Current forecast models from sources including the National Weather Service and European ensembles indicate Mexico City’s overnight low on September 21 will reach 12–14 °C, driven by the city’s 2,240 m elevation, clear to partly cloudy skies allowing efficient radiative cooling, and light northwesterly flow. Historical September minima cluster near 13 °C, with the 25th–75th percentile range spanning roughly 11–15 °C; dew points near 10–12 °C limit further drops absent stronger high-pressure subsidence. Trader consensus remains closely split between the 17 °C+ and sub-8 °C bins because model runs show minor sensitivity to exact cloud cover and any late-evening showers, while the tight clustering around 12–13 °C reflects the narrow expected diurnal range under typical late-summer conditions.