Opdyke tornado. (Courtesy viewer Dennis Fick)
Doyle Rice, USA TODAY
USA TODAY - The USA is seeing its quietest year for tornadoes in more than a decade.
Of course, for the victims of the savage EF-5 tornado that blasted Moore, Okla., in May, the season was anything but quiet. That violent tornado killed 24 people as it roared across the Oklahoma City suburbs.
Tornadoes have killed a total of 45 Americans this year, according to data from the Storm Prediction Center. Forty-two of those killed were in May.
Still, the actual number of tornadoes is clearly low.
"If we estimate the 2013 count at about 610 tornadoes through Aug. 20, the last year with fewer tornadoes through Aug. 20 was 2002, with 579," reports Gregory Carbin, warning coordination meteorologist with the Storm Prediction Center in Norman, Okla.
Typically, more than 1,000 tornadoes have hit the USA by this point in the year.
Will this be one of the calmer years for tornadoes since accurate records began in 1950? That depends on how you count them.
Carbin says that 2013 is not on track to be the year with the least tornadoes on record. "There have been at least 20 years in the past 60 years with fewer tornadoes through June than this year," Carbin says.
However, using another method for measuring tornadoes, meteorologist Greg Forbes of the Weather Channel says 2013 is the third-quietest year, in records dating back to 1950. His method takes into account the fact that more tornadoes are being reported now than in decades past, because of the expanding population and better observations.
According to Forbes, the only years with fewer tornadoes to date were 2002 and 1988.
Whatever the exact number, Carbin says the lack of powerful thunderstorms this year is keeping the number of tornadoes low.
A key ingredient in tornado formation is wind shear, which is the change in direction or speed of wind with altitude that creates rotation in the atmosphere, which helps spawn tornadoes, according to AccuWeather.
"Since tornadoes require strong shear, the low numbers are a result of a lack of shear, and that is tied to the lack of strong and intense weather systems, " Carbin says. "So, in a sense, the lack of powerful storm systems, especially during the late winter/early spring, is why the tornado numbers are lower than average."
The chance of catching up to normal or average tornado numbers through the end of the year is low, Carbin says, as the heart of the typical severe storm season is over. "However, a combination of an increase in tropical cyclone landfalls, and the possibility of a late-autumn tornado outbreak could turn the curve, so to speak."
Through June, states seeing unusually quiet years for tornadoes included Florida and Texas, Carbin says. States with above-average numbers include Oklahoma, Arkansas and Tennessee.