**The Hong Kong Observatory’s September 2026 monthly outlook, released August 31, positions rainfall as normal to below normal, directly supporting the market’s leading <275 mm outcome at 43% implied probability.** Normal September totals average near 300 mm; the forecast reflects a strengthening high-pressure ridge over southern China in the month’s second half that suppresses convective activity, combined with the early development of a strong-to-very-strong El Niño event expected to peak later in the year. El Niño conditions in this phase typically reduce monsoon rainfall and tropical cyclone impacts over the northern South China Sea. Early-month instability from a broad low-pressure trough and Tropical Cyclone Saudel (Standby Signal No. 1 in force early September) is adding localized showers, but these are not expected to offset the broader drying trend. Model consensus and objective indices reinforce the below-normal tilt, though inherent uncertainty in tropical cyclone tracks and exact El Niño timing leaves room for upward revisions if multiple systems track closer to Hong Kong. Traders are therefore weighting the official agency guidance and climatological El Niño response more heavily than isolated early-September rain events.