**Short-term forecast uncertainty around potential afternoon convection and cloud cover is the main driver of the closely matched Polymarket odds (30°C at 37%, 31°C at 32.5%, 29°C at 20.5%).** As of late August 31, 2026, official and ensemble guidance for Shenzhen shows daytime maxima clustered near 29–32°C on September 1, consistent with early-September climatology (long-term average high ~31°C in this humid subtropical regime). Persistent northeasterly flow, high boundary-layer moisture (dew points near 24–26°C), and 40–60% thunderstorm probabilities introduce variability: thicker cloud decks or earlier showers can reduce insolation and trigger evaporative cooling, capping the peak near or below 30°C, while clearer breaks allow brief warming toward 31–32°C before convection develops. Model spreads arise mainly from differences in the timing and coverage of these mesoscale features rather than large-scale pattern shifts. With resolution imminent, traders are weighting the latest runs that favor a modest peak near 30°C while assigning non-negligible probability to the adjacent bins.