Official National Weather Service measurements from Chicago's primary airport stations recorded a daily maximum of 84-85°F on September 15, locking in the market's unanimous trader consensus. This outcome stems from direct thermometer readings under standard protocols, with no remaining forecast uncertainty once the calendar date passed. Climatological context for mid-September shows typical highs near 75°F, yet recent atmospheric conditions supported the observed peak without pushing into higher bins. Minor station discrepancies or rounding differences represent the only realistic scenarios that could theoretically alter resolution, though agency data have historically aligned closely on this threshold.