Extreme Water Levels
1619000 Johnston Atoll, United States of America
The monthly extreme water levels include a Mean Sea Level (MSL) trend of 0.75 millimeters/year with a 95% confidence interval of +/- 0.56 millimeters/year based on monthly MSL data from 1947 to 2003 which is equivalent to a change of 0.25 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.
