Chicago: The United States is experiencing its deadliest year for tornadoes since 1936 and the fifth worst year on record, officials said on Thursday.
With the tornado season far from over, the 2011 toll rose to 523 on Thursday.
It includes four new victims who died in hospital nearly two weeks after the deadliest single twister in more than six decades destroyed much of Joplin, Missouri.
That toll now stands at 138, the Missouri Department of Public Safety said in a statement.
The mile-wide May 22 twister reduced a third of the Missouri town of Joplin to rubble, tearing apart homes, businesses, a hospital, churches and schools along a four-mile (six-kilometre) path of destruction.
It came after a horrific outbreak left 361 dead in several US states in April.
At least four people were killed on Wednesday when a rare twister outbreak struck Massachusetts.
The deadliest year on record is 1925, when tornadoes killed 794 people in the United States, followed by 552 deaths in 1936, 551 deaths in 1917, 540 in 1927 and 537 in 1986, according to the National Weather Service.