Recent ensemble forecasts from the ECMWF and GFS models show wide uncertainty in Shanghai’s minimum temperature on May 21, with possible outcomes ranging from cool advection behind a weak frontal boundary to warmer southerly flow tied to early East Asian monsoon influences. This bimodal spread explains why market-implied odds currently price the extremes of 15 °C or below and 25 °C or higher nearly equally at 41 %, while narrower bins around the climatological 18–20 °C range command lower probabilities. Shanghai’s subtropical location and urban heat-island effects amplify sensitivity to small shifts in 850 hPa winds and boundary-layer moisture. The next high-resolution model runs and updated China Meteorological Administration guidance, expected within 24 hours, will likely refine these probabilities ahead of the May 20–21 verification window.