Our six-hour update just came in and it’s not good news for New York City and areas of New Jersey inside NY/NJ Harbor — the model forecast for Monday night rose two feet higher at The Battery, so now it calls for a flood elevation of 10.8 ft above MLLW (normal daily low tide), which is 1.3 ft higher than Irene’s flood elevation and about 3 ft above the area’s lowest seawalls. This is bad news, but the forecast is still highly changeable.
The flood elevation is a combination of a 6.5 ft storm surge coinciding with the time of a high tide, so slight changes to storm surge timing could help reduce the forecast water level. The forecast flood elevation at Kings Point is still about 13 ft, but it is noteworthy that it would be a 7 ft storm surge that is arriving three hours before the time of high tide, so it could be worse with a slight delay in arrival time.
If the Battery forecast should hold or worsen, these would be the highest flood elevations in the 80 years that records have been kept at that location. Unless efforts are taken to protect vulnerable infrastructure, this would cause some subways, highway tunnels, and electrical infrastructure to be filled with salt water and shut down for weeks. And beyond that, I would estimate the 10% likelihood exceedance flood elevation (sort of like a mild version of a worst-case scenario) for Sandy to be about 3 feet higher than this forecast. This could occur if the peak surge coincides with high tide and the storm ends up making landfall with stronger peak winds than are currently predicted.