**Forecast model spread and overnight radiational cooling under a high-pressure system are the main drivers of closely matched market odds around 13–14°C for São Paulo’s September 3 minimum.** An advancing high-pressure ridge following recent extratropical activity has established stable, drier conditions across southeast Brazil, favoring clear to partly cloudy skies and light winds that promote surface cooling after sunset. Official guidance from INMET and IPMet points to a minimum near 9–11°C, while other numerical models (including ensemble outputs) show a wider range centered on 11–17°C depending on exact cloud cover, dew-point depression, and boundary-layer mixing. Historical early-September climatology for the city centers near 12°C, so current conditions sit slightly below average but not extreme. The tight clustering of probabilities at 13°C (43.0%), 14°C (39.5%), and 17°C-or-higher (38.5%) directly reflects this model divergence and the narrow margin between modest versus stronger nocturnal cooling. Lower-probability outcomes below 12°C remain discounted because most guidance keeps the urban heat-island effect and light northerly flow from allowing deeper drops. Traders are effectively pricing the latest model runs and any intraday updates to cloud or wind fields that could shift the overnight low by 1–3°C.