**Trader sentiment for Mexico City’s August 21 maximum temperature centers on 25–26 °C because recent model guidance and climatological context place the most likely outcome in that narrow band.** Mexico City’s high elevation (~2,240 m) keeps daytime maxima moderate even in the rainy season; August averages hover near 24–25 °C, with occasional peaks to 26–27 °C when morning sunshine is strong before afternoon convection develops. The slight edge for 26 °C (42 % market-implied probability versus 33.5 % for 25 °C) reflects the latest deterministic and ensemble runs showing a window of clearer skies or delayed thunderstorm initiation that would allow one extra degree of warming. Key differentiating factors include the precise timing and coverage of convective clouds and showers—earlier or more widespread rain cools the boundary layer and caps the maximum near 25 °C, while a later or more isolated storm pattern permits brief additional insolation. Low-level moisture, wind direction (typically light southeasterly), and the strength of the subtropical ridge also modulate the outcome by a degree or two. Because resolution hinges on the official Servicio Meteorológico Nacional observation at the standard station, traders are watching the final 24–48 h model updates and any shifts in the 12–18 UTC convective initiation timing. Historical analogs from similar late-August setups confirm that the difference between 25 °C and 26 °C often comes down to just a few hours of extra sun before the usual afternoon storms.