Oct 27, 2012

As the future track of Sandy starts to become clearer, the potential for damaging, dangerous storm surge is starting to become somewhat clearer as well.

There are still important uncertainties in Sandy's future path, but the prospect of a sharp northwestward turn toward the coast and a landfall somewhere from Long Island to the Delmarva Peninsula is becoming increasingly likely.

Michael Lowry, hurricane specialist at The Weather Channel, notes that despite the clearer picture of Sandy's future path, the details can play an important role in the final result. 

"Even though the models are coming into agreement on track and to some degree in intensity, subtle differences in the size of the storm, the intensity, the angle at which it hits the shore, the pressure -- these things that we have little skill in forecasting -- are the factors that determine exactly how the surge will unfold," Lowry said.

Lowry, whose stint at the National Hurricane Center's Storm Surge Unit included issuing official storm surge forecasts for last year's Hurricane Irene and this year's Hurricane Isaac, noted that storm surge is a phenomenon that's very sensitive to local variables. "It has to do with the topography -- barrier islands, flows, channels; the orientation of the coast, the slope of the ocean floor; and whether or not you live in a bay or an estuary where you get this piling up of water.

"It's all of these things that make it difficult to pinpoint specifics of surge."

That said, let's take a look at the range of possibilities for coastal residents and property owners.

New York City area

Referencing Saturday's 11 a.m. advisory, Lowry said that "a track slightly north of the official track [South Jersey landfall] would push more water intoNew York Harbor. The higher winds would be situated in that area."

Water would be pushed north into New York Harbor from Lower Bay, and westward from Long Island Sound via the East River.

Lowry noted that Raritan Bay will be "more of a focus for storm surge, in terms of relative magnitudes," with a track into New Jersey. This would concern places like Perth Amboy, N.J. 

But the entire New York City area will be at risk. "You can look at what Irene did," Lowry added, in terms of the potential threat to the subway system. "The planning should be similar to that for Irene at this point in the forecast."

The team of hurricane specialists at The Weather Channel notes that there is the potential for Sandy's storm surge to be worse than that of Irene if the New Jersey track verifies.

New Jersey

"They're in the bull's-eye right now," Lowry said early Saturday afternoon. "If you're anywhere along the Jersey shore, you need to pay close attention to this."

Easterly winds blowing directly onshore would lead to high surge.

Lowry added that wave action on top of the surge would only add to the damage. "Water tears things apart, but the waves make it worse in terms of the possible destruction," he said.

In the event of an offshore wind, there could be some water buildup on the bay side of New Jersey's Barnegat Peninsula, a long barrier peninsula that includes resort towns such as Seaside Heights and Point Pleasant Beach. Barrier islands further south along the Jersey shore could face the same problem.

Long Island and Southern New England

"A lot of what's happening in Long Island Sound is dependent on the direction of the track at landfall, which will obviously affect the wind direction," Lowry said.

A southeasterly wind will pile water up on the northern end (the Connecticut side) of the sound. During Irene, both sides dealt with water rise as the wind changed direction with the passage of Irene due north through the region.

If the New Jersey landfall scenario verifies, staying south of New York City, Lowry said the Connecticut and Rhode Island coasts would be more affected.

Speaking of Rhode Island, a more northerly track (still depicted by a few computer forecast models) would create more of a concern there. Lowry noted that we can't rule out a "significant storm surge" intoNarragansett Bay.

Even if the storm takes the New Jersey track, some water level rise would still occur based on the large size of Sandy's wind field.

"But there's a difference between 8 feet and 4 feet of storm surge," Lowry said. "Right now they're not under any forecast for even those kind of numbers."

The bottom line for southeast New England is that it's too soon to assign great confidence to specific storm-surge values.

Background

Storm Surge Forecast 

Delaware Bay to North Carolina

Delaware Bay should expect significant surge, even if the ultimate landfall is in New Jersey, because while Sandy is farther south, water can be pushed in.

Indeed, by late Saturday afternoon, water levels were already over a foot above normal at several tidal gauges in Delaware Bay well in advance of Sandy.

Likewise, the Atlantic coast of the Delmarva Peninsula can expect water rise as long as winds are onshore, mainly while Sandy remains to the south.

For Chesapeake Bay, while there could be 1 to 2 feet of water rise in the upper Chesapeake, the most vulnerable area is farther south. The Hampton Roads area around Norfolk could see 2 to 4 feet of storm surge because of a northeasterly component to the wind, pushing water toward shore there.

Finally, for eastern North Carolina, the situation is complex.

"I've done site surveys and built surge models for this area," Lowry noted, saying the interaction of Albemarle and Pamlico Sounds and the barrier islands of the Outer Banks creates that complexity.

North of Hatteras, he said, dune erosion and possible overwash of Highway 12 will be a concern.

Then, as the storm moves by, northwest wind flow will push water from the sounds toward the ocean, creating problems for Cedar Island and parts of Carteret County.

http://www.weather.com/news/weather-hurricanes/sandy-storm-surge-ne...

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