What was Andrea, the first named storm of 2007, doing off the Georgia coast, so far ahead of the official June 1 start date of the Atlantic hurricane season?
The early appearance of a subtropical storm like Andrea has happened before, though it's somewhat unusual. Storms of this type can even evolve into full-blown hurricanes, but Andrea was expected to dissipate this weekend.
Still, Andrea's emergence gives scientists an opportunity to study the storm's unusual features – its early timing, its formation outside the usual tropical-storm-incubation zone – to gain a better understanding of a class of storm that until recently has been overlooked and, therefore, difficult to forecast accurately.
The challenge of forecasting storms like Andrea – which carried tropical-storm-force winds and lashing rains – stems in part from the fact that they originate outside the tropics, in places forecasters aren't looking. Many have their genesis in mid-latitude storm systems coming off the North American continent, which masks their formation. As a result, forecasters have sometimes underestimated their intensity and failed to give adequate warning to areas subsequently hit hard.
Occasionally, subtropical cyclones can appear in unexpected places. A storm that crossed Lake Huron in September 1996 came to be known informally as "hurricane Huron." Like Andrea, it evolved out of a continental storm system. It displayed the high winds, tightly formed eye, and rain bands typical of tropical cyclones.
Although subtropical cyclones appear in various guises in other ocean basins, "the Atlantic basin seems to receive these subtropical-type storms with the most frequency," says James Elsner, a geography professor and tropical-cyclone specialist at Florida State University in Tallahassee.
Typically, they occur in spring and fall when a broad patch of low pressure spins off of a deep southerly trough in the jet stream. When this eddy meets up with cold air aloft and warm seawater below, the system can spawn thunderstorms. At some point – and this is where forecasting gets tricky, researchers say – the storm spawns enough thunderstorms to significantly warm the air at high altitude, allowing the storm to take on the characteristics of a tropical cyclone.
Because this formation usually happens at a latitude of about 30 degrees north, such storms are called "subtropical." But they also have appeared in the southern Atlantic. In 2004 hurricane Catarina evolved via this mechanism off the coast of Brazil. It was short-lived, lasting only four days. But it made landfall as a Category 2 hurricane, killing as many as 10 people and inflicting $360 million in damage.
Scientists have known of these storms for decades, but they've had a tough time figuring out where they fit in the storm-system spectrum. "There's no unique definition of what they are," acknowledges Christopher Davis, a researcher at the National Center for Atmospheric Research in Boulder, Colo. For a long time, the subtropical cyclone tag was tied to "anything that looks weird," he says.
As a result, some systems didn't get the attention they deserved, adds Jenni Evans, professor of meteorology at Penn State University in State College. That changed in 2002 when the National Hurricane Center began naming out-of-season subtropical storms.
Only within the past few years have researchers systematically reviewed weather records to try to identify these storms. In the North Atlantic alone, they have found 123 subtropical cyclones over the past 50 years.
That's an average of about two a year – "a significant contribution" to a season's storm total, says Dr. Evans. This is prompting researchers to better understand these storms to improve forecasts, she adds.