Aon Benfield said the US insured losses in April and May caused due to severe weather outbreaks have reached an estimated $15bn, nearly triple the annual average over the last 20 years.

The total economic losses during the same period were estimated at $21.65bn, according to the Aon Benfield, the reinsurance intermediary of Aon Corporation.

The intermediary said 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 over $1bn.

Aon said out of the eight timeframes examined in the report titled as ‘US April and May 2011 Severe Weather Outbreaks’, the two most notable stretches occurred between April 22-28 and May 21-27.

The late-April period witnessed the largest tornado outbreak, comprising 334 separate tornado touchdowns which led to catastrophic damage across the Southeast and the Tennessee Valley.

Impact Forecasting president Steve Jakubowski said the late-May stretch was highlighted by an outbreak that spawned a massive EF-5 tornado that destroyed a large section of Joplin, Missouri.

"In addition, it is worth noting that the Tuscaloosa and Joplin events will go down as two of the costliest singular tornadoes ever recorded," Jakubowski said.

As of mid-June 2011, at least $15.5bn in US severe weather insured losses had been recorded since the start of the year – 303% above the 1990-2010 annual average of $5.11bn, added Aon Benfield.