December in Isahaya averages a daily high of 13.2°C. Expect roughly 10.3 wet days totalling 78.7mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit Isahaya is April–May.
Based on NAGASAKI, JA (20.5km), 1995–2024. 20 years of December data.