Mean High Water (MHW)

Mean High Water (MHW) is the arithmetic mean of all high-water heights observed at a station over the 19-year National Tidal Datum Epoch. In a semidiurnal regime it averages both highs of each tidal day; in a mixed regime it sits below Mean Higher High Water. MHW is the tidal datum that defines the legal shoreline on US charts and the upper limit of the foreshore in many jurisdictions. The 19-year averaging period spans the 18.61-year lunar nodal cycle, removing its modulation of tidal range so the datum is stable between epoch updates.