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NSSL
Tornado Climatology
Where and when do tornadoes occur?
Tornadoes occur in many parts of the world, including Australia, Europe, Africa, Asia, and South America. Even New Zealand reports about 20 tornadoes each year. Two of the highest concentrations of tornadoes outside the U.S. are Argentina and Bangladesh. Both have similar topography with mountains helping catch low-level moisture from over Brazil (Argentina) or from the Indian Ocean (Bangladesh). About 1,000 tornadoes hit the U.S. yearly. Since official tornado records only date back to 1950, we do not know the actual average number of tornadoes that occur each year. Plus, tornado spotting and reporting methods have changed a lot over the last several decades.
Tornado season usually refers to the time of year where the U.S. sees the most tornadoes. The peak "tornado season" for the southern plains -- often referred to as Tornado Alley -- is during May into early June. On the Gulf coast, it is earlier during the spring. In the northern plains and upper Midwest, tornado season is in June or July. But, remember, tornadoes can happen at any time of year. Tornadoes can also happen at any time of day, but most tornadoes occur between 4-9 p.m.
Tornado Alley is a nickname for an area that consistently experiences a high frequency of tornadoes each year. The area that has the most strong and violent tornadoes includes eastern SD, NE, KS, OK. Northern TX, and eastern Colorado. The relatively flat land in the Great Plains allows cold dry polar air from Canada to meet warm moist tropical air from the Gulf of Mexico. A large number of tornadoes form when these two air masses meet, along a phenomenon known as a "dryline."
The dryline is a boundary separating hot, dry air to the west from warm, moist air to the east. You can see it on a weather map by looking for sharp changes in dew point temperatures. Between adjacent weather stations the differences in dew point can vary by as much as 40 degrees or more. The dryline is usually found along the western high plains. Air moving down the eastern slopes of the Rockies warms and dries as it sinks onto the plains, creating a hot, dry, cloud-free zone. During the day, it moves eastward mixing up the warm moist air ahead of it. If there is enough moisture and instability in the warm air, severe storms can form - because the dryline is the "push" the air needs to start moving up! During the evening, the dryline "retreats" and drifts back to the west. The next day the cycle can start all over again, until a larger eather system pushes through and washes it away.
Tornadoes kill an average of 60 people per year, mostly from flying or falling debris. The Tri-State Tornado of March 18, 1925 was the deadliest tornado in history, killing 695 people. It is also the longest tornado track ever known - 219 miles - across parts of Missouri, Illinois and Indiana.
Codell, KS was struck by a tornado on May 20 three years in a row: 1916, 1917, and 1918.
Understanding the threat posed by tornadoes in the United States - particularly the threat of strong and violent tornadoes - is valuable knowledge to everyone, but especially to weather forecasters and emergency management people. Knowledge about long-term patterns helps us be better prepared for natural disasters and could also help scientists detect shifting patterns in severe weather events caused by climate change.
SOURCE: NSSL
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Map indicating highest concentration of sig. tornadoes in the US for May (DFW/OKC Tours)
Map indicating highest concentration of tornadoes in the US for June (DEN Tours)
Other Tornado Climatology Facts
More tornadoes occur in the US than any other country in the world.
More tornadoes occur in Texas than any other state.
More tornadoes have occurred in Tarrant County, Texas (home of Tempest Tours) than any US county since 2000.
The peak of tornado season for North Central Texas occurs in early May.
Dallas-Ft. Worth is the largest metropolitan area in Tornado Alley.
The highest average annual concentration of violent-class (F4-F5) tornadoes occurs between Dallas-Ft. Worth and Oklahoma City.
The peak of tornado season for Central Oklahoma occurs in mid to late May.
The peak of tornado season in Northeast Colorado occurs in late June/early July.
June is the most active tornado month of the year for Northeast Colorado, Nebraska and the Dakotas.
Northeast Colorado is home to the highest concentration of tornadoes in the US for June.
More tornadoes occur in Weld County, just north of Denver, than any other county in Colorado.
Longer tours offer a greater chance of seeing tornadoes and other spectacular weather. You can increase your odds by chasing more days.
The impact of global warming on tornado activity
by Martin Lisius
It will probably take at least another decade before we are certain that global warming or a "climate change" is really underway across the plains of Tornado Alley. This region, which stretches from Texas to the Dakotas, is where more tornadoes occur than anyplace on earth thanks to the common interaction of three air masses. It is here that we can most easily study any significant climate change and its impact on tornado activity.
Just a few years ago, March 1st was considered by many storm chasers to be the unofficial start of tornado season on the Plains with the peak occuring from about mid-April to early July. However, since 2005 it is clear that spring is starting a little earlier. In fact, I have seen more tornadoes in March and April during this period, all in Texas. And, in 2009 I intercepted my first February tornado since I began chasing in 1987. It appears that the tornado spring season in Tornado Alley is now longer.
If global warming actually exists, then its logical that the Gulf of Mexico will be warmer than the previous norm. A warmer Gulf would lead to sufficient instability across Tornado Alley probably for a longer period of time each year. An earlier return of warm, moist air onto the Plains means an earlier spring. As long as rich Gulf air and moderate to strong westerly winds aloft come together, tornado season will remain in place.
For now, I believe strongly that the tornado season for Tornado Alley begins approximately mid-February and ends in late July with the peak running from mid-March to mid-July. Texas will continue to experience more tornadoes than any state, on average. There may also be a slight easterly shift to average tornado activity favoring areas of lower elevation than the previous norm, although I would like to monitor activity for a few more years before committing to that opinion.
March-April tornado intercept summaries since 2005:
Lamb-Hale-Swisher County, Texas tornado intercept, April 21, 2007
Brisco - Donley County, Texas tornado intercept, March 28, 2007
Thornberry, Texas tornado intercept, April 24, 2006
Paris, Texas tornado intercept, March 21, 2005
Tempest Tours is a corporate member of the American Meteorological Society, National Weather Association, National Tour Association and the National Safety Council. Storm footage can be licensed through StormStock a division of Prairie Pictures. We support and contribute to Autism Awareness and the Texas Severe Storms Association. Trademark and Copyright © Tempest Tours, Inc.
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