Though commonly thought of as a product of hot spring afternoons on the plains of North America, tornadoes have a deadly habit of wintering over in the Southeastern United States.
Last year, 471 tornado outbreaks were reported in the region, setting a record over the past 50 years. According to new research, the uptick is indicative of a trend of increasing tornado outbreaks that dates back at least 50 years.
Pound for pound the cool season is also more dangerous — over the past 20 years in Georgia, more than 50 percent of tornado-related deaths and damage have come between November and February, according to a database maintained by the National Oceanic and Atmospheric Administration.
The main reason for that is an increase in wind shear, Stephen Konarik of NOAA said. Tornadoes typically require three main ingredients to form: warm, moist air, instability in the atmosphere, and wind shear.
During the spring and early summer months the sun takes care of the first two by heating the air and wind shear over the plains states makes the region a perfect breeding ground for tornadoes.
That all changes in the late fall and winter. Normally coursing high over Canada, the fast-moving polar jet stream dips south into the Southeastern United States. And while the atmosphere isn't very warm, the winds are strong enough to take what little instability there is and whip it into a frenzy.
Since they don't rely on atmospheric heating, cool season twisters also tend to form late at night, between 1 and 7 a.m.
"There's added danger from these tornadoes because they form at night," Konarik said. "And because the public is not expecting tornadoes in the winter time — that's another element."
Konarik is presenting a series of guidelines this week at the Severe Local Storms Conference in Savannah, Ga. to help meteorologists forecast cool season tornado outbreaks.
But the increase in overall number of tornadoes, which culminated in last year's huge twister turnout, may have nothing to do with the weather. In a separate study to be presented at the conference, researchers show that the overall number of reported tornadoes has gone up significantly in the past five decades.
"I hesitate to call last year a 'record' though, because I think we're seeing more people reporting tornadoes than ever before," Gregory Carbin of NOAA's Storm Prediction Center in Norman, Okla., said.
The predicted effects of global warming will likely add more heat to the atmosphere, and that could lend more fuel to thunderstorms and tornadoes in the form of atmospheric instability, Carbin said. But other ingredients, like wind shear, might wane in a warmer world.
"It's not clear at all what the ramifications of climate change are for tornadoes 10 to 20 years down the road," he said.