In many places, forecasters warned, the storm surge could be as destructive as the hurricane itself, flooding low-lying areas before the storm even arrives with its winds and pelting rain.
"Storm surge will raise water levels by as much as 4 to 8 feet above ground level within the hurricane warning area from the North Carolina/Virginia border northward to Cape Cod, including southern portions of the Chesapeake Bay and its tributaries," said Daniel Brown, the warning coordination meteorologist for NOAA's National Hurricane Center, in a forecast this evening.
"Near the coast, the surge will be accompanied by large, destructive, and life-threatening waves," Brown said.
Near Kill Devil Hills, N.C., on the Outer Banks of North Carolina, water in Pamlico Sound was reported as much as 13 feet above normal, though precise numbers were hard to verify.
"The Manteo Causeway was 5 feet under water by the time it was too dark to observe," said Scott Summers of local radio station WCXL. "Water is chest high at the docks in Manteo."
A storm surge is, in its simplest terms, a bulge of water that a hurricane pushes out in all directions as it passes over the ocean. It can add several feet to the height of floodwaters created by the storm; in extreme cases, such as Hurricane Katrina in 2005, it can be more than 20 feet.
But the effect can vary tremendously from one place to another, especially along the nooks and crannies of the coast of the eastern United States. Some places may, in fact, see the eight-foot surges that NOAA warned about today; others may see very little.
Complicating matters is, of all things, the moon, whose gravity has such a powerful effect on high and low tides.
There is a new moon just after 9 p.m. on Sunday -- with the likely result that tides, shifting rhythmically, could be especially high during the daytime Sunday as the moon passes overhead.
"Higher than normal astronomical tides are occurring this weekend," Brown said. "Coastal and river flooding will be highest in areas where the peak surge occurs around the time of high tide."
Historically, NOAA warns, people do not die in hurricanes as they hit; instead, the most deaths happen when people come out after the storm has passed and try to make their way through floodwaters.
New York City, taking actions unprecedented in modern times, ordered 370,000 people to evacuate areas with the lowest elevations, especially along shorelines and near the wetlands along the south shore of Long Island. Smaller cities from Virginia to New England ordered their own evacuations.
Connecticut ordered people off state roads. Mass transit systems -- including New York's subways and buses, and Amtrak service from Boston to Washington -- were shut down until the storm passes.
Shelters were set up for people taking refuge from the floodwaters. New York City said it had space for 70,000 people. But as of Saturday evening, the city's office of emergency management said only 5,500 people had taken it up on the offer.
Part of that was probably a matter of timing. While the storm punished the coasts of North Carolina and Virginia, winds in the New York area were not forecast to pick up until well after midnight. AccuWeather, the private forecasting service, said they would spike in New York at 73 mph around 11 a.m. Sunday, and again at 76 mph around 5 p.m.