Official meteorological observations and short-range forecast models from agencies including Turkey’s MGM and international sources show Istanbul’s peak temperature on September 25 reaching 21°C under mostly cloudy skies, light northeasterly winds, and minimal precipitation. This consensus, reinforced by real-time station readings and model agreement on maximum values near 21–22°C, underpins the market’s near-certain outcome for that exact threshold. Historical September climatology in the region supports typical daily highs in the low 20s, aligning with current conditions. Late-day revisions in official reporting or differences between measurement stations remain the only realistic factors that could alter resolution, though both appear unlikely given the advanced stage of the diurnal cycle.