**Forecast uncertainty in overnight radiative cooling and boundary-layer moisture drives the near-even split between sub-21 °C and 31 °C+ minima for Guangzhou on 28 September.** Subtropical high pressure and light southwesterly flow favor clear to partly cloudy skies, allowing strong nocturnal cooling toward the 26–28 °C range typical of late-September climatology, yet scattered showers or increased low-level humidity can limit that drop and keep the minimum closer to daytime values. Model spreads in cloud cover and timing of any convection create the balanced market-implied odds, with traders weighting the potential for either enhanced radiative cooling under clear conditions or suppressed cooling from moisture and weak southerly advection. No major synoptic shift is indicated before resolution.