A strengthening El Niño event, with Niño 3.4 anomalies already exceeding +2°C and models favoring a historic peak by late 2026, represents the dominant driver tilting trader sentiment toward below-normal September rainfall in Hong Kong. The Hong Kong Observatory’s seasonal outlook assigns a modestly elevated probability of normal to below-normal totals, aligning with the market’s 30.5% implied probability on the <275 mm bin and the broad distribution across higher ranges. Key variables include tropical cyclone frequency and steering patterns, which can deliver 100+ mm events in days and push outcomes into the 400–550 mm+ bins, versus suppressed monsoon convection under persistent warm equatorial Pacific SSTs that favors the drier tail. Historical September variability remains high, with typhoon tracks and short-term model consensus on early-month troughs likely to dictate near-term shifts in the 275–375 mm cluster.