Recent forecasts from multiple models indicate Mexico City’s overnight low on September 19 will likely fall between 11–14 °C, consistent with the city’s 2,240 m elevation and typical late-rainy-season radiative cooling under variable cloud cover. Light northerly winds and scattered showers are expected to limit extreme drops, while urban heat-island effects and high humidity moderate minimums near the official measurement site. Historical September lows average ~12–13 °C, with recent observations showing similar values. The tight clustering of market-implied odds around 13–14 °C versus the 5 °C-or-below and 15 °C-or-higher tails reflects genuine model spread in exact timing of the minimum and sky cover, rather than any dominant cold-air outbreak or anomalous warming. Updated runs from regional and global ensembles will likely narrow the range before resolution.