Extreme Water Levels
8545240 Philadelphia, PA

At this location, the tidal range has increased significantly over the period of record. The monthly extreme high water levels include a Mean High Water (MHW) trend of 4.41 millimeters/year
with a 95% confidence interval of +/- 0.24 millimeters/year based on monthly MHW data from 1900 to 2011 which is equivalent to a change of 1.45 feet in 100 years. The monthly extreme low water levels include a Mean Low Water (MLW) trend of 0.95 millimeters/year
with a 95% confidence interval of +/- 0.23 millimeters/year based on monthly MLW data from 1900 to 2011 which is equivalent to a change of 0.31 feet in 100 years.

The plots show the monthly highest and lowest water levels with the 1%, 10%, 50%, and 99% annual exceedance probability levels in red, orange, green, and blue. The plotted values are in meters relative to the Mean Higher High Water (MHHW) or Mean Lower Low Water (MLLW) datums established by CO-OPS (1 foot = 0.3 meters). On average, the 1% level (red) will be exceeded in only one year per century, the 10% level (orange) will be exceeded in ten years per century, and the 50% level (green) will be exceeded in fifty years per century. The 99% level (blue) will be exceeded in all but one year per century, although it could be exceeded more than once in other years.