© Reuters/Gene Blevins
A severe thunderstorm wall cloud is seen over the area of Canton, Mississippi in this April 29, 2014 file photo.
Tornadoes in the United States are increasingly coming in swarms rather than as isolated twisters, according to a study by U.S. government meteorologists published on Thursday that illustrates another trend toward extreme weather emerging in recent years.
Looking at tornado activity over the past six decades, the study in the journal Science
found the total number of tornadoes annually remaining rather steady, averaging 495. Since the 1970s, there have been fewer days with tornadoes but plenty more days with many of them, sometimes dozens or more.
On the list of the 10 single days with the most tornadoes since 1954, eight have occurred since 1999, including five since 2011. That year alone had days with 115, 73, 53 and 52 twisters.
The meteorologist who led the study, Harold Brooks of the U.S. National Oceanic and Atmospheric Administration's National Severe Storms Laboratory in Norman, Oklahoma, said emergency management agencies and insurers should be prepared to deal more often with days with lots of tornado damage.