September is the month everything lines up, which is why it is named ahead of the rest of the year. The nortada dies off through the month, so the offshores come back and a west coast afternoon becomes usable again. The first proper Atlantic groundswells of the autumn arrive at the same time. And the European holidays end in the first week, so the beaches empty.
None of those three would be enough alone. Together they give the swell of the winter half on the water temperature of the summer half with nobody on it. The sea is at 18°C, its yearly high, and a 3/2 covers a long session. Peniche, Ericeira and the Algarve are all in play.
