Official meteorological stations, including INMET networks, recorded a daily maximum of exactly 20°C in São Paulo on September 12 amid persistent heavy rainfall totaling over 75 mm in 24 hours at key sites like Mirante de Santana. A polar-origin cold front and associated moisture surge suppressed daytime heating, keeping readings well below typical early-spring averages near 23–26°C and aligning with forecasts of maxima capped at or below 20°C through mid-month. This data-driven outcome produced near-certain market pricing at 20°C, as traders incorporated verified observations rather than models. Only an unreported station anomaly or post-hoc data revision could alter resolution, though such shifts remain improbable given multi-station consistency.