Devastating December MidWest twisters “generational”

The recent swathe of devastating tornadoes in US are being compared to the historic Tri-State tornado outbreak of 1925 – and could be a potent indicator of climate change, according to experts.

“The events of Friday [10th December] are historic, legendary, generational – pick your favourite adjective,” said Guy Carpenter research partner, Dr Victor Gensini (pictured) of Northern Illinois University.

“You have to go way back to 18 March 1925, the Tristate Tornado that unfortunately killed 695 people, to find anything even remotely close to what happened on Friday evening.”

“I would expect it to be rated EF4 or EF5 on the Fujita scale, with a path length of very close to potentially 250 miles, making this really the most historic tornado event in history,” he added.

On average, there are 1,500 tornado events per year in the United States, with only 25 occurring in December. However, this particular outbreak was exceptionally rare in terms of the season, the intensity and the length of the storm paths, according to Guy Carpenter, with tornadic ‘supercells’ forming in Arkansas and Missouri, and then tracking hundreds of miles through Illinois, Tennessee and Kentucky.

A question of attribution

Some 1,200 twisters hit the US each year, according to the NOAA National Severe Storms Laboratory, but the fact remains that attributing a specific storm to the effects of climate change is difficult.

Less than 10% of severe thunderstorms produce tornadoes, which makes drawing conclusions about climate change and the processes leading up to them tricky, according to Harold Brooks, a tornado scientist at the National Severe Storms Laboratory.

However, there appears to be a degree of consensus. In the aggregate, extreme storms are becoming more common because we have a lot of warmer air masses in the cool season that can support these types of severe weather outbreaks, while the US is also likely to see more tornadoes occur in the winter as national temperatures rise above the long-term average.

Furthermore, although tornadoes in December are unusual, they are not unheard of – though the intensity of these twisters has raised eyebrows. Indeed, one of them likely broke a nearly 100-year-old record for how long a tornado stayed on the ground in a path of destruction.

Another Costly Severe Convective Storm Year

With expectations of a multi-billion dollar industry loss from the outbreak, this will be the tenth billion dollar severe convective storm industry loss in 2021, the second highest number of annual events behind last year with 13 severe convective storm (SCS) billion dollar loss events, according to Guy Carpenter.

The broker added that sggregate losses for 2021 look to be well behind the two highest SCS loss years of the last ten, which were 2011 and 2020: “Leveraging NOAA’s billion-dollar database, this year will likely be more comparable to the loss years of 2012, 2017 and 2016 from an SCS perspective. With only billion dollar or higher events being counted in the NOAA database, seasonal totals will be underestimated as smaller loss events less than $1 billion are not included in seasonal totals.”