He says the severe weather “funneled across a corridor that stretched from Lake Superior to central Texas and east through Missouri, Tennessee, Kentucky, Ohio and to the East Coast, impacting more than 20 states in all.”

Doggett states that the individual meteorological elements that caused the tornadoes are not unusual. “Large, strong jet stream disturbances happen occasionally; persistent low-pressure frontal systems are common, especially in spring. And the storms that developed occurred where they are expected to occur at this time of year.”

However, he adds, “What is unusual is for all of the factors that contribute to the development of severe thunderstorms to have aligned themselves so optimally in the same place at an opportune time. To get optimal intense instability, shear and lift all in the same place for a long period of time is a relatively rare circumstance.”

AIR says Missouri was most severely hit from May 20 to May 27. Damage to the state includes the May 22 tornado strike in Joplin.

Kansas saw 14 tornadoes touch down on May 21 alone, with the city of Reading hit the hardest. Texas, Indiana and Minnesota were also impacted by the storms.

Meanwhile, Pembroke, Bermuda-based reinsurer Partner Re Ltd. says it expects losses related to the tornado outbreak in April to be approximately $50-$70 million

Partner Re says the net loss for this event, which will be recorded in the company's second-quarter 2011 results, will primarily impact the company's catastrophe, North America and global (Non-U.S.) specialty subsegments.

Loss estimates are pre-tax, net of retrocession and reinstatement premiums, and relate to all reinsurance contracts and insurance-linked securities expected to be impacted by these events, Partner Re adds.

In late May, catastrophe modeler Risk Management Solutions said losses from the April storms will likely be in the range of $3.5 billion to $6 billion.