Along the mid-Atlantic coast, storm surge is already starting to build, Uccellini said. NOAA's Coastal Services Center chief Margaret Davidson said to expect "bodacious impacts" from both surge and inland flooding.
The surge — in which water steadily increases from the ocean— will be worst in the areas north of where Sandy comes ashore.
New York will have the most intense storm surge if Sandy comes ashore anywhere in New Jersey, Uccellini said. Only if it arrives farther south, such as Delaware, will New York see a slightly, only slightly, smaller storm surge.
In general, areas to the south and west of landfall will get the heaviest of rains. Some areas of Delaware and the Maryland and Virginia peninsula will see a foot of rain over the several days the storm parks in the East, Uccellini said. The rest of the mid-Atlantic region may see closer to 4 to 8 inches, NOAA forecasts.
The good news about inland flooding is that the rivers and ground aren't as saturated as they were last year when Hurricane Irene struck, causing nearly $16 billion in damage, much of it from inland flooding in places like Vermont, Uccellini and Masters said.
The storm, which threatens roughly 50 million in the eastern third of the country, began as three systems. Two of those — an Arctic blast from the north and a normal winter storm front with a low-pressure trough— have combined. Hurricane Sandy will meld with those once it comes ashore, creating a hybrid storm with some of the nastier characteristics of a hurricane and a nor'easter, experts have said.
AP writer Jennifer Peltz in New York contributed to this story.
Seth Borenstein can be followed at http://twitter.com/borenbears
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