King tides.

What to expect at the top of one

A normal Duxbury high runs nine to eleven feet above the low-water datum. Past 11.75, the bay starts doing things it usually doesn't: the marsh disappears under a sheet of water, floats ride level with their pilings, and the low spots along the beach road and the landings can go wet, especially when weather stacks on top. That last part matters: these are predictions for calm conditions, and a storm's wind and low pressure can add a foot or more on the day. A king tide plus a northeaster is a different animal from a king tide alone. Check the forecast before you park anywhere low.

The consolation prize is spectacular: every king tide is paired, about six hours later, with an unusually deep low. The biggest floods buy the widest flats. If a date below has a ▲, the flats page that day will be showing one of the best walks of the year.

The schedule

NOAA predictions, station 8446166 (Duxbury Harbor), MLLW datum, local time · ▲ marks 12 feet and up · Predicted heights; weather adds or subtracts on the day · Outlook extends each year

Why they happen

Tides are the ocean answering the pull of the moon and sun. When the two line up at new or full moon, their pulls add, and the tides run bigger: spring tides, nothing to do with the season. When that alignment also lands near perigee, the point in the moon's slightly oval orbit where it swings closest to Earth, the pull is strongest of all, and the spring tide becomes a king. That is why the schedule clusters: a run of two or three big days around certain new and full moons, then months of ordinary water. The moon's orbit does the arithmetic; NOAA writes it down; the bay obeys.