Weather
Rain across most of the country, heaviest on the Andaman. The Gulf is transitioning too. It rarely rains all day; the pattern is a hot morning and a heavy late-afternoon shower.
Why go anyway
Half-price hotels, no queues at the Grand Palace, and Bangkok's food scene at its best. If your trip is about cities, temples and eating rather than sunbathing, September is underrated.
Plan around it
Schedule indoor things for 3–5pm, boats in the morning, and keep one flexible day for a washout. Travel insurance that covers weather delays is worth it.
