Japan Meteorological Agency ensemble models currently cluster Tokyo’s May 20 overnight low near 18–21 °C, driving the tight spread in trader-implied probabilities across those exact outcomes. Mid-May climatology shows a gradual seasonal rise in minimum temperatures from roughly 14 °C early in the month toward 17–19 °C by the 20th, modulated by persistent high pressure that favors clear skies and efficient radiative cooling under light winds. Subtle differences in forecast guidance hinge on whether a weak frontal boundary or increased low-level moisture arrives late on the 19th, potentially limiting cooling to 19–20 °C, versus continued dry northerly flow that could allow a dip to 17–18 °C. Updated JMA runs and international model consensus expected within the next 48 hours will narrow the uncertainty range ahead of resolution.