Traders have concentrated implied probability on 26–28°C lows for Shanghai on July 20 because current model guidance and seasonal climatology place typical overnight minima in this narrow band during late July, after the plum-rainy season ends and subtropical high pressure strengthens. Subtle differences among these outcomes hinge on forecast details such as nocturnal cloud cover, boundary-layer moisture, and light wind regimes that modulate radiative cooling rates over the urban heat island. Ensemble spreads remain modest this close to the date, with any late adjustments from updated ECMWF or GFS runs able to shift the exact minimum by 1–2°C. Historical July lows average near 26–27°C, providing a firm baseline against which traders weigh short-term deviations.