**Trader sentiment for Shanghai’s August 21 overnight low centers on 27°C (55% implied probability) because current numerical weather prediction models and climatological baselines converge on that value.** Shanghai’s subtropical monsoon climate keeps late-August nighttime temperatures elevated by persistent high humidity, warm southerly flow from the western Pacific subtropical high, and the urban heat island effect that limits radiative cooling. Typical August lows average 25–27°C, with the final third of the month often showing modest declines only if clearer skies or light northerly winds develop. Recent forecasts reinforce this range: Met Office guidance lists a 27°C minimum for August 21, while longer-range products from AccuWeather and climatological summaries place overnight lows between 25–27°C amid continued muggy conditions and scattered showers. No typhoon or strong cold-surge signals appear in the short-term outlook that would drive the thermometer below 25°C or allow it to reach 28–29°C. Lower-probability outcomes (≤25°C or ≥28°C) reflect residual model spread from cloud-cover or wind variations over the next 48 hours, but the consensus of operational guidance supports the market’s heavy weighting toward 26–28°C. Updated model runs and official briefings from Chinese meteorological agencies over the coming day will provide the next clear signal for any shift in these probabilities.