Recent ensemble runs from the European Centre for Medium-Range Weather Forecasts and Global Forecast System, aligned with China Meteorological Administration guidance, show Shanghai’s May 18 maximum centered near 27°C under a weakening high-pressure ridge and light southerly flow that favors modest daytime warming without strong advection. This places 26°C and 27°C as the tightly contested leading outcomes, each carrying 29.5% implied probability, while the 22.5% on 28°C reflects residual model spread from potential afternoon cloud cover or slightly stronger sea-breeze cooling. Mid-May climatology for the city yields daily highs of 25–27°C on average, so the current distribution remains anchored to historical baselines rather than anomalous heat. Final resolution will hinge on official observations at Shanghai Pudong International Airport, with the last major forecast update expected within 24 hours.