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Tornadoes are one of the most awe-inspiring yet deadly weather phenomena in the world. The United States is home to the most reported tornadoes on the planet, averaging more than 1,000 each year since 1990. So, why is that? Due to its topography, especially the flat terrain in the central regions, along with a mix of additional factors, including moisture from the Gulf, thunderstorms are created, and from them tornadoes can form. With expert insight from meteorologist Tony Merriman, we will do a deep dive into what makes the U.S. environment ripe for tornadoes and which regions of the country are most likely to experience them, from how tornadoes form to the states with the highest odds of seeing a tornado.
How Tornadoes Form: The Atmospheric Ingredients
Meteorologist Tony Merriman has studied tornadoes throughout his career, and they remain his favorite weather phenomenon to predict. In his words, here are the "atmospheric ingredients" needed for a tornado to form:
Atmospheric Moisture
Atmospheric moisture (also called water vapor) is one of the main ingredients needed in order for thunderstorms to develop. Without water vapor, there wouldn't be enough moisture for clouds to form. Surface dewpoint temperatures are used to assess the amount of available moisture. Dewpoints in the 60s and higher indicate ample available moisture for thunderstorms. The primary source of water vapor in the U.S. is the marine layer over the Gulf of Mexico. Oftentimes, this moisture layer can drift as far north as the U.S.-Canadian border during the late spring and summer months.
Instability
The greater the temperature difference (hot at the surface, cold aloft, or both), the more unstable the atmosphere. Oftentimes, it will be a really hot day before thunderstorms develop. It might sound counterintuitive, but clear skies are often needed to warm the Earth's surface before big thunderstorms develop.
Lift
The primary lifting mechanism for tornadic storms to initiate is usually along cold fronts. However, any kind of front would suffice to initiate thunderstorms. A cold front is the boundary between a warm, moist air mass (think marine layer from the Gulf of Mexico) and cold, dry air rushing in from colder climates, mainly from Canada. Orographic lifting is another lifting mechanism, primarily observed in the west. Mountains and mountain ranges force air upwards, leading to thunderstorms to initiate over the tallest peaks.
Spin
The most vital ingredient for tornadoes is atmospheric spin. Thunderstorm updrafts can begin to spin when there is sufficient wind shear (a change in both wind speed and direction with height) throughout the atmospheric column.
States With the Highest Odds of Seeing a Tornado
Now that we have an overview of how tornadoes form, here are five of the top tornado-prone states along with tornado statistics for each one:
1. Texas
- Average Annual Number of Tornadoes (1991-2020): 142
Due to its sheer size and the perfect mix of atmospheric ingredients, Texas has the highest average number of tornadoes of any other state. Its panhandle and parts of northern Texas are part of Tornado Alley, but that doesn't mean that the rest of the state is immune from deadly storms. A mix of warm, moist air from the Gulf of Mexico and cold, dry air from the Rocky Mountains to the west is a recipe for tornado formation. The peak tornado season in Texas, Kansas, and Oklahoma runs from May through June, when conditions are most favorable for tornadoes to form, though tornadoes can occur at any time of the year.
2. Kansas
- Average Annual Number of Tornadoes (1991-2020): 87
If you've ever seen the classic movie, "The Wizard of Oz," it probably comes as no surprise that Kansas is one of the top states for tornado activity. In fact, many people often think of tornadoes at the mere mention of Kansas (but we know the state is much more than that!). Grouped into Tornado Alley along with Texas, Oklahoma, and Illinois, which are also on this list, Kansas's location in the flat, wide-open Great Plains makes it enticing for storm chasers to view some of its largest and strongest tornadoes at a "safe" distance. However, the state has had its fair share of deadly tornadoes. Check out the most historic Kansas tornadoes for more.
3. Oklahoma
- Average Annual Number of Tornadoes (1991-2020): 66
Like in Texas, tornadoes in Oklahoma typically form when warm, humid Gulf air mixes with colder air from Canada and the Rockies. The western portion, plus the panhandle of Oklahoma, is within Tornado Alley, and the state has experienced some of the strongest tornadoes on record. The NOAA National Severe Storms Laboratory is actually located in Norman, Oklahoma, an ideal spot in Tornado Alley for researching and forecasting tornadoes. If you're curious to see which Oklahoma county experiences the most tornadoes on average per month, visit the National Weather Service website.
4. Florida
Average Annual Number of Tornadoes (1991-2020): 58
Because Florida experiences frequent thunderstorms, hurricanes, and tropical storms, the likelihood of tornadoes is higher than in most other states. While it's not located within the traditional Tornado Alley, it's almost like a miniaturized version of it. And sometimes, waterspouts that form off the coast over tropical waters turn into destructive tornadoes once they reach land. These mainly occur during the summer rainy season, although tornado season is year-round in Florida.
5. Illinois
- Average Annual Number of Tornadoes (1991-2020): 53
Illinois has seen more tornadoes than usual in recent years, with most occurring between April and June, making it part of the evolving Tornado Alley area. The combination of warm air from the Gulf and cold air from Canada and the Northern Plains, as well as a strong jet stream, produces an ideal environment for tornadoes to form in the Land of Lincoln.
Why Does Tornado Alley Keep Shifting?
Traditionally, Tornado Alley has included the Central Plains states, including parts of Texas, Oklahoma, Kansas, Nebraska, and South Dakota. However, there are no strict boundaries where tornadoes occur or which states are included during peak tornado season in the United States, though it seems to be shifting eastward toward other states in the Midwest along with the Southeast. Included among these states, often referred to as "Dixie Alley," are Mississippi, Louisiana, Arkansas, Georgia, Tennessee, North Carolina, South Carolina, Kentucky, and Missouri.
Common Tornado Warning Signs to Watch For
So, how do you know if a tornado is headed your way? There are several ways communities are warned, and then there are signs and sounds to look for in the sky. Here are some of the most common tornado warning signs to look out for:
- Listen for outdoor sirens (if your town or city has them).
- Look for a rotating cloud shaped like a funnel or a rotating cloud of debris.
- Listen for a sound similar to a freight train.
Check the National Weather Service Tornado Safety page and the NOAA Storm Prediction Center website for any issued warnings, watches, or emergencies, as well as severe weather safety information.
Tornado Safety Tips
It is best to have a plan in place before tornadoes are even a threat. If you're out on a tornado watch day, be aware of your surroundings and have a plan to take shelter immediately if the weather turns violent. Good options for sheltering are a tornado storm shelter (underground), a basement, or a windowless interior room of a well-constructed building. Bad options for sheltering are mobile homes, manufactured housing, large open rooms like gymnasiums, vehicles, or under a highway overpass. Check the FEMA Tornado Preparedness page for shelter-in-place guidance.
States With the Highest Chances of Seeing a Tornado
Whether or not you live in one of the top five states with the most tornadoes, it's always good to check your local weather and websites like the NOAA Storm Prediction Center. Remaining alert and prepared during bad weather can help keep you and your family safe. If you live in a state that's prone to getting hurricanes, learn about the ways you can prepare for hurricane season. You can also learn about the wildest weather states in America.
