**Trader sentiment for Hong Kong's October 2026 rainfall reflects the Hong Kong Observatory's September 30 forecast of normal to below-normal totals amid a strengthening El Niño event.** The 1991-2020 climatological average stands at 120.3 mm, with daily normals declining through the month as the northeast monsoon strengthens and high-pressure systems suppress convection. El Niño episodes have historically favored drier autumn conditions across southern China by altering Walker circulation and reducing moisture transport. Market-implied odds concentrate on the drier side of the distribution, with the <20 mm bin leading at 20% amid this outlook, though probabilities remain dispersed across multiple bins due to inherent forecast uncertainty in monthly totals. Key variables include the timing of any late-month trough weakening and tropical cyclone activity, which official guidance currently rates as limited.