I haven't been watching the tide charts, but the tides come in a cycle, which is dictated by the relative positions of the moon and sun. The position in the sky of the moon and sun also go through cycles, but have slightly different periods (the sun is twenty-four hours, the moon, I think, is twenty-two). Occasionally these line up, and produce exceptional tides. These exceptional tides may be rare on a monthly basis, but they are just as predictable as ordinary tidal cycles.