A surge of polar air from southern South America, reinforced by a cold front, drove the dominant trader consensus toward an 11°C low for São Paulo on September 7. Official Inmet data from the Mirante de Santana station recorded a minimum of 10.1°C—the lowest September reading in four years—consistent with radiative cooling under clear, calm overnight conditions. This aligns with climatological expectations for early-spring cold outbreaks in the region and matches pre-event model consensus from agencies tracking the air mass trajectory. The 90% implied probability for 11°C reflects high confidence in these verified observations, though minor discrepancies in station choice, rounding protocols, or post-event data revisions could still shift resolution toward 10°C.