**A cold front advancing across Southeast Brazil is the dominant driver pushing trader consensus toward 15–16°C as the most probable high for São Paulo on September 6.** Official guidance from INMET and consensus from global models (including Yr and Met Office) indicate the front will deliver persistent cloud cover, southeasterly winds, and widespread rain or showers, sharply limiting daytime warming after a milder September 5. This setup favors maximum temperatures clustered around 14–16°C, with 15°C and 16°C holding the highest implied probabilities because model runs show only modest differences in frontal timing and boundary-layer mixing. Key scientific factors differentiating the narrow 15–17°C band include the precise strength and speed of the post-frontal high-pressure ridge, which controls how quickly drier air arrives, the depth of low-level cloud suppressing solar heating, and local effects near the official observation site (typically Congonhas or INMET stations). Stronger southeasterly flow and heavier rain favor the lower end (14–15°C), while any earlier clearing or weaker winds could allow brief warming toward 17°C. Historical September averages near 23°C highlight how anomalous this event is, and traders are weighting the latest model consensus heavily given the short time to resolution. New forecast updates or radar trends on frontal position over the next 12–24 hours remain the primary variables that could shift probabilities within this tight range.