Response of extreme storm tide levels to long-term sea level change

The occurrence of dangerously high or low water levels at coastal locations is an important public concern and is a significant factor in coastal hazard assessment, navigational safety, and ecosystem management. The monthly highest and lowest water levels at 117 NOAA/National Ocean Service water level stations show a clear response to local mean sea level trends. The extreme levels reached by hurricanes and extra-tropical storms of the past can be adjusted for sea level trend, so that unbiased comparisons can be made. A data set of the annual highest and lowest water levels is derived from the monthly data and used to determine the expected frequency of future storm tides rising above or falling below any given level. The same analysis is also applied to the data for each individual month in order to estimate the varying likelihood of extreme high or low levels by season. The results are a set of annual and monthly exceedance probability levels relative to the tidal datums for each station. This information should prove useful for identifying, in real time, when a rare event threshold has been crossed. The exceedance probability levels can be adjusted in the future to reflect newly-updated tidal datums.