**Seoul's September precipitation typically averages 140–180 mm, driven by the tail end of the East Asian monsoon, stationary fronts, and occasional typhoon influences.** Current market-implied odds favor the 175–200 mm bin at 42% as traders weigh near-normal totals against variability. Recent late-August heavy rain—exceeding 150–200 mm in parts of central and southern Korea from a stationary front strengthened by anomalously warm Yellow Sea surface temperatures (29°C, +4°C above average)—highlights moisture availability that could extend into early September. However, long-range forecasts point to a typical transition toward drier autumn conditions, with reduced monsoon influence and uncertain typhoon tracks (historically ~5 form in the western North Pacific during September, though none have yet affected the peninsula in 2026 due to blocking highs). Lower bins (<75 mm or 75–100 mm) reflect drier scenarios if high-pressure systems persist, while higher outcomes (200+ mm) hinge on one or more typhoon landfalls or persistent fronts. Model consensus and climatological baselines support the leading 175–200 mm range as the most probable without major anomalies.