Hurricane season gets an early start

Andrea, the first named storm of 2007, appeared well ahead of the June 1 start date of the Atlantic's official season.

from the May 11, 2007 edition

Page 2 of 2

Page 1 | 2

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.

1 | Page 2