The tornadoes that hit the US in April and May will cost the insurance industry $15bn, estimates reinsurance broker Aon Benfield.

The amount is nearly three times the 1990-2010 annual average for severe weather losses, the broker said in a report titled ‘United States April and May 2011 Severe Weather Outbreaks’.

The report examines the active stretch of severe weather that occurred across areas east of the Rocky Mountains, where at least eight separate timeframes saw widespread severe weather activity – including five separate outbreaks with losses in excess of $1bn.

The two most notable stretches occurred between April 22-28 and May 21-27, Aon Benfield said.

Risk modelling firm AIR Worldwide estimated the insured losses from the late May tornadoes would be between $4bn and $7bn. Fellow risk modeller Risk Management Solutions put the April tornado loss at between $3.5bn and $6bn.