Forecast uncertainty across numerical models is the main driver behind the tight clustering of Polymarket odds around 27–32°C for São Paulo’s September 27 maximum. Official and ensemble guidance shows a range of outcomes tied to variable low-level moisture, southeasterly flow, and limited cloud cover that could either suppress or enhance daytime heating. Some runs project clearer, warmer conditions favoring 32°C or higher, while others indicate slightly more stable air or earlier cloud development capping the peak nearer 27–29°C. Historical late-September climatology for the city centers near 26–28°C, so current model spread places the most likely outcomes just above or below that baseline. Updated runs from INMET and global ensembles over the next 24–48 hours will be the key catalyst for any shift in trader positioning.