Official National Weather Service observations at Chicago O’Hare recorded a daily maximum of 80 °F on September 9, 2026, two degrees above the 1991–2020 normal of 78 °F under partly cloudy skies with light rain and fog. Market-implied odds of 100 % on the 78–79 °F bin reflect trader consensus anchored to pre-event model guidance and the narrow resolution window around climatological norms. This positioning aligns with historical September 9 highs averaging 78.6 °F and the low probability of significant late-summer heat anomalies once steering patterns and moisture advection stabilized. A realistic challenge would require revised station data, a different official reporting site, or measurement adjustments exceeding 1–2 °F, scenarios traders currently assign negligible weight given the final NWS summary.