**Strong El Niño conditions, now strengthening with >90% odds of a very strong event through fall 2026-27, represent the dominant large-scale driver shaping September precipitation probabilities in Hong Kong.** This phase typically correlates with suppressed or redirected western North Pacific tropical cyclone activity and a more westward-extended subtropical ridge, which can reduce the frequency or proximity of typhoon-related heavy rain episodes that normally account for a substantial share of monthly totals. September climatology features a normal of roughly 277–321 mm, driven by the monsoon trough, occasional tropical cyclones (peak influence month for signals in Hong Kong), and embedded disturbances. The current market distribution—led by the <275 mm bin at 31.5% with notable probability mass across mid-range bins and tail risk above 500 mm—reflects high uncertainty around whether one or more systems will track close enough to deliver extreme rainfall or whether El Niño steering will keep activity suppressed or farther offshore. Key variables include: typhoon genesis and track in the South China Sea and western Pacific (proximity within ~300–500 km often produces 100+ mm events); steering flow from the subtropical high; local sea-surface temperatures supporting convection; and any mid-latitude interactions or monsoon surges. Early-month model guidance points to near-normal accumulation absent major cyclone impacts, while the spread in implied probabilities captures the potential for rapid shifts from revised forecasts or late-month developments. Resolution hinges on official Hong Kong Observatory totals through September 30.