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Waves crashed over a road in Winthrop, Mass., as Hurricane Sandy moved toward coastal areas Monday.
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Hurricane Sandy may be grinding closer to the East Coast with 90 mph winds and torrential rains, but the most devastating aspect is likely to be storm surge.
Simply put, storm surge is wind-driven water that is forced against the shore, piling up in low-lying areas where it can cause dangerous flooding. A number of factors can make storm surge worse: a massive storm with high winds headed straight for a region full of shallow coastal bays and inlets.
Sandy seems to have them all, says Chris Landsea, science and operations officer at the National Hurricane Center.
"If warnings about storm surge aren't heeded, there's the potential for horrific loss of life," Landsea emphasizes. "That's what happened during Hurricane Katrina."
The impact could be catastrophic when Sandy, which is about 1,000 miles in diameter, hits the continental shelf, where the water suddenly becomes shallower along the U.S. East Coast.
"It's like putting a fan on a plate full of water," Landsea says. "The fan will push the water right off. If you do the same thing with a bowl full of water, that's less likely to happen."
That only gets magnified when the surge reaches small inlets and triangular-shaped bays, which act as funnels for the water. This is why places such as Long Island Sound and Raritan Bay, which separates northern New Jersey and Lower Manhattan, are forecast to get storm surge as high as 11 feet above normal tide.
At the southern tip of Manhattan, Sandy's surge is expected to be more than a foot higher than it was when Tropical Storm Irene raked the area last year. New York Gov. Andrew Cuomo warned during a Monday news conference that the storm surge "is already at Irene levels."
As Hurricane Sandy moves closer to landfall, strong winds will pile water up at the southern tip of Manhattan. An already higher-than-usual high tide caused by the full moon will add to this large storm surge, potentially leading to record-breaking storm tide heights.
The destructive force of that much moving water — which weighs nearly 1,700 pounds per cubic yard — is enormous, especially if it is sweeping a debris field ahead of it.
Because of the nature of hurricane winds, storm surge is worse in areas that are to the right front side of the storm's eye as that's where the water gets driven ashore. As the winds swirl around, water is sucked back in a "blow out" tide on the left side, says Jeff Masters, chief meteorologist for Weather Underground. A good illustration of crushing storm surge is what happened on the Chesapeake Bay in 2003 during Hurricane Isabel.
"With Isabel, the winds were forcing water up the Chesapeake," Masters says. "With Sandy, if it stays east of the bay, it will actually do the opposite. You'll probably see low water once it passes."
But, he warns, Sandy is such a behemoth that it's still difficult to know what will happen. "Whenever you put that much water in motion that is being pushing up, it's going to be hard to predict," Masters says.
Low pressure near the center of the storm can exacerbate the surge.
"Think of it like sucking water through a straw," Masters says. "That's a secondary effect, about 5 percent. The wind is by far the biggest factor."
While much has been made about the full moon and its effect on the surge, Masters says he crunched the numbers and thinks that impact will be minimal — only a few inches.