Robert Stolarik for The New York Times
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Like a hand pushing water in a bathtub, the winds of a hurricane push the water of the Atlantic Ocean. When the windblown water runs up against land, the water piles up and flows inland. That describes a storm surge.
"It's almost a little bit like a tsunami," said Klaus H. Jacob, a scientist at Columbia University's Lamont-Doherty Earth Observatory.
When Hurricane Sandy makes landfall late Monday or early Tuesday, the pulse of windblown water will be "in a word, bad," Dr. Jacob said. "It's of course still somewhat uncertain. It all depends on the exact timing."
Storm surges like those accompanying Hurricane Sandy as it churns north are, at their simplest, a function of strong winds driving too much water into too small a space. But other factors, some of which will come into play as this storm approaches the New York area, can combine to make surges higher and more destructive, experts said.
"A storm surge is really caused by one thing," said Pat Fitzpatrick, an associate research professor at Mississippi State University who studies the phenomenon. "When a storm is approaching land, it starts to encounter shallow water. The water tends to pile up."
The height of a surge depends to a great extent on how shallow the water is near the coast. "The shallower the water is, for longer distance, the more vulnerable an area is," Mr. Fitzpatrick said.
The New York area has extensive shallow water offshore, and was expected to see some of the largest surges — National Weather Service computer models were predicting a storm surge of 6 to 11 feet at Battery Park at the lower end of Manhattan. The surge also could coincide with high tide at about 9 p.m. Monday, with tides even higher than usual because of the full moon.
"It's kind of a worst-case scenario for the New York Harbor area," said Alan Cope, a meteorologist with the National Weather Service in Mount Holly, N.J.
Hurricane Sandy is much larger, with greater surge potential, than the devastating Tropical Storm Irene last year, with winds already up to 60 miles an hour over an area more than 500 miles northeast of its center. Adding to the potential for damage was the hurricane's unusual track. The expected path northwest will trap water against the shores of New York and New Jersey, Mr. Cope said.
"It's going to blow the water from east to the west and pile it up in Raritan Bay and also pile it up in the western end of Long Island Sound," he said.
Many Atlantic hurricanes move parallel to the coast for long distances and then, pushed by high-level winds, veer northeast, eventually "spinning out," or losing their energy, over the North Atlantic. But by the time it nears New York on Monday, Hurricane Sandy will have traveled hundreds of miles from the coast for several days, picking up enormous amounts of water over the open ocean.
The storm's winds, which are rotating in counterclockwise, are creating an area of strong ocean waves north and east of the storm's center. "That amplifies the surge," said Louis Uccellini, director of the National Centers for Environmental Prediction, part of the National Weather Service.
As Hurricane Sandy approaches New York, forecasters expect it to hit a roadblock. Instead of veering northeast and out to sea, the storm will be forced to move west by a strong and persistent region of high-pressure air over southeastern Canada and southern Greenland.
"It's quite unusual to have this westward component of motion to a hurricane track," said Jeff Masters, director of meteorology for the Web site Weather Underground.
As it moves west, the hurricane is expected to make landfall in New Jersey, perpendicular to the coast. Such a head-on hit can produce worse surges than a glancing blow because more water can be driven into estuaries and harbors.
"The bottom line for New York and New Jersey and Long Island Sound is that they are going to have the worst of the surge and coastal inundation," Mr. Uccellini said.
But strong surges are only one aspect of this storm that makes it potentially deadly, he added.
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