**Forecast models and surface observations indicate an overnight low near 12–14°C for São Paulo on August 21, with trader probabilities closely clustered around 11–14°C and notable weight on 9°C or below reflecting uncertainty in nocturnal cooling.** Late-winter subtropical conditions allow polar air intrusions via cold fronts, enhanced by clear skies that promote strong radiative heat loss after sunset, while increased cloud cover and showers from an approaching frontal boundary can limit the drop by trapping longwave radiation. Current August anomalies run +2°C above the 17.9°C climatological mean, consistent with El Niño influence favoring milder air masses, yet local elevation effects around the city and model spread in boundary-layer moisture keep exact minimums uncertain. Updated NHC-style guidance and station data through the night will clarify whether the low settles near the 12–14°C cluster or dips cooler.