Official National Weather Service observations at San Francisco International Airport recorded a minimum temperature of 58°F on September 4, 2026, establishing the primary driver behind the market's near-certain positioning on the 58-59°F outcome. This reading occurred amid typical late-summer marine layer conditions, with clear skies and light winds allowing radiative cooling to that level while daytime highs reached 72°F. The value sits slightly above the 1991-2020 climatological normal low of 56°F for the date. Realistic challenges to resolution include any post-event data revisions from the reporting station or resolution criteria referencing a different official site, though such adjustments remain uncommon once preliminary climate summaries are issued.