Official National Weather Service observations at San Francisco International Airport recorded a minimum of 56°F on September 15, 2026, aligning precisely with the seasonal normal of 56°F and driving the near-certain market outcome. Mild marine layer conditions, light westerly winds, and typical late-summer fog patterns maintained temperatures near average without significant cold-air advection or radiative cooling extremes that could have pushed readings into the mid-50s or lower. Historical data show this range as common for the date, with traders incorporating verified station reports and model consensus on stable overnight conditions. Resolution hinges on the primary airport reading, though minor station-to-station differences or post-event quality control revisions remain theoretically possible but unlikely to alter the outcome.