The market consensus centers on the official National Weather Service maximum of 66-67°F recorded at Chicago’s primary observation sites on September 21. This outcome was driven by a stable high-pressure ridge that produced clear skies and light winds, limiting daytime heating and keeping readings aligned with late-September climatological normals for the region. Ensemble forecasts from the National Centers for Environmental Prediction showed strong agreement on this narrow range in the final 48-hour period, with minimal spread among guidance. An unexpected warm-air advection event or stronger urban heat-island influence could have shifted the high into the upper 60s or low 70s, yet surface observations and quality-controlled data confirmed the 66-67°F threshold without revision.