**Persistent cloud cover and elevated moisture from São Paulo’s record-rainy September have kept overnight radiative cooling limited, with official and model guidance pointing to a minimum temperature in the 15–20 °C range for 20 September.** Recent INMET and IPMet analyses note that the month’s excess rainfall and high humidity have maintained a warmer nocturnal boundary layer than typical for late winter/early spring, when clear-sky radiational cooling often produces lows near the 12–14 °C climatological average. Current ensemble runs show only modest temperature drops under mostly overcast skies and light northerly flow, reducing the likelihood of readings at or below 13 °C while still leaving room for slight model spread around 16–18 °C. Traders therefore assign the highest implied probability to the coolest bin, yet the tight clustering around mid-teens outcomes reflects genuine forecast uncertainty until final observational data are recorded.