**Cooler post-frontal air and northerly flow are the main drivers keeping Chicago’s September 13 high in the low-to-mid 70s, with National Weather Service guidance and local forecasts centering on 72–75 °F.** A cold front that passed the previous day advected a drier, less humid airmass northward, while a mid-level shortwave trough is promoting afternoon cloud cover and scattered showers (40 % chance after 2 p.m. CDT). These factors suppress daytime heating relative to the late-summer climatological average of ~77 °F. Model consensus and official point forecasts cluster tightly in the 72–75 °F range, producing the market’s heavy weighting on 74–75 °F (55 %) and 72–73 °F (25.5 %). A modest tail of probability on 76–77 °F (13.5 %) reflects residual uncertainty in how much any breaks in cloud cover or slightly delayed shower timing could allow additional warming before peak heating. Afternoon winds shifting northwest at 10–15 mph with gusts to 20 mph further limit temperature rise by enhancing mixing and evaporative cooling from any showers. No major model disagreements or rapid changes have emerged in the latest guidance, so trader consensus aligns closely with the current NWS forecast of a high near 72 °F under partly sunny skies with isolated to scattered showers.