A cold front advancing across São Paulo state has become the dominant driver of market-implied odds, with forecasts showing a sharp cooldown after late-September heat. Official guidance and ensemble models indicate the overnight minimum on October 1 will likely fall to 14–18 °C as cooler maritime air replaces the prior warm, humid airmass, accompanied by widespread rain and increased cloud cover that limits nighttime radiative cooling. Historical October climatology places average lows near 14 °C, but the frontal timing introduces genuine uncertainty: stronger or slower-moving systems could push readings below 14 °C, while partial clearing might keep them near 17 °C. Traders are weighting the balance of these near-term model runs and the precise arrival of the frontal boundary.