for National Geographic News
The Atlantic hurricane season just began Monday, and already forecasters are tweaking their predictions.
With an El Niño looking increasingly likely later this summer, Colorado State University meteorologists lowered their hurricane forecast this week.
Forecasters Phil Klotzbach and William Gray now predict 11 named tropical storms will form in the Atlantic Basin in 2009.
Five of the storms should be hurricanes, meaning they'd have sustained winds of at least 74 miles (119 kilometers) an hour. Two of the hurricanes should be major hurricanes, with winds of at least 111 miles (179 kilometers) an hour, they said in a statement.
In April, Klotzbach and Gray had predicted 12 tropical storms, 6 hurricanes, and 2 major hurricanes. An average Atlantic hurricane season, which ends each year on November 30, sees about ten tropical storms, six hurricanes and two major hurricanes.
(See hurricane pictures.)
How El Niño Tames Hurricanes
An El Niño is an unusually warm flow of Pacific waters that in some summers forms off the northern coast of South America.
The phenomenon causes a band of upper-level prevailing winds known as the jet stream to move southward.
Blowing over the Atlantic Basin, which includes the Caribbean Sea and the Gulf of Mexico, the jet stream creates wind shear—changes in upper-level wind speed or direction—which can disrupt hurricane formation, said Rusty Pfost, meteorologist in charge of the National Weather Service office in Miami.
The last El Niño formed in 2006, and that summer's hurricane season was uneventful for the U.S. Gulf and Atlantic coasts.
Why an El Niño Looks More Likely Now
Meteorologist Jeff Masters, producer of the Weather Underground Web site, said waters in the equatorial eastern Pacific have been steadily warming all year, and this makes it more likely that an El Nino will form.
"As of this week, it's right at the threshold of El Niño conditions," Masters said. "If it stays the way it is for three months, it will be classified an El Niño."
Still, the National Weather Service's Pfost noted, powerful hurricanes sometimes form in otherwise uneventful seasons.
Hurricane Andrew, the third most powerful recorded storm to make U.S. landfall, formed in the quiet summer of 1992.
"It only takes one storm like Andrew to make it a bad hurricane season," Pfost said.
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