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“It’s likely that you’re going to have a lot of this uninsured storm surge,” Mr. Keogh said.

Hurricane Sandy delivered storm surges worse than most previous storms, with water surging nearly 14 feet above normal in New York City, so the flooding is expected to be greater than usual.

For Hurricane Irene, insured flood losses accounted for only about $1 billion of the devastation, but for Hurricane Katrina the number was closer to $20 billion, according to Mr. Hartwig.

As of June, the National Flood Insurance Program had about 5.5 million policyholders with $1.2 trillion worth of coverage.