We tend to focus on watching wind speed as a hurricane approaches, but as the monster storm that is Hurricane Sandy is reminding us, it’s not the winds themselves that cause the most damage—it’s the storm surge. The waves that come crashing in look (and are) impressive, but the real problem is the higher-than-normal sea level that boosts those waves up onto the land.
This is mostly due to a fairly simple process—the constant and violent winds that push water landward and keep it piled up there. Think of blowing across the surface of a hot cup of tea. As long as you keep blowing, the “tea level” on the far side of your cup will remain higher. Since Hurricane Sandy is spinning, the storm surge will be focused on its northwestern side, where the winds are blowing toward the coast. That means northern New Jersey, New York, and Connecticut.
The angle at which the seafloor drops away from the coastline affects the size of the storm surge, as demonstrated by these animations on the National Hurricane Center’s website. The shallower the slope, the easier it is for wind to pile up the water since it’s unable to escape downward and back out to sea.
This explains some of the local variation in expected storm surge around the area of the East Coast where Sandy will make landfall. Maps of those projections can be found on the National Hurricane Center’s website as well as Google’s crisis map for Sandy (which also includes information on weather conditions and emergency shelters).
Another factor in storm surge is atmospheric pressure. While it plays a small role in relation to the effects of winds, it illustrates the remarkable magnitude of these dangerous weather systems. The ocean surface bulges upward near the low atmospheric pressure at the center of the hurricane, as water from the surrounding area (where atmospheric pressure is higher) is pushed in. Given Sandy’s current conditions, barometric pressure will add from several inches to as much as one foot of storm surge.
Unfortunately, Sandy’s landfall coincides with tonight’s spring tide—the peak tide that occurs every two weeks with a new or full moon. At that time, the tide will push the storm surge a little bit higher than it might have been on a different day.
All this adds up to projections of storm surge that could reach 10 to 12 feet in some locations. Peak tides are expected to arrive in most of the affected areas this evening.
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