**Market-implied odds show notable uncertainty around the daily minimum temperature in São Paulo on September 4, 2026, with the 13°C-or-below outcome leading at 37.5% and 23°C-or-higher close behind at 34.5%.** Specific values cluster tightly between 14–17°C (27–31% range), reflecting trader assessment of model spread rather than a single consensus. The primary driver is the timing and strength of a cold front or polar air mass advection interacting with the subtropical high-pressure ridge. Recent observations indicate a shift from warmer, drier northwesterly flow (supporting higher overnight minima around 17–18°C via reduced radiative cooling) toward southeasterly winds and increased cloud cover or moisture, which could enhance cooling and push the minimum lower. Historical September climatology shows average lows near 12°C, so current guidance sits above normal but with high variability. Key differentiating factors include nocturnal cloud fraction (clear skies accelerate cooling via longwave radiation loss), boundary-layer wind speeds (calm conditions favor colder surface temperatures), and dew-point depressions that influence fog or stratus formation. Numerical weather prediction models diverge on the exact arrival of cooler air overnight, creating the balanced probabilities. Resolution depends on official INMET or equivalent station readings of the 24-hour minimum; fresh model runs and satellite trends over the next 24 hours will likely narrow the range.