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A Brief Summary and Explanation of Terms
- Extratidal High Water (XHW) and Extratidal Low Water (XLW) are observed water-level extremes measured above the tidal datum HAT (XHW) or below the tidal datum LAT (XLW). These extremes occur only in the presence of a weather-induced residual water level or "storm surge" (observed water level minus the astronomic tide), a sea-level anomaly (m30-MSL vertical difference) or, more often, a combination of the two. The astronomic tide—the water level change occurring at tidal frequencies—will not exceed either datum in the absence of a sea-level anomaly.
- m30 is the 30-day mean water level covering the period of observation shown. It represents a time-local departure from MSL that is due to a combination of the seasonal tide, the decadal change in sea level, and the long-term sea-level trend relative to the land. The vertical reference for the astronomic tide is m30.
- HAT83 is the elevation of Highest Astronomical Tide above MSL (1983-2001) at the station shown.
- LAT83 is the elevation of Lowest Astronomical Tide below MSL (1983-2001) at the station shown.
- MHHW, MSL, and MLLW are the tidal datums of Mean Higher High Water, Mean Sea Level, and Mean Lower Low Water as defined by NOAA/NOS for the 1983-2001 National Tidal Datum Epoch (See http://tidesandcurrents.noaa.gov).
For a more extensive explanation, read "What is Extratidal High or Low Water?" (pdf).
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