Recent National Weather Service guidance and ensemble model runs show Chicago’s overnight low on August 22 centered in the mid-60s, with 66–67 °F emerging as the market favorite at 50 % implied probability. Key variables driving the tight spread versus the 45 % odds on 74 °F or higher include uncertain cloud cover and wind speeds that control radiational cooling, plus the moderating influence of Lake Michigan’s surface temperatures. Historical August normals near 65 °F provide context, yet short-term model disagreement on a possible warmer, more humid air mass creates the observed split. Updated NWS forecasts and next model cycle will likely sharpen resolution before the market closes.