When is tornado season?
The season reaches its peak at different times in the United States.
Fri, Oct 07, 2011 at 02:46 PM
Photo: Wikimedia Commons
In April of 1998, right in middle of tornado season, a particularly destructive tornado hit Dunwoody, Georgia, causing the tragic death of John Janisch, destroying homes and businesses, and uprooting trees that were centuries old.
Michelle Melamed, a 24-year-old student at Emory University, remembers that night vividly. The way she sees it, a miracle of timing saved her brother’s life. “At about 10 o’clock at night, the dogs started going crazy and we noticed the sky was pale pink and it was very windy,” she said. Lightning covered the sky, and no one was able to sleep, she added.
Melamed’s older brother Eli got out of bed to go downstairs, and moments later a giant tree fell through the roof and across his bed. A total of 22 trees fell on the Melamed’s home, the first one they had lived in after emigrating from South Africa several years earlier.
Because she lives in Georgia, Melamed will likely face future storm warnings each year when tornado season reaches its peak in the spring months.
Tornado season by region
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 in 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 their almost daily thunderstorms, as well as the many tropical storms and hurricanes that affect the Florida 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 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?
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.
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 theories and results from the Vortex program 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 any of these signs are spotted, 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
NOAA says that the details of how a tornado dissipates are debated by scientists, 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.
It’s been over a decade since the Dunwoody tornado, but for the Melamed family and many other tornado victims, life changed forever in one night. Of course, tornado season returns each spring in Georgia.
“For several years after the tornado, I wouldn’t sleep away from home,” says Michele Melamed. “I’ve grown out of the fear, but I still experience heightened concern during thunderstorms.”
Related weather stories from MNN & partners:
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- Safety precautions for tornadoes
- Extreme protection for extreme weather (from Allstate)