There is another important factor in the surge’s ultimate impact: coastal geography. "Storm surge is like real estate: location, location, location," Rhome says. In New York Harbor, the surrounding coastline acted as a funnel, channeling more and more of the incoming water into a narrower and narrower region. When a massive volume of water gets confined in that way, "it has no choice but to spill out and flood the surrounding land," Rhome notes. And, in places where the shore gently slopes out to sea, rather than precipitously drops off, an even larger storm surge results. New York City, with some 305 square miles of area, is particularly vulnerable to storm surge because of its more than 500 miles of coastline feature small bays, inlets and other potential funnels that can channel rising seawaters far inland.
The art of surge prediction
An important part of coping with such floodwaters is knowing how likely they are to hit, and how high they will be when they do come ashore. The National Hurricane Center's Storm Surge Unit bases its projections on the amount of water that will physically move atop land, called the "wet" line above sea level. Of course, predictions can never be perfect, Rhome (who is also a former hurricane specialist) notes of his unit, as the parameters that influence storm surge change hour to hour: precise location of landfall, strength of the winds, the angle of approach to the coast, how fast the storm is moving, how big it is, among others.
In fact, the NHC is one of the few such facilities in the world that offers multiple predictions of storm surge to help emergency planners cope. It starts with a computer model that takes into account data on the coast itself, including its contours, its depths, natural structures and man-made ones, and where the rivers enter and other factors. The computer then simulates storm surge based on input wind speeds, the speed of the storm itself and its total size, which are in turn based on the best projection of the NHC's human hurricane specialists. That single best guess is where most storm surge predictions end.
But even the best meteorologists with the best tools and the most experience cannot precisely predict any of those things, so the NHC runs the model multiple times with multiple variations of the storm inputs, such as wind speed or the total area of the storm. The level of a storm surge can change quickly with relatively small fluctuations in such factors. "It's very tricky," Rhome says. "Just a subtle change in the meteorology makes a huge difference."
For example, Hurricane Ivan in 2004 shifted its track, and its eye passed to the east of Mobile Bay rather than just to the west, where it had been expected based on forecasts. This directional change of less than 30 miles cut the actual storm surge by 10 feet, according to Rhome, pushing water out of the bay rather than into it. "Anyone who thinks they can predict landfall within 30 miles two to three days in advance doesn't know what they're doing," Rhome says.
Or take Sandy, which remained only the weakest level of hurricane, boasting sustained winds above 74 mph, despite having the lowest pressure ever recorded for any storm north of North Carolina—943 millibars just prior to landfall in New Jersey. Instead Superstorm Sandy's sheer size—with winds spread over a massive area of more than 1,000 square miles—generated the enormous surge of ocean waters. To appreciate the difference, think of a smaller storm as like running a finger through a bathtub—it won’t disturb much water—whereas a larger storm is like moving a whole arm through—you can make a significant swell.