tornado outbreak

April's ferocity was one for the record books: There were more tornadoes in April 2011 than in any month in U.S. history. The National Oceanic and Atmospheric Administration estimates there were more than 600 tornadoes in April, shattering the record of 543 set in May 2003.

With an estimated 327 deaths, the tornado outbreak April 25-28 was the third-deadliest on record, behind 1925 with 747 and 1932 with 332.

The staggering death toll from the tornado outbreak in April 2011-- the deadliest U.S. natural disaster since Hurricane Katrina in 2005 -- comes as the National Weather Service reports that the average lead time for tornado warnings was 24 minutes, and that warnings were in effect for more than 90% of the tornadoes. Tornado warning lead time is the difference from when the warning was issued and the time the tornado was on the ground, said weather service meteorologist John Ferree in Norman, Oklahoma.

Ferree said that during the past few years, average tornado warning lead time has been about 14 minutes. This compares with 3 to 5 minutes in the 1980s and early 1990s, before the installation of a national network of Doppler radars and the weather service's modernization.


Tornado warning accuracy is the percentage of tornadoes that occur within areas under warnings. The percentage has increased from around 30% to 40% in the 1980s and early 1990s to around 75% by the early 2000s, Ferree said.

Were it not for tornado warnings, the average annual death toll in the USA from tornadoes would be about 1,000 people, said Kevin Simmons, an economist at Austin College in Sherman, Texas. The Storm Prediction Center says an average 60 people are killed by tornadoes in the USA each year.

There have been an estimated 887 tornadoes in 2011. The year with the most was 2004, when 1,817 were reported, according to the Storm Prediction Center.