Forecast models from the National Weather Service and private outlets show Chicago's August 21 overnight low settling in the low-to-mid 60s under mostly cloudy skies, light southwest winds, and a 50% chance of evening thunderstorms that could limit radiative cooling. This range aligns with the tightly bunched market leaders at 60-63 °F, where modest differences in cloud timing or dew-point recovery create the observed spread across bins. Daytime highs near 79 °F and elevated humidity further constrain how far temperatures can fall before sunrise, while historical climatology places the date's average minimum at 66 °F. Traders are weighting the latest model consensus and any late-day observational updates that could shift the exact minimum by a degree or two before resolution.