The term "Tornado Alley" has become a common term to describe the region in America most affected by twisters, but a closer look reveals that Tornado Alley is actually composed of four extreme wind expressways.
The clash of warm, moist air and cooler, dry air coupled with a strong jet stream are the key ingredients for severe storms capable of producing a tornado. When you have a strong jet stream from the west or southwest high in the atmosphere and weaker, southern winds at the surface, you get a twisting motion in the atmosphere. The strength of a tornado depends upon how much of each ingredient is present as well as the topography of the land.
Long-tracking tornadoes, or the storms most commonly found in box office hits, are the "one percent" of the extreme weather world, because tornado paths of 25 miles or longer can only account for one percent of the total amount of recorded twisters. This one percent, however, creates an estimated 46% of total destruction and 70% of deaths caused my tornadoes in the United States, reports National Geographic.
The most frequently used funnel freeway in America is the Great Plains area, which includes Iowa, Oklahoma, Nebraska, Kansas and Texas. The geography of these states provides the perfect recipe for destruction, as the nation's breadbasket contains flat fields as far as the eye can see. Western winds riding the North American jet stream over the Rocky Mountains have roughly 500 miles to meet with dry air coming out of the north from Canada. "When looking at the population of the Plains, it's not surprising that there are so many tornadoes," said Rayno. "With major cities so spread out, there is a great deal of space for winds to start whipping."
Kansas has had the highest amount of F-5 tornadoes since 1880, the year America started to keep record of its tornado sightings, and Iowa has had the highest percentage of "violent" tornadoes, which consist of F4 and F5 twisters.
The deadliest tornadoes in America come from Dixie Alley, a region that spans from the Mississippi Valley to the Tennessee Valley. Here, most tornadoes are violent, long-tracked storms. There is no other tornado alley in the world quite like Dixie Alley, in that there is a tornado threat three seasons out of the year.
Warm, moist air comes out of the Gulf of Mexico to clash with the dry air coming from the deserts of Arizona and the west side of Texas at the center of Dixie Alley, which is where more lives are lost due to tornadoes than anywhere else in the United States. In the fall, winter and spring months, the North American jet stream is in prime real estate to get the air churning enough to create some wicked windstorms. "Normally, people associate tornadoes with the summer months, but at that period in the year, the jet stream is too far south to create tornadoes" said Bernie Rayno, Expert Senior Meteorologist. The presence of moist air will enjoy its starring role over the summer in hurricane season.
Simpson County, Miss., has historically been the most vulnerable to long-tracked storms. The area places second in the nation for F-3 to F-5 tornadoes, averaging about one every 25 years. The northern Mississippi and northern Alabama region is also a haven for heavy wind.
Hoosier Alley is the region from southern Michigan to southern Indiana, and from eastern Illinois to western Ohio, an area that turns into a hotbed for tornadoes from late spring to the early summer months. This region is the last place a tornado can begin to churn before the Appalachian Mountains in Pennsylvania and West Virginia muddle the opportunity to generate enough wind speed to become a tornado. Hoosier Alley is not as active as the first two tornado hotspots and is at its peak in the spring and early summer months due to location of the jet stream.
Georgia to North Carolina: A Newer "Alley"
Finally, a newer "alley" is starting to be more recognized. This new alley includes the northern portion of Georgia through the top of South Carolina extending toward the coast into the northeastern part of North Carolina. This region offers two impacting tornado threats, including active cold fronts in the spring and passing tropical cyclones in the summer and fall months. The double threat allows this region to be the fourth most tornado-ridden area in the country.
Since 1990, South Carolina has averaged around 28 tornados per year, and North Carolina has averaged 16 tornadoes per year. Although the region has not experienced an F5 tornado in recent years, a strong threat is always on one of the horizons. In 1998, North Carolina experienced a record 66 tornadoes, and South Carolina's record stands at 54 in 1995.
Tornados can strike anywhere at any given time as long as there is potential for dry air to clash with a predominant jet stream overhead. Make sure to check the AccuWeather.com Severe Weather Center when severe weather is a threat.