-5, moved through the center of the town of Guin, Marion County, destroying much of the community. Downtown Jasper, Walker County, took a direct hit from an EF-4 tornado in that same event, and other violent twisters touched down near Cullman and Huntsville.
The largest number of tornadoes ever recorded in Alabama in one month occurred in April 2011, when 107 touched down. A severe weather event on April 15 produced 45 tornadoes, and this was surpassed on April 27, when 62 tornadoes touched down in a single 24-hour period, a world record. The April 15 event is sometimes described as the "forgotten outbreak" because it was overshadowed by the subsequent outbreak, but on that day four people were killed in Marengo County and four more died in northern Autauga County. An EF-3 touched down in the southern part of Tuscaloosa County, but there were no fatalities from that storm. The outbreak on April 27 that produced 62 tornadoes was the largest on record in the United States. It killed 252 people in Alabama, despite timely warnings.
The tornadoes came in two distinct waves: one during the early morning, and the other during the afternoon and evening. The morning event caused the loss of commercial power for almost 250,000 people, which affected the ability of some citizens to receive tornado warnings for the next round later in the day. These occurrences were part of a larger scale event from April 25-28 that produced 362 tornadoes in 21 states from Texas to New York to southern Canada. The storm produced three EF-5 tornadoes, including one that moved through the communities of Hackleburg, Marion County, and Phil Campbell, Franklin County, before moving up into the Tennessee Valley. Winds were estimated by the National Weather Service to be 210 mph, and a total of 18 people were killed. The most publicized tornado, however, was one of the eight EF-4 twisters that day; it moved through rural west Alabama, then made a direct hit on the city of Tuscaloosa and parts of the Birmingham metropolitan area. Estimated winds were 190 mph, killing more than 65 people and injuring more than 1,500. The town of Cordova, Walker County, took a direct hit from two different tornadoes in that outbreak. An EF-3 touched down there around 5:30 a.m., followed by a stronger, EF-4 around 5:00 that evening. Thirteen were killed by the second tornado, and the downtown area was destroyed.
Tornado watches and warnings are designed to give people notice about the potential for severe thunderstorms and tornadoes. Watches are issued by the National Oceanic and Atmospheric Organization's (NOAA) Storm Prediction Center in Norman, Oklahoma; these cover fairly large areas and are designed to let people know that conditions are favorable for severe weather. Warnings are issued when tornadoes are identified on radar or actually reported. Lead times can vary from seconds to more than 30 minutes, and in some cases, tornadoes can touch down without warning. These are generally the weaker, short-lived EF-0/F-1 tornadoes, however.
In Alabama, NOAA operates National Weather Service forecast offices in Huntsville, Madison County; Calera, Shelby County; and Mobile, Mobile County, and these offices also issue tornado warnings. It is crucial for people in Alabama to have at least two ways of receiving tornado warnings; the two preferred methods are a NOAA Weather Radio and a smartphone app designed for sharing warnings. Although most communities also have outdoor warning sirens, they reach only a limited number of people and should never be a primary way of receiving warnings.