Recent National Institute of Meteorology forecasts and ensemble models project a daytime maximum near 22 °C for São Paulo on May 18, driven by a typical mid-autumn pattern of moderate Atlantic moisture, light easterly flow, and possible brief showers that limit surface heating. Historical climatology shows May averages of 21–23 °C, with occasional frontal passages producing 1–3 °C cooling. Current model consensus favors 20–22 °C peaks, explaining the market’s clustering around those outcomes, while uncertainty in exact cloud cover and timing of any rain keeps probabilities spread across 19–21 °C. Updated runs from INMET and global ensembles are expected before resolution and could shift implied odds if drier conditions allow stronger afternoon warming.