Predicting When the Next Sandy Will Hit

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The day after Sandy hit, New York Governor Andrew Cuomo repeated a joke he said he had told President Barack Obama in the days leading up to the storm: “We have a 100-year-flood every two years now."

Mayor Michael Bloomberg, in a later interview, suggested that Sandy was a rare event. As a result, he said the Metropolitan Transportation Authority should spend its money on expanding service instead of on steps to harden its current transit network against future flooding.

“If this only happens once every 110 years,” he said on WOR radio’s John Gambling Show, “and they got it back as quickly as they did, is this a good use of your money?”

The question of just how soon the tri-state area will see a storm of Sandy’s strength again remains largely unanswered, even though governments and institutions are being forced to decide just how much money to invest in preparing for the next big storm.

If you look at a map showing the actual and expected flooding, you can see that Sandy’s storm surge exceeded many of the 100-year flood zones, seeping into places considered "safe" like Canarsie and Sheepshead Bay in Brooklyn. The Federal Emergency Management Agency, which draws the maps, could have underestimated how powerful a 100-year-flood is. (FEMA is in fact revising the flood zone boundaries.) But also, Sandy could have been a truly exceptional storm.

Timothy M. Hall, a senior scientist at the NASA Goddard Institute for Space Studies in upper Manhattan, believes he has part of the answer. In a paper co-written with Adam H. Sobel, a professor at Columbia University, that's under review at a scientific journal, Hall estimates that Sandy’s trajectory is a one-in-a-700-year-event.

Hall says most hurricanes move north from the equator and, if they get near New York, glide north along the New Jersey Coast. Sandy, by contrast, made landfall at a perpendicular angle. He says that’s part of what made Sandy so powerful.

“The winds are just pushing this wall of seawater directly onto the coast rather than sliding up parallel to the coast,” he said.

And by 1-in-700-years, Hall doesn’t literally mean that the next storm with a similar trajectory won’t hit until 2712, but instead that one has a 0.14 percent chance of hitting New Jersey in any given year.

Another research team from the Massachusetts Institute of Technology and Princeton University, came up with a similar answer, while asking a slightly different question. In an article that appeared in the journal Nature Climate Change months before Sandy hit, the team calculated the likelihood of different levels of storm surges hitting the Battery at Manhattan’s southern tip.

Like the Manhattan researchers, the MIT/Princeton created “synthetic” storms based on real ones to come up with the probabilities of surges that are much too rare to forecast based on the historic record alone.

That MIT/Princeton team determined that a 9- or 10-foot storm surge—which turned out to be roughly equivalent to the surge from Sandy’s if the high tide is not counted—to be a one-in-800-year-event. (When factoring in how the storm hit at high tide, Sandy’s surge maxed out at nearly 14 feet, making the total surge several times rarer.)

“I was surprised too,” said Kerry Emanuel, a professor of atmospheric science at MIT, about how unusual Sandy appears to be. He led the research team with post-doctoral fellow Ning Lin.

But both research teams caution that other factors could increase the odds of another devastating hit: a weaker storm carrying more rain, for instance, or a storm that approached Long Island instead of New Jersey—and Emanuel says his calculations were based on hurricanes, while Sandy had characteristics of a nor'easter. The other big variable, they say, is climate change. Once the sea level rises two feet or so, a large, but not exceptional, storm could have a similar impact as Sandy had because it will be that much easier for the ocean to breach the bulwarks at the Battery and elsewhere.

“That will mean, all other things being equal, a 100-year storm will be a storm of just a couple of decades,” Hall said. “A 500-year storm will become the 100-year-storm.”