Recent National Weather Service and ensemble model guidance points to an overnight low in the mid-to-upper 60s °F for Chicago on September 18, with the minimum most likely occurring in the pre-dawn hours amid increasing cloud cover and a chance of showers. Persistent warm advection from the south and elevated dew points near 60 °F have limited radiative cooling, keeping temperatures several degrees above the 1991–2020 climatological average low of 57.7 °F. Model spread in boundary-layer mixing and precipitation timing creates the tight clustering of market-implied probabilities around the 64–69 °F bins, as even modest shifts in cloud timing or wind could move the realized minimum by 2–3 °F. Updated short-range guidance and hourly observations through September 17 will be the key near-term catalysts for any repricing.