In this satellite image provided by National Oceanic and Atmospheric Administration, Hurricane Sandy's huge cloud extent of up to 2,000 miles churns over the Bahamas, as a line of clouds associated with a powerful cold front approaches the U.S. East Coast on Friday.
Three days before reaching land — a time when the National Hurricane Center usually puts a bulls-eye on a small stretch of coast — government forecasters were still talking about the possibility of the storm striking anywhere from Maryland to New York.
Their uncertainty was especially surprising because hurricane track forecasts have become so good in the past couple of decades. They are usually accurate five or more days out.
Yet during a press conference on Friday, James Franklin of the NHC was still deflecting reporters' questions about Sandy's track. "We cannot be precise at this stage about exactly where it will come in," he said.
Forecasters say Sandy just isn't like other hurricanes.
"The whole thing is unprecedented," Henry Margusity of AccuWeather told NPR's Melissa Block. "We've never seen anything like this."
For one thing, Sandy is about to start moving the opposite direction from a typical storm system.
Tropical storms in the Atlantic usually make a right turn and head out to sea as they travel north up the coast. But Sandy is expected to turn left and head straight inland.
That's because the storm is expected to encounter a highly unusual weather pattern. This pattern "blocks the system from going north and east and forces it to go more westward," said Louis Uccellini of the National Oceanic and Atmospheric Administration.
Another odd thing about Sandy is that it is expected to get slightly stronger as it moves north. Usually, hurricanes weaken as they reach cooler waters.
The reason for this strengthening is that Sandy is about to undergo a strange metamorphosis, forecasters say. Shortly before it reaches land, it will begin to encounter cold air from the north, and this cold air will change Sandy to something more like a winter storm.
As a practical matter, this means that instead of drawing its power primarily from warm ocean water the way a hurricane usually does, Sandy will be powered by pressure and temperature differences in the atmosphere, Uccellini says. So it can pick up strength when most storms would be weakening.
Becoming a winter-type storm also means Sandy won't have the usual structure of a hurricane, with a clearly defined eye. Instead it will be an enormous swirling mass of wind and rain, and even snow.
Of course Sandy isn't the first late-season storm to behave oddly. In 1991, right around Halloween, a hurricane named Grace merged with a powerful nor'easter to become what's still known as "the perfect storm."