Terrifying twister clusters a sign of the future, say scientists

On Tuesday, 5 tornadoes touched down in the Atlanta, Georgia region, all part of the same storm system. If scientists are correct, terrifying clusters of twisters like this are becoming the norm.

In a new study published this week in the journal Science, researchers looked at 60 years worth of weather data across the United States, counting up all tornadoes and found that while in the 1970s there were about 150 days a year with at least one twister, that number has now dropped to about 100 days.

While on the surface this may appear to be great news for folks in tornado country, the study discovered that on the days when there is tornado activity, they appear to occur in clusters, making them potentially much more devastating.

“We started from looking at the monthly records and then went back to looking at daily distributions, and what we found was there was a variability of tornado occurrence that appears to be increasing without changing the average occurrence,” explained study lead author Harold Brooks, research meteorologist with the National Oceanic and Atmospheric Administration’s (NOAA) National Severe Storms Laboratory in Norman, Oklahoma in an interview with Yahoo Canada News.

According to NOAA, there have been 966 tornadoes across the U.S. in 2014 so far, ranking second lowest in number since 2005. In terms of absolute tornado counts, the U.S. has had the most by far with an average of over 1,000 twisters recorded annually, while Canada has only about one-tenth of that per year.

The study also found that F0 tornadoes, the weakest on the scale used to measure their intensity, appears to have increased from about 100 per year in the 1950s to about 800 a year in the 2000s. Brooks attributes this rise to a number of factors, including better awareness and better efforts to collect reports.

The data on tornadoes ranking at least F1 and above shoes that they, too, are appearing on fewer days per year, but on days when they do appear, there are more likely to be multiple tornadoes.

‘Takeaway is that we’re seeing an increase in variability of tornado occurrence without a change in the average,” said Brooks.

“More periods with lots, more periods with very few or none.”

However Brooks and his team are not sure yet what environmental conditions are exactly driving this clustering effect. “How such a change would relate to the increase in global temperature, if it relates at all, is unknown at this time,” Brooks and his team wrote in their paper.

"Nevertheless, if the variability continues to increase, it could lead to an even greater concentration of tornadoes on fewer days.”

At this point, the jury is still out on a definite link between tornado clustering with climate change. But if we want to explore that possible link, Brook says we will have to improve our statistical models and understanding of the relationships between the large scale weather conditions, and the much smaller-scaled tornado occurrences so that we can draw more informed conclusions.

In terms of direct, practical implications of these findings, it may end up affecting tornado preparedness, including response and recovery.

“If there are more “big” [tornado acitivty] days, then emergency management on the state and federal level and insurance/reinsurance industries may need to adjust their approach to resource management,” warned Brooks.

“They’ll need resources to respond to big events more often, but those resources will set around unused for long periods of time.”

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