Warm Winter Fuels Deadly Southeast Tornadoes
Scientists wonder if it's climate change or coincidence that's behind the region's string of tornado outbreaks, citing lack of long term data and conflicting theories about storm dynamics.
The unseasonably warm winters that have brought shirtsleeve temperatures to much of the Southeast have also had a downside.
The balmy, springlike weather has spawned record numbers of winter tornadoes the past two years, according to regional statistics - including an outbreak that killed 20 people in Mississippi and southern Georgia in late January. But researchers say it's too difficult to say whether the spike is likely to be a sign of things to come in an era of climate change.
The winter of 2015-2016 saw 71 twisters across the Carolinas, Georgia, Florida and Alabama, said Jordan McLeod of the Southeast Regional Climate Center in Chapel Hill, N.C. The current season has produced 79, and it's barely two-thirds of the way through, he said.
"We haven't even seen what February has in store yet, but we've already blown past the winter record," said McLeod, the center's regional climatologist.
For meteorologists, winter means the months of December through February. The previous seasonal high mark since 1995 was 59, a tie between the winters of 2007-2008 and 1997-1998. Most years in that period saw fewer than 30.
"It's not going to stand alongside the really extreme springtime months, but it's one of the most active wintertime seasons that we've seen, and it certainly does stack up to some of the more active springtime seasons that we've seen," McLeod said.
The past three years have set global temperature records. Sixteen of the hottest seventeen years have occurred since 2000. Each of the last three decades has been warmer than one prior to it. Warmer air carries more moisture, adding more fuel and energy for a potential storm to tap into. When warm, moist air collides with colder weather, that raises the potential for thunderstorms - and tornadoes.
That normally starts to happen in the spring. But with the mercury above normal across most of the Southeast so far this year, the number of twisters have been going up.
"We're definitely seeing warmer winters, and this winter is certainly no exception," McLeod said. "We're seeing warmer temperatures all across the Southeast region. That by itself adds extra fuel and energy that can be tapped into by any migrating storm systems."
But, while scientists have been more willing to pin some extreme weather events on climate change in recent years, they're much less likely to draw a connection between warming and tornadoes for several reasons.
First, detailed data on tornadoes dates back only to the 1950s. In the mid-1990s, the widespread use of Doppler radar systems started giving researchers a better count - which is why McLeod started his count in 1995.
"Nowdays, very few tornadoes go undetected," said Luigi Romolo, McLeod's counterpart at the Southern Regional Climate Center in Baton Rouge, La. Before the 1970s, many tornadoes either went unnoticed or weren't reported unless they hit something, he said.
That relatively short and spotty record makes scientists wary of identifying a trend, said Jeff Trapp, an atmospheric scientist and professor at the University of Illinois. And it takes a combination of circumstances to produce the rotation that marks a tornado.
But some recent studies point to a higher risk of heavy thunderstorms that can produce tornadoes, while others indicate there's more of a risk of still-damaging but less dramatic effects. Others note an increase in the number of tornadoes spawned by individual outbreaks, like the deadly chain of tornadoes that ripped through the South and Midwest in April and May 2011.
That April saw 364 people killed in a total of 758 twisters, including an outbreak that ripped through Tuscaloosa and Birmingham, Ala., killing 64 and inflicting an estimated $2 billion in damage. The next month, another 326 tornadoes left another 178 dead-most of them in Joplin, Mo., where 157 died.
But other recent research suggests the increasing number of tornado clusters might be the result of factors other than climate change.
"We don't have a lot of confidence that we can distinguish these conditions and break apart, say, only storms that produce tornadoes," Trapp said. So while researchers can't say the odds of a tornado aren't getting worse, they can't say they're getting any better, either.
"We look at this general, generic category of severe thunderstorms, and our projections are an increase in the frequency of the conditions that allow for their formation," Trapp said. "But how does this all shake down in terms of their distributions across hazards - if it means more hail and fewer tornadoes, or more wind and fewer tornadoes, or does it mean more of everything? That's the question we're still working on."
And while there's been a spike in Southeastern tornadoes the past couple of winters, McLeod said they don't seem to be getting bigger. In fact, the number of tornadoes that clocked in as a 2 or higher on the scale meteorologists use to rate the storms has gone down since the same period in the 1970s, he said.
But he warned that tornadoes can be just as deadly in the winter as in the summer - perhaps more so, because fewer people expect them.
"They think about winter as the season of snow or ice," he said. "They're not thinking about severe thunderstorms as a hazard they need to pay attention to." And when the nights are longer, "that can be a recipe for disaster."
"It's bad enough if you have a tornado coming at you during the day, but if it's at night and you're sleeping?" McLeod said. "The majority of the Georgia fatalities in this recent event were in the overnight hours when people were sleeping and caught off-guard."
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