Recent National Weather Service and private model guidance points to a minimum temperature in the low-to-mid 70s for Chicago on September 4, amid a persistent warm, humid air mass with easterly flow off Lake Michigan moderating overnight cooling. Highs are forecast near 87–89 °F under partly to mostly cloudy skies and a 30–50 % chance of showers or thunderstorms, which could limit radiational cooling and keep the low closer to 73–77 °F. Historical September 4 averages are near 61–63 °F, underscoring the anomalous warmth, while model spreads in cloud cover, wind shifts, and precipitation timing create the tight clustering of market-implied probabilities across the 70–77 °F bins. Traders are weighting the consensus forecast range against residual uncertainty in the final overnight minimum.