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Extreme Water Levels

Please click on the arrow to access additional information about that station.

Legend for station map above

Extremely high or low water levels at coastal locations are an important public concern and a factor in coastal hazard assessment, navigational safety, and ecosystem management. Exceedance probability is the likelihood that water levels will exceed a given elevation based on a statistical analysis of historic values. The Product provides annual and monthly exceedance probability statistics for select CO-OPS water level stations with at least 30 years of data. When used in conjunction with real time station data, exceedance probability statistics can be used to evaluate current conditions and determine when a rare event is occurring. This information may also be instrumental in planning for the possibility of dangerously high or low water events on a local level. Because these statistics are station specific, their use for evaluating surrounding areas may be limited.


The extreme levels measured by the CO-OPS tide gauges during storms are called storm tides which are a combination of the astronomical tide, the storm surge, and the wave setup caused by breaking waves. They do not include wave runup, the movement of water up a slope. Therefore, the 1% annual exceedance probability levels shown on this website do not necessarily correspond to the Base Flood Elevations (BFE) defined by the Federal Emergency Management Administration (FEMA), which are the basis for the National Flood Insurance Program. The 1% annual exceedance probability levels on this website more closely correspond to FEMA's Still Water Flood Elevations (SWEL). The peak levels from tsunamis, which cause high-frequency fluctuations at some locations, have not been included in this statistical analysis due to their infrequency during the periods of historic record.

Comparison plots of 10-year and 100-year exceedance probability levels
Northern Atlantic stations
Northern Atlantic stations
Northern Pacific stations
Northern Pacific stations
Tropical and Gulf of Mexico stations
Tropical and Gulf of Mexico stations

September, 2013 -- In late October 2012, Hurricane Sandy struck the coast of New Jersey. The storm's large area, direction of motion relative to the coastline, strong winds, and low barometric pressure caused a large storm surge along the Mid-Atlantic coast from Cape Cod to Cape Hatteras. At most long-term NWLON stations, the peak water levels were high, but not the historically highest levels, with the exception of two stations in New York Harbor (The Battery and Sandy Hook) and at Bridgeport on Long Island Sound. Because these new maxima are so far above the previous maxima, they cause a significant change in the shapes of calculated exceedance probability curves especially at the longer return periods. Therefore, the high water exceedance probability levels for these three stations have been recalculated with the inclusion of the highest levels for 2011 and 2012, which were the peak levels reached by Hurricane Irene and Hurricane Sandy at all three stations.

Data and Resources

  • NOAA Technical Report NOS CO-OPS 36, "Sea Level Variations of the United States 1854-1999".
  • The mean sea level trends for each station are also available as a table in millimeters/year or a table in feet/century.
  • "Response of Extreme Storm Tide Levels to Long-term Sea Level Change", Chris Zervas, CO-OPS, NOAA/National Ocean Service.
  • Coastal flooding terms as defined by FEMA.
  • Statistics of Weather and Climate Extremes, National Center for Atmospheric Research (NCAR) website.
  • "Analyzing Seasonal to Interannual Extreme Weather and Climate Variability with the Extremes Toolkit", Eric Gilleland and Richard W. Katz, Research Applications Laboratory, NCAR, Boulder, CO.
  • "Extremes Toolkit: Weather and Climate Applications of Extreme Value Statistics", a tutorial by Eric Gilleland and Richard W. Katz, NCAR.
  • R Programming Language, a free software environment for statistical computing and graphics which compiles and runs on a wide variety of platforms.

    Contact Information

    For additional information, please contact CO-OPS.

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