From Dr. Jeff Masters of WeatherUnderground.com
Last night's 9:30 pm EDT H*Wind analysis from NOAA's Hurricane Research Division put the destructive potential of Sandy's winds at a modest 2.6 on a scale of 0 to 6. However, the destructive potential of the storm surge was exceptionally high: 5.7 on a scale of 0 to 6. This is a higher destructive potential than any hurricane observed between 1969 - 2005, including Category 5 storms like Katrina, Rita, Wilma, Camille, and Andrew. The previous highest destructive potential for storm surge was 5.6 on a scale of 0 to 6, set during Hurricane Isabel of 2003. Sandy is now forecast to bring a near-record storm surge of 6 - 11 feet to Northern New Jersey and Long Island Sound, including the New York City Harbor. This storm surge has the potential to cause many billions of dollars in damage if it hits near high tide at 9 pm EDT on Monday. The full moon is on Monday, which means astronomical high tide will be about 5% higher than the average high tide for the month. This will add another 2 - 3" to water levels. Fortunately, Sandy is now predicted to make a fairly rapid approach to the coast, meaning that the storm surge will not affect the coast for multiple high tide cycles. Sandy's storm surge will be capable of overtopping the flood walls in Manhattan, which are only five feet above mean sea level. On August 28, 2011, Tropical Storm Irene brought a storm surge of 4.13' to Battery Park on the south side of Manhattan. The waters poured over the flood walls into Lower Manhattan, but came 8 - 12" shy of being able to flood the New York City subway system. According to the latest storm surge forecast for NYC from NHC, Sandy's storm surge is expected to be several feet higher than Irene's. If the peak surge arrives near Monday evening's high tide at 9 pm EDT, a portion of New York City's subway system could flood, resulting in billions of dollars in damage. I give a 50% chance that Sandy's storm surge will end up flooding a portion of the New York City subway system.
An excellent September 2012 article in the New York Times titled, "New York Is Lagging as Seas and Risks Rise, Critics Warn" quoted Dr. Klaus H. Jacob, a research scientist at Columbia University’s Earth Institute, on how lucky New York City got with Hurricane Irene. If the storm surge from Irene had been just one foot higher, "subway tunnels would have flooded, segments of the Franklin D. Roosevelt Drive and roads along the Hudson River would have turned into rivers, and sections of the commuter rail system would have been impassable or bereft of power," he said, and the subway tunnels under the Harlem and East Rivers would have been unusable for nearly a month, or longer, at an economic loss of about $55 billion. Dr. Jacob is an adviser to the city on climate change, and an author of the 2011 state study that laid out the flooding prospects. “We’ve been extremely lucky,” he said. “I’m disappointed that the political process hasn’t recognized that we’re playing Russian roulette.”