Nagasaki weather in September

Forum posters consistently rate April as one of the best months to visit Japan, generally warm and springlike, though posters are direct that conditions can still turn variable enough to want a coat on some days, whether from cooler temperatures or rain. Posters describe typical April days as comfortable in shirtsleeves with a light jacket in the daypack for evenings, rather than requiring a heavy winter coat, and note that on a sunny day temperatures can climb into the mid-20sC. Posters discussing Kyushu specifically, where Nagasaki sits, describe the region as reliably warm by late March into April compared with much of the rest of Japan. Posters generally rate April as a comfortable, mild time to visit Nagasaki, with a light jacket the main packing recommendation for cooler mornings and evenings.

October is widely rated one of Japan's two best months, and Kyushu — Nagasaki, Fukuoka, Kumamoto, Kagoshima — is the part of the country the forum points at when someone wants October warmth specifically. Nagasaki bears that out: days near 23°C and nights that stay unusually mild at about 18, well above what Honshu offers by then, with the sea still at 25°C. Early October runs warm and late October cools, so a light jacket for the evening covers it. The one risk worth planning around is typhoons, which can deliver a day or two of heavy rain before things return to normal.