September is the swell of August with the sky of the green season at its worst. The Southern Hemisphere is still sending groundswell and the Pacific coast is still getting it, so the surf holds up. What changes is everything on land: this is the wettest stretch of the Costa Rican year, roads on the Nicoya get harder, and storms can turn a clean morning into a grey week.
That is what puts September a notch below June, July and August, despite the swell. The compensation is real: it is the quietest month of the season by a distance, prices drop, and empty peaks are normal. The water is at 29°C and the rain is warm.
