**Recent forecasts position 14–15°C as the most probable overnight low for São Paulo on September 18, 2026, aligning with the market’s leading implied probabilities of 46% for 15°C and 30.5% for 14°C.** Multiple operational models and agency outlooks, including runs consistent with ECMWF and GFS guidance referenced in current briefings, show minimum temperatures stabilizing near 13–16°C under persistent high pressure, easterly winds of 10–15 km/h, and extensive low-to-mid cloud cover that reduces nighttime radiative cooling. Sao Paulo’s subtropical highland climate (elevation ~760 m) typically sees September minima around 12°C historically; the current pattern reflects a continuation of below-average conditions observed earlier in the month, with dew points near 12–14°C limiting further drops. The 21% probability assigned to 9°C or below reflects residual tail risk from any unexpected clear-sky breaks or stronger radiative cooling, while the 21% on 19°C or higher captures upside uncertainty if daytime heating lingers longer into the evening than expected. Updated model guidance and official monitoring agency briefings over the next 12–24 hours will be the key catalysts likely to refine these market-implied odds.