Japan Meteorological Agency forecasts hold Tokyo’s official overnight minimum at 18 °C on May 17 under a persistent high-pressure ridge that drives southerly warm advection and suppresses nocturnal radiative cooling across the Kanto Plain. Successive model runs have remained within a tight 1 °C envelope, producing the market’s overwhelming 99.7 % implied probability for that outcome. Historical May climatology for central Tokyo shows typical lows near 14–16 °C, so the current warmth represents a clear departure from baseline. The primary risk to this consensus lies in a late forecast shift from operational models or minor local siting effects at the official observing station that could nudge the recorded value outside the 18 °C bin.