Number of tornadoes in outbreaks is increasing

Thursday 10th Mar 2016 by the Weather Club

Photo: The EF4 tornado that hit parts of Alabama in the 2011 tornado outbreak. Source: Wiki

The average number of tornadoes associated with any one outbreak has risen from 10 to 15 over the last 50 years. 

The study, published in Nature Communications, at the end of February, was co-authored by Michael Tippet and Joel Cohen, director of the Laboratory of Populations, based jointly at Rockefeller University and Columbia's Earth Institute.

To calculate the results, the authors calculated the the mean number of tornadoes per outbreak for each year since 1954. Their findings revealed that not only has the average number of tornadoes in each outbreak risen, but the chance of extreme outbreaks has also increased. 

The findings are of particular interest to both the forecasting and insurance industries, because the most destruction tends to come from tornadoes that are part of outbreaks. But what defines an ‘outbreak’?. Outbreaks consist of several tornadoes that are part of a large-scale weather event, that can last for days, and can affect areas spanning hundreds of miles. Tornado Alley, covering the states of the Great Plains in the US, is one of the areas most affected by tornado activity. The largest outbreak over the last few years occurred in 2011, with 363 tornadoes across the US and Canada, claiming more than 350 lives, and causing $11 billion worth of damage.

Previous research published in autumn 2014 found that the tornado season is beginning ever earlier, advancing by an average of one week compared with start times in 1950’s. Peak activity, on average, used to occur on 26th May, however, this has now shifted back to the 19th May. 

The question of climate change is always prominent, when discussing extreme weather events. Although scientists expect climate change to increase the frequency of atmospheric conditions that favour tornado outbreak, Tippet and Cohen are unable to confirm the cause in this case. It is accepted that global warming could well be a factor, however the computer models currently used are unable to pinpoint what is driving the changes.