When Is Tornado Season?

This tornado was spotted seven miles south of Anadarko, Oklahoma, in May 1999. NOAA/Wikipedia

Tornado season may reach its peak in the spring months, but it lasts a lot longer than that and depends on where you live.

In the U.S., tornado season tends to move northward from late winter to mid-summer. In Southern states, tornado season is typically from March to May. In the Southern Plains, it lasts from May to early June. On the Gulf Coast, tornadoes occur most often during the spring. And in the Northern Plains, Northern states and upper Midwest, peak season is June or July.

The two regions with a disproportionately higher incidence of tornadoes are Florida and Tornado Alley. Florida's high tornado frequency is credited to the area's almost daily thunderstorms, as well as the many tropical storms and hurricanes that affect the peninsula.

Tornado Alley refers to a strip of land going north to south that covers the northern region of Texas, Oklahoma, Kansas, Nebraska, Iowa, the eastern edge of Colorado, southwest tip of South Dakota and the southern edge of Minnesota. Tornadoes in this area typically occur in the late spring.

In the Gulf Coast region, Dixie Alley — which has been experiencing a growing number of storms — refers to West Tennessee, West Kentucky, North Mississippi and North Alabama. These states experience a significantly later tornado season that occurs in the late fall from October through December.

What causes tornadoes?

Green skies and a wall of clouds accompany this tornado in El Reno, Oklahoma.
Green skies and a wall of clouds accompany this tornado in El Reno, Oklahoma. Daniel Rodriguez/Flickr

The most common explanation for the formation of tornadoes is the result of warm, moist air meeting cooler, dry air, and creating instability in the atmosphere. When the wind changes direction and increases in speed and height, it creates an invisible horizontal spinning effect in the lower atmosphere. Inside this updrift, rising air tilts the spinning air from horizontal to vertical, forming tornadoes that can be miles wide.

Tornadoes typically form during thunderstorms, but they can also accompany tropical storms and hurricanes. These tornadoes will most often be at the right and ahead of the storm path as it comes ashore.

But according to Roger Edwards of the Storm Prediction Center, the common answer given to how tornadoes are formed — warm, moist air meeting cool, dry air — is a gross oversimplification. He writes that recent results from the Vortex program, which is focused on studying and understanding tornadoes, suggest that tornado development is related to temperature differences across the edge of downdraft air, but mathematical modeling studies of tornado formation also indicate that it can happen without such temperature patterns. "Very little temperature variation was observed near some of the most destructive tornadoes in history on May 3, 1999," notes Edwards in the National Oceanic and Atmospheric Association's webpage on tornadoes.

Scientific explanations aside, there are some simple clues that indicate a tornado may be on its way. Signs of an oncoming tornado include a dark, greenish sky, wall cloud, large hail or a load roar, similar to the sound of a freight train rolling by. If you spot any of these signs, or if there is a tornado warning in your area, get to a basement, interior room or under a sturdy piece of furniture immediately.

Tornado duration and intensity

The details of how a tornado dissipates are debated by scientists, according to NOAA, but what is known is that tornadoes rely on a source of instability and a large-scale property of rotation. Tornadoes can range from a few seconds to an hour long, but most last less than 10 minutes. As a tornado is weaving its destructive path, varying storm conditions can cause it to weaken and die or pick up strength and become more destructive. On rare occasions, two tornadoes can hit the ground together in the same place, but the larger tornado will typically draw in and absorb the less powerful one.

Every year, tornadoes come and go, resulting in stories of personal loss and incredible near-misses. The best we can do is to prepare for them keep learning.