for National Geographic News
Hurricane Bill became the Atlantic's first major hurricane of the 2009 season earlier today, when it intensified into a Category 4 storm with sustained winds of more than 135 miles (217 kilometers) an hour (hurricane categories guide).
Thanks to rare weather conditions, Bill is expected to remain a major hurricane for an unusually long stretch as it heads north on a course roughly parallel to the U.S. East Coast, according to meteorologist Keith Blackwell of the University of South Alabama.
Hurricanes don't typically remain major—with wind speeds of at least 111 miles (179 kilometers) an hour—after passing north of about the latitude of Cape Lookout, North Carolina (map). After this point cooler waters and other hurricane-suppressing conditions typically prevail.
Bill's success is especially remarkable during a year notably unfriendly to Atlantic hurricanes, thanks mainly to the storm-disrupting effects of El Niño.
"Any storm that does well this year is lucky," Blackwell said. "Bill is lucky."
Hurricane Bill: Storm Track
As of 11 a.m. ET today, Hurricane Bill's eye was east of the Leeward Islands (map) at the northeastern edge of the Caribbean Sea.
The National Hurricane Center's forecast predicts Bill will remain an intense Category 4 hurricane through Saturday morning, when it's forecast to pass between Bermuda and the U.S. East Coast (map). Even if Bermuda doesn't suffer a direct hit, the British-administered island may be in for rough waves and swells, according to the National Hurricane Center.
After that, Bill will probably weaken slightly as it takes a northeasterly turn, though the storm is expected to remain a major hurricane at least through Sunday, when it should pass offshore of New Jersey.
That turn is expected to keep the hurricane well away from the U.S., though rip tides and strong waves are expected through the weekend.
Hurricane Bill Escapes El Niño
El Niño conditions over the Pacific Ocean have largely kept a lid on the 2009 hurricane season, Blackwell had said in an earlier interview.
El Niño is an unusually warm flow of water that sometimes forms off the northwestern coast of South America. The phenomenon causes a band of upper-level prevailing winds known as the jet stream to shift southward.
When the jet stream blows over the Atlantic Basin—which includes the Caribbean Sea and the Gulf of Mexico—the current creates wind shear, upper-level winds that can disrupt hurricane formation and development.
El Niños are especially unkind to Cape Verde storms like Hurricane Bill, which have the entire ocean to cross before they reach the Americas. Such storms form from a tropical depressions that blow off Africa's west coast near the island nation of Cape Verde.
Despite the hurricane-unfriendly conditions, Blackwell had warned that "oases of favorable conditions" could exist in the Atlantic Basin long enough to allow a powerful storm to form.
Hurricane Bill seems to have found just such an oasis.
By contrast, tropical storm Ana—the remains of which are soaking South Florida today—chugged into the Caribbean, where it ran into strong wind shear that tore the storm apart.
Lucky Hurricane Bill's northerly storm track, though, intersected with a jet stream traveling in a similar direction as Bill's winds. That stream won't necessarily supercharge the hurricane, but it won't weaken it, as opposing winds would have, Blackwell said.
Right now, he said, "Conditions are somewhat more favorable for hurricane formation in the tropical eastern Atlantic than we typically see when there's an El Niño."
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