Official observations from Brazil’s Instituto Nacional de Meteorologia confirmed a daily maximum temperature at or below 20°C in São Paulo on May 15, anchoring the market-implied odds at essentially 100% for that outcome. A persistent polar air mass, reinforced by strong southerly winds and extensive cloud cover with relative humidity near 90%, suppressed daytime heating and produced stable atmospheric conditions that kept readings well below the month’s climatological average of roughly 23°C. High-resolution models from INMET and CPTEC converged on subdued highs around 19°C, with only limited clearing possible late in the day. While station-to-station micro-variations or post-processing revisions could theoretically nudge the final recorded value, the broad observational consensus across automated networks leaves little realistic scope for a higher outcome.