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Extreme tornado outbreaks have become more common
A tornado west of Laramie, Wyoming, on June 15, 2015. It passed over mostly rural areas and lasted some 20 minutes.
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Most death and destruction inflicted by tornadoes in North America occurs during outbreaks --large-scale weather events that can last one to three days and span huge regions. The largest ever recorded was in 2011 and spawned 363 tornadoes across the U.S. and Canada, killing more than 350 people and causing $11 billion in damage.
A new study shows the average number of tornadoes in these outbreaks has risen since 1954, and that the chance of extreme outbreaks like the one in 2011 has also increased.
While the researchers who conducted the study do not know the exact cause for these changes, Michael Tippett, lead author of the study and a climate and weather researcher at Columbia University's School of Applied Science and Engineering and Columbia's Data Science Institute, says, "It could be global warming, but our usual tools, the observational record and computer models, are not up to the task of answering this question yet."
Dozens of tornado outbreaks occur in North America annually. Some are small, spawning only a few twisters, but others, such as the 2011 "super outbreak," can generate hundreds.
Tornados are ranked by intensity on a scale of zero to five called the Fujita scale. Lower grades are less severe, causing light damage, while higher grades can tear bark off trees, rip houses from their foundations and turn cars into missiles.
For the study, the authors calculated the mean number of tornadoes per outbreak for each year as well as the variance, or scatter, around this mean. They found that while the total number of tornadoes rated F/EF1 and higher on the Fujita scale each year hasn't increased, the average number per outbreak has, rising from about 10 to about 15 since the 1950s.
Results of the study are expected to help insurance and reinsurance companies better understand the risks posed by outbreaks, which can also generate damaging hail and straight-line winds and have caused an average of $12.5 billion in insured losses annually over the past 10 years.
This research was supported in part by the National Science Foundation.
To learn more about the study, see the Columbia news story Extreme tornado outbreaks have become more common, says study. (Date image taken: June 2015; date originally posted to NSF Multimedia Gallery: July 12, 2016)
Credit: John Allen, IRI, Columbia University
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