Recent ensemble forecasts from global models reveal substantial uncertainty in Tokyo's minimum temperature on May 21, with projected lows spanning roughly 12–22°C depending on the timing of a weak frontal passage and overnight radiative cooling under variable cloud cover. Typical May climatology places average lows near 16°C, yet current synoptic patterns show a split between cooler maritime air advection and warmer subtropical ridging, producing the market's near-even odds between sub-11°C and 21°C-plus outcomes. Japan Meteorological Agency updates over the next 48 hours, along with refined high-resolution model runs, will likely narrow this spread as the event window approaches.