**The official National Weather Service minimum of 58°F at San Francisco International Airport (KSFO) on September 16, 2026, recorded at 5:55 a.m. PDT, directly anchors the 99.5% market-implied probability for the 58-59°F bin.** This value sits 2°F above the 1991-2020 normal low and reflects typical late-summer marine-layer influence that moderates overnight cooling along the coast without allowing deeper drops. Recent model guidance and surface observations showed no anomalous cold-air advection or clear-sky radiative cooling that would push temperatures into the 56-57°F range. The 0.5% probability on the lower bin accounts for minor station differences or measurement uncertainty, but the authoritative KSFO reading aligns precisely with the dominant outcome. A shift would require revised official data or an alternate station definition outside standard Polymarket criteria.