**Official temperature observations from Chicago-area stations, including O'Hare, show a daily maximum reaching the 70-71°F range amid overcast skies and widespread showers driven by a slow-moving frontal system.** National Weather Service guidance and real-time data align with this peak, reflecting mild southerly flow and high humidity that capped further warming while preventing significant cooling. This positions the 70-71°F bin as the near-certain outcome in trader consensus, consistent with historical late-September variability where highs often cluster in the upper 60s to low 70s. The strong market-implied probability reflects aggregated confidence in verified measurements over model spreads. A realistic challenge would require a late upward revision in the official NWS daily climate summary or discrepancies in station data exceeding typical observational error.