**Persistent Meiyu (plum rain) conditions from the East Asian monsoon front are the dominant driver capping Shanghai’s daytime maximum near 25–26 °C on June 15.** Overcast skies and frequent light showers or drizzle reduce incoming solar radiation, limiting surface heating compared with clearer June climatology (average highs ~28–29 °C). Recent model consensus (ECMWF, GFS, and local CMA runs) shows a moist, stable airmass with limited breaks in cloud cover through the afternoon, keeping the peak in the mid-20s °C range. **Key differentiating factors between the closely matched 25 °C and 26 °C outcomes include the precise timing and coverage of any convective elements or temporary cloud thinning.** A slightly earlier or more pronounced clearing could allow an extra degree of warming, while sustained stratiform rain or thicker low cloud would favor the lower end. Humidity levels near 80–90 % and light easterly winds further suppress the daily range, with urban heat-island effects adding only modest additional warmth. Traders are weighting the latest 24–48-hour forecast updates and ensemble spreads heavily, as small shifts in the Meiyu boundary or model bias can swing the exact maximum by 1 °C. Resolution hinges on verified observations from official Shanghai stations (typically the highest 5-minute or hourly reading), so any late-day clearing or missed shower remains the main uncertainty.