Recent ensemble forecasts from the Japan Meteorological Agency and global models show substantial uncertainty in Tokyo's overnight minimum for May 21, with temperature guidance diverging between a mild 18–21 °C range under weak high pressure and cooler 12–14 °C readings if a stronger frontal passage occurs. This split directly explains the near-even market split between the 21 °C-or-higher bucket and the 11 °C-or-below outcome. Historical May climatology for the Kanto Plain places average lows near 15 °C, so traders are weighting the latest model runs that could shift either way before the event. Updated 00Z and 12Z guidance expected over the next 48 hours will likely narrow the range and move probabilities.