A cold front advancing across southern Brazil on August 21 is the main driver behind the tightly clustered Polymarket odds around 11–15°C for São Paulo’s daily minimum. Official forecasts from INMET and international models converge on a low near 12°C under increasing cloud cover, light showers late in the day, and southerly winds that limit daytime heating while promoting radiational cooling overnight. Subtle differences among outcomes hinge on the precise timing of frontal passage, the depth of the post-frontal air mass, and variable urban heat-island effects at official stations, all of which can shift the recorded low by 1–2°C. With typical August climatological lows near 13–14°C, this event sits slightly below average, keeping probabilities evenly distributed across the central bins and leaving little room for extremes below 10°C or above 16°C.