Recent forecasts from multiple models indicate Mexico City's overnight low on September 21 will likely settle near 12–14°C, driven by typical late-summer radiative cooling under partly cloudy skies at the city's 2,240-meter elevation. Light northerly winds and moderate humidity levels around 70–90% limit further heat loss after sunset, while urban effects and any residual moisture from patchy showers can raise minima by 1–2°C. Historical September averages near 13°C provide context, yet small differences in cloud cover timing or wind speed create the tight clustering around 12°C and 13°C outcomes. Updated model runs overnight will refine the exact threshold as resolution approaches.