**Trader sentiment for Seoul's September 2026 precipitation favors the 175-200 mm range (49% implied probability), above the 1991-2020 climatological normal of ~140-150 mm.** This positioning reflects the tail end of the East Asian monsoon combined with peak typhoon season, when tropical cyclones frequently track near the peninsula and deliver heavy rainfall episodes. Recent analyses document an emerging “autumn rainy season” pattern, with September–October totals rising ~42% since the late 1990s as typhoon formation zones shift westward and the North Pacific high retreats later, sustaining moisture transport. A strong El Niño currently in place historically correlates with reduced September rainfall via a more suppressed western North Pacific subtropical high, introducing countervailing downward pressure that helps explain why sub-100 mm outcomes still attract meaningful volume. Korea Meteorological Administration outlooks and updated typhoon guidance through early September will be key near-term catalysts for any shifts in these market-implied odds.