The night Feltwater Ness had no low water

Two Tuesdays ago the duty phone took four calls about Feltwater Ness, all of them the same call. The app said low water at 03:14, at 0.4 m. The gauge sat above a metre all night and never turned.

Nothing was broken. It is the clearest example we have of what a tide prediction is and is not, so here is what happened.

Feltwater is the marginal case

Feltwater sits close to a point where the twice-a-day tide almost cancels out. On a neap, the whole astronomical range there is under half a metre. That is the entire signal the moon and sun produce at that port: less than the freeboard of a dinghy.

What the weather added

That night a 992 millibar low was sitting over the coast, and a south-westerly had been blowing onshore for two days. Low pressure lets the sea surface rise by roughly a centimetre for every millibar below average, so about 20 cm. The wind held water against the shore and added more. Together they were larger than the tide.

What we changed

The prediction was right about the moon and the sun. It had no way of knowing about the weather, and at most ports that does not matter much. At Feltwater it matters more than the tide.

That night is the reason surge watch exists. Since version 4.6, Pro reads the pressure and wind forecast and flags any day when they are likely to move the water by 20 cm or more, with the direction. If you walk or fish at Feltwater, it is the most useful setting in the app.