Persistent northeasterly flow and extensive cloud cover from a broad high-pressure system are suppressing daytime heating across the Chicago area, holding projected highs for September 24 well below the 1991–2020 average of 73 °F. Official and ensemble guidance from NWS and private models converge on mid-60s readings, with the precise maximum hinging on the timing of any partial clearing, boundary-layer mixing depth, and Lake Michigan’s moderating influence on onshore winds. This range of uncertainty—spanning roughly 62–69 °F—explains the tightly bunched market-implied odds across those bins, as small shifts in insolation or wind speed could tip the daily high by a degree or two. Updated model runs and afternoon observations on the 23rd will provide the next key data points for resolution.