**Trader sentiment for São Paulo’s September 18 high temperature centers on the narrow 22–24 °C band because current model runs and recent observations show only modest daytime warming under persistent southeasterly flow.** High pressure and onshore winds have kept the city cooler than seasonal norms, with highs on the 16th–17th limited to the mid-teens °C amid widespread cloud cover and dew points near 12–14 °C. Forecasts for the 18th converge on afternoon peaks of 22–24 °C as gradual warming occurs, yet thick low- to mid-level clouds and light-to-moderate easterly winds are expected to cap insolation and prevent stronger heating. Small differences in cloud timing or wind backing could shift the daily maximum by 1–2 °C, which explains why the three central outcomes command roughly equal implied probability while tails remain thin. Updated model guidance and real-time observations on the morning of the 18th will be the key near-term catalysts.