Researchers warn more emergency services are needed to respond to twisters – which appear to be grouping together.

Twin Tornadoes Hit In Nebraska

A twin tornado system tears across Nebraska in June

Source: Nwo Report

Decades of US tornado data has suggested twisters are ganging up and grouping together, according to meteorologists.

If their analysis is correct, residents living in high-risk areas are experiencing fewer tornado days and deadlier storms.

Researchers led by Harold Brooks, senior scientist at the US National Severe Storms Laboratory, compiled data on tornadoes that hit the country between 1964 and 2013.

Friends and family work to clean up debris at the home of Mary Ann Wemhoff outside of Pilger, Nebraska

The trend could have implications for insurers and emergency services

Their task was made difficult as the way twisters are observed and reported has changed over the years.

They discovered that although the average number of tornadoes per year remained relatively constant, there has been an increase in clusters of tornadoes since the 1970s.

In 1973 there were 187 tornado days. But by 2011 that had reduced to 110 days with nine of those days days experiencing more than 30 tornadoes each.

Emergency personnel walk along North Gloster Street after a tornado went through Tupelo Mississippi

Climate change could be responsible but the link has not yet been proven

“In effect, there is a low probability of a day having a tornado, but if a day does have a tornado, there is a much higher chance of having many tornadoes,” the researchers write.

“Concentrating tornado damage on fewer days, but increasing the total damage on those days, has implications for people who respond, such as emergency managers and insurance interests.

“More resources will be needed to respond, but they won’t be used as often.”

It is thought climate change could be responsible for the trend but scientists have yet to prove the correlation.

U.S. flag sticks out window of damaged hot rod car in suburban area after a tornado near Vilonia, Arkansas

In 2011 there were 110 tornado days, nine of which experienced 30 tornadoes

“The links in the chain connecting them aren’t complete yet,” said Mr Brooks.

James Elsner, an atmospheric scientist at Florida State University in Tallahassee, who was not involved in the study, said: “The greater heat and moisture in the atmosphere is a direct result of a warming planet, and the warming is greater at the poles than at lower latitudes, amplifying and slowing the jet stream.

“The climate change signal may be hidden in this idea of more concentrated tornadic activity.”