At the start of September 2026, trader consensus assigns the highest implied probability (31%) to below 275 mm total rainfall in Hong Kong, reflecting broad uncertainty in seasonal outlooks rather than a single dominant forecast. Primary variables include the strength and position of the monsoon trough, which can steer moisture-laden air over the region, and the frequency and tracks of tropical cyclones in the western Pacific, where small shifts in steering flow determine whether systems bring heavy rain or pass offshore. Sea-surface temperature anomalies and the evolving ENSO state further modulate atmospheric moisture and storm genesis, while climatological baselines show September averages near 300–350 mm with high interannual variability. Official agency model runs and updated typhoon guidance through mid-month will likely drive the largest near-term shifts in these probabilities.