Recent ensemble forecasts from Brazil’s INMET/CPTEC and the European Centre for Medium-Range Weather Forecasts place the May 18 maximum in São Paulo near 21–22 °C under typical Southern Hemisphere autumn subsidence and light easterly flow. Small differences in the timing and strength of a weak cold front expected late on the 17th or early the 18th, combined with variable low-level moisture and cloud cover, create the tight clustering around 19–21 °C. These factors determine whether daytime solar heating reaches the upper or lower end of that narrow range, while climatological May highs of 22–23 °C provide the baseline against which model spread is calibrated. Updated runs tomorrow morning will refine the exact steering pattern and precipitation timing ahead of market resolution.