Recent forecasts from models including ECMWF and GFS indicate Mexico City’s overnight minimum on September 18 will likely settle near 12–14 °C, driven by persistent high pressure, northerly flow, and extensive cloud cover that limits radiative cooling at the city’s 2,240 m elevation. Light rain or drizzle in some runs adds modest evaporative cooling and maintains higher humidity, while calm winds reduce mixing that could otherwise prevent the surface layer from reaching its lowest values. Historical September lows average 10–13 °C, so current conditions sit slightly above the climatological baseline. Differences between 13 °C and 14 °C hinge on the precise timing of any clearing and the strength of the nocturnal inversion; updated model runs and local observations through the evening of September 17 will refine these thresholds before resolution.