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Why Sandy Wasn’t Worse

Vol. II, No. 71

Hurricane Sandy's storm surge floods a driveway near the Mill Pond in Westport. Photo by John Kantor.

Sandy was a 100-year storm. In our area that means a flood height measuring 12 feet or more. Initial reports indicate it varied between 12 and 13 feet. But why was it not the 16 to 19 foot flood predicted by the media? Two reasons: 1) Some media meteorologists do not fully understand storm surge and how it is calculated and 2) Sandy suddenly picked up speed just before making landfall.

Meteorologists are weather people. They study the atmosphere. Where the atmosphere interacts with the sea, some meteorologists get foggy (pun intended). I heard one TV meteorologist repeatedly predict a “double tide” in the days before Sandy struck. In oceanography there is no such term. What the meteorologist was trying to say was that there would be storm surge arriving at the same time as the regularly occurring high tide, making it higher than normal. There was just one high tide which combined with Sandy’s storm surge to cause the historic coastal damage. It occurred at night. The following high tide, the next day, was a minor flooding event by comparison.

Some other broadcast meteorologists were even more confused and further out of their element. They took the National Weather Service tide height predictions, which had storm surge already factored in, and then added the surge predictions to that – in effect doubling the surge factor and drastically over-predicting the flood height. The surge, alone, turned out to be about 4 feet locally. That combined with the 8-ish foot astronomical high tide gave us the 100-year flood height.

But an interesting thing happened. The flood waters crested before high tide.  At the time of the astronomical high tide the water level was actually starting to go down. That was because the storm sped up and moved inland sooner and faster than predicted. It had less time and opportunity to work on the Atlantic waters and drive them into the Sound, so the surge was not as great as originally predicted. Anecdotal evidence corroborates the finding. Most of the trees which came down (and there were plenty) fell hours before high tide. Why? The worst clusters of wind gusts, the ones which tipped over the trees, were over with before high tide. The surge, a product of wind, started to ease up as the wind field moved inland – prior to local high tide.  So, a phenomenon I have only witnessed once before on the Sound (in another hurricane) occurred with Sandy. The water level was actually falling at high tide.  How could that be? Peak storm surge had already passed about 1-2 hours earlier.

A tree down, most likely as a result of the strong winds before high tide. Photo by John Kantor.

We who live, work and depend on the water know how important good weather information is. We also know that weathermen sometimes get it wrong. They have many computer models to choose from, and not all can be correct. Some vary widely from the others. For that reason, when a serious weather system approaches, I take in weather info from as many sources as possible and make an independent judgment. Some forecasts are too regional to be accurate for a specific locale, so I combine their broader view with other data and my own local experience to formulate my own local forecast. When my battery operated radio informed me that Sandy had sped up in the late afternoon, I smiled, knowing the Sound would not be in the living room that night.

Sandy’s coastal flooding might have been far worse if it had stuck to the schedule. Fortunately for us all, it was in a hurry to make landfall.