The developing strong El Niño event, with central and eastern Pacific sea surface temperatures rising sharply and models projecting peak intensity by late 2026, forms the main driver behind trader sentiment favoring below-normal September rainfall in Hong Kong. Hong Kong Observatory seasonal and monthly outlooks explicitly assign higher probability to normal-to-below-normal totals, consistent with El Niño suppression of the southwest monsoon and reduced tropical cyclone influence near the South China Sea. Historical September averages of 321 mm provide context, while early-month troughs and potential systems introduce upside risk that explains the spread across bins below 350 mm. Key variables include anticyclone strengthening in the latter half of the month, exact typhoon tracks, and any model revisions ahead of resolution.