**Unstable atmospheric conditions driven by a low-pressure system and developing extratropical cyclone are limiting daytime heating across São Paulo on October 15, 2026.** Persistent cloud cover, scattered thunderstorms, and high humidity—forecast by INMET and global models—are capping solar insolation and holding afternoon highs primarily in the 24–26°C range, consistent with mid-October climatology. Recent model runs show minor disagreements on rain timing that could allow brief clearer periods and slightly higher peaks near 27–28°C. Traders appear to weigh this uncertainty, assigning the strongest implied probabilities to 28°C (43.1%) and 27°C (25.5%) while discounting both cooler outcomes below 25°C and extremes above 29°C. Official updates from INMET and revised model guidance later today will further refine resolution criteria.