**Observed meteorological data from official São Paulo stations, including those managed by the Centro de Gerenciamento de Emergências Climáticas (CGE) and INMET, recorded a daily maximum of 17°C on September 17, 2026.** Persistent cloud cover, elevated humidity near 90%, and a polar air mass advected from the south suppressed daytime warming, keeping temperatures between roughly 14–17°C with minimal thermal amplitude. This aligns with the broader pattern of an unusually cool and rainy September driven by frequent frontal passages. Trader consensus at near-100% on 17°C reflects the finalized observational record rather than forecasts. A realistic challenge would require an official data revision or differing station reading reaching 18°C, though current reports show low likelihood of such an adjustment.