Catastrophe modeller AIR Worldwide estimates the severe thunderstorms that struck the United States between May 20-27 will cause insured losses of between $US4-7 billion ($3.8-6.6 billion).

Principal Scientist Tim Doggett says May is normally the most active month for tornadoes and in that week more than 150 were confirmed.

More than 20 states were impacted and more than 1000 people injured. The city of Joplin, Missouri, was worst hit, with 151 people killed by the deadliest tornado since records began in the 1950s.

The May Catastrophe Study from Aon Benfield’s catastrophe model development centre, Impact Forecasting, says the Joplin tornado was confirmed at EF-5, the highest level on the Enhanced Fujita Scale.

Impact Forecasting President Steve Jakubowski says the first half of 2011 has seen substantial severe weather and flood activity in the US, causing billions of dollars of losses.

“To put this period into perspective, the last year to see more than one EF-5 or F-5 tornado touchdown in the US was 1998, yet in 2011 five EF-5 tornadoes have already been recorded.”

He says the American Farm Bureau Federation has reported insured crop losses of at least $US1 billion ($941 million) from flooding along the Mississippi River and similar-sized losses from river flooding in Canada during May.

AIR Worldwide’s Dr Doggett says none of the individual weather elements that gave rise to severe thunderstorms is unusual, but it is rare for them to align. He says this year is on track to rival the very active tornado season of 2008.

“It is also becoming quickly apparent that 2011 will surpass 2008 in terms of insured losses from severe thunderstorm activity,” he said. “Indeed, the two major outbreaks of this year – the first in late April, the second in late May – are the costliest on record.”