**Overcast skies and northerly flow are the dominant drivers keeping Tokyo’s October 5 high tightly clustered around 21–22 °C.** Multiple forecast sources show persistent cloud cover through the afternoon, reducing incoming solar radiation and capping daytime warming despite a modest seasonal baseline near 24 °C. Light-to-moderate northerly to northwesterly winds (roughly 10–15 km/h) advect cooler air from higher latitudes, while high humidity (75–85 %) further limits temperature rise through evaporative cooling and reduced sensible heat flux. Model runs and official guidance align on a peak near 20–22 °C, with only low-probability scenarios for brief clearing that could allow 23 °C. The tight market spread between 21 °C and 22 °C reflects this narrow uncertainty band in the latest observational trends and ensemble guidance, with resolution hinging on whether any late-day breaks in cloud cover materialize before the daily maximum is recorded.