Recent forecasts from multiple models indicate Mexico City’s minimum temperature on September 22 will most likely fall between 12–14 °C, reflecting the primary driver behind the closely matched market probabilities. At the city’s 2,240 m elevation, strong nocturnal radiative cooling under partly cloudy skies can produce these values, while increased cloud cover or light rain—expected in several runs—tends to limit further drops by trapping heat. Historical September climatology shows average overnight lows near 13 °C, providing a baseline against which current ensemble guidance sits slightly cooler. Model spread in boundary-layer moisture and wind patterns creates realistic uncertainty that keeps 15 °C or higher and single-digit outcomes viable, with new model runs and observational updates from the coming 48 hours likely to narrow the range.