Official National Weather Service observations recorded a 66°F minimum at Chicago O’Hare on September 17, 2026, aligning with the market’s leading 66-67°F outcome. Elevated overnight humidity near 100 percent, combined with 0.71 inches of rainfall and light winds, limited radiational cooling and kept the low several degrees above the 1991-2020 September average of 57-58°F. These conditions, typical of a moist late-summer air mass, explain why probabilities cluster tightly around 64-67°F ranges while lower outcomes remain discounted. Final resolution depends on the precise station and rounding rules used by the market.