The water is rising.

There’s been a lot of media chatter about the potential for Hurricane Irene’s storm surge, which may be more dangerous than the average Category 1 hurricane because of the storm’s unusually large size and slow pace. But for us denizens of the concrete jungle, “storm surge” isn’t exactly a term we use in everyday parlance. So what does it mean, really? Here is the definition from the National Oceanic and Atmospheric Administration (NOAA):

The onshore rush of sea or lake water caused by the high wind and the low pressure centers associated with a landfalling hurricane or other intense storm. The amplitude of the storm surge at any given location is dependent upon the orientation of the coast line with the storm track, the intensity, size and speed of the storm, and the local bathymetry. In practice, storm surge is usually estimated by subtracting the normal or astronomical tide from the observed storm tide at tide stations.

As of 4 a.m. today, the storm surge at NOAA’s station in Battery Park was showing that the waters around Lower Manhattan had risen about 4 feet above normal. In the chart above, the blue line graphs the predicted water levels, the red line graphs the actual levels, and the green line measures the difference between them. This chart is online and updated every 15 minutes; you can find it here.

At 1 a.m. today, Reuters was reporting that the FDR Drive was already beginning to flood, “with heavy pooling and tow trucks strategically idling on the sides of the road.”