The devastating two-month period, taken as one block, now ranks as one of the worst in Allstate's history, alongside events like the 1994 Northridge earthquake in California and Hurricane Andrew in 1992.
In all of 2010, Allstate's catastrophe losses totaled $2.21 billion.
The company, which reported $1.4 billion in pretax losses for April, said on Thursday it lost an additional $600 million from seven events in May.
Allstate began reporting monthly disaster losses for the first time in April. It had been under pressure from analysts to offer more disclosure on its exposures.
For the industry as a whole, severe weather events in just one week in late May may result in insured losses of $4 billion to $7 billion, according to risk modeling agency AIR Worldwide. The two major U.S. tornado outbreaks in May and April were the costliest in history, the agency said.
Last week Travelers Cos Inc
(Reporting by Ben Berkowitz in New York and Rachel Chitra in Bangalore; Editing by Joyjeet Das and John Wallace)