It would take a rare combination of currents, winds and timing to wash leaking oil from the Gulf of Mexico ashore anywhere above South Florida, scientists say.
It's far from impossible. Such a sequence has occurred before on the North Carolina coast, with devastating impact.
In 1987, an outbreak of toxic algae called a "red tide" slipped into the Loop Current that circulates in the Gulf of Mexico.
The water carried the algae out of the gulf, around the tip of Florida and up the Atlantic Coast on the Gulf Stream.
The warm, fast-moving Gulf Stream normally flows far off the Carolinas. Three tongues of the current carrying the algae flicked toward the coast. Winds shifted out of the northeast, pushing the outbreak into North Carolina's Onslow Bay, two months after it first appeared near Naples, Fla.
The result: 200 miles of contaminated shellfish beds and fisheries shut down; beaches deserted because toxic ocean spray caused respiratory problems in people; millions of dollars lost.
"You need the right - or the wrong - series of circumstances to get [oil] onto the North Carolina beaches, but it could happen," said Leonard Pietrafesa, a retired oceanographer from N.C. State University who researched the red tide. "It did happen, and so it could happen again."
Scientists say the leaked oil afloat now, like the red tide, may be whisked around the southern tip of Florida and up the East Coast by the Gulf Stream.
Beyond that, so many factors would determine its path - including weather, unpredictable eddies and the Gulf Stream's proximity to the coastline - that it's impossible to say whether or where it could come ashore.
Florida most at risk
If the oil reaches the Gulf Stream, the most imperiled area will be South Florida, from the Keys to a point close to West Palm Beach where the current is immediately adjacent to land, said Ruoying He, an N.C. State University oceanographer. He leads the Ocean Observing and Monitoring Group, which runs an elaborate and constantly updated model of ocean circulation off the Southeast.
Waves and sun-triggered daytime sea breezes that spring up close to shore could easily push the oil to land there, he said.
North of Palm Beach, the Gulf Stream angles farther offshore before coming relatively close to North Carolina's Cape Hatteras. He thinks it unlikely that spills moving up the stream would reach the U.S. coast anywhere north of Cape Lookout on the central N.C. coast.
Above Cape Hatteras, the Gulf Stream veers more eastward across the Atlantic toward Ireland.
The first step would be for the oil to reach the gulf's Loop Current, which becomes the Gulf Stream on the Atlantic coast.
A tendril of the current crept to within 25 or 30 miles of the spill Tuesday. By Wednesday afternoon, forecasts predicted winds would push the oil away from the current, said Robert Weisberg, an oceanographer at the University of South Florida.
"It's a waiting game," he said. "It's imminent that they come together, and I think it will happen, but when I don't know."
And if oil gets into the loop, he said, there's no question it will come up the East Coast.
Pietrafesa, the N.C. State professor emeritus, envisions this scenario: Several days of wind blow the gulf oil into the Loop Current. Once around the Florida Keys, the oil is on the shoreward side of the Gulf Stream as it moves north along the Atlantic Coast.
Southeast of Charleston, the stream meets an underwater formation called the Charleston Bump. The bump deflects the current offshore, but it then sometimes curls back and forms tongues of warm water.
If that happens, winds blowing from the north could blow those tongues at a 45-degree angle to the right of the wind's direction, toward Carolina beaches.
"The question really is what the local winds are going to be," said Susan Lozier, an oceanographer at Duke University.
Predominant winds in the Carolinas are out of the west, she added, making it more likely oil would be pushed offshore. Northeasterly winds that pound the Outer Banks - and could push the oil toward shore - are usually fiercest in winter.
The Outer Banks would likely form a protective shield around the rich nursery grounds of North Carolina's Pamlico and Albemarle sounds, said Richard Luettich, director of UNC's Institute of Marine Sciences in Morehead City. The barrier islands are broken only by narrow inlets.
Still, Luettich added, "The thing people don't realize is just how long this oil is going to be around, maybe a year or more. We may be dealing with this next fall, we could be dealing with this next spring."
Some of the chemicals in the oil will evaporate, he said, leaving a goo that might not be toxic but would coat everything it touches: beaches, shorebirds and turtles.