Recent Japan Meteorological Agency forecast models and ensemble runs show Tokyo’s overnight minimum on May 20 likely falling between 18–21 °C, with probabilities tightly bunched because of modest uncertainty in cloud cover, wind speed, and the precise timing of any weak frontal passage. Typical May climatology places average daily lows near 17 °C, but current atmospheric conditions—warm, moist southerly flow ahead of a slowly approaching trough—support a slightly elevated baseline. Small differences in boundary-layer mixing or urban heat-island retention could easily shift the recorded minimum by 1–2 °C, explaining why traders assign nearly equal weight to the 18 °C, 19 °C, and 20 °C outcomes while assigning sharply lower odds to extremes below 16 °C or above 23 °C.