Recent cold air advection from a polar front, combined with clear skies and radiative cooling under high pressure, drove the strong trader consensus around a 13°C minimum in São Paulo on September 8. Official monitoring from Inmet and the Centro de Gerenciamento de Emergências showed temperatures bottoming near that threshold after the sharper 10.1°C reading on September 7, consistent with seasonal September climatology and model consensus that the coldest air had already passed. Surface observations and ensemble forecasts aligned on limited overnight cooling once winds shifted and humidity rose slightly, producing the market-implied probability above 90%. Scenarios that could still alter resolution include post-analysis revisions from official stations or unaccounted microclimate effects at the primary measurement site.