**Forecast model spreads and atmospheric dynamics create tight clustering of implied probabilities around 58–67°F for Chicago’s September 8 minimum.** Official National Weather Service guidance and ensemble outputs show overnight lows most likely in the mid-to-upper 60s, with some guidance holding near 65°F under clearer skies while others trend toward the low 70s amid increasing cloud cover and southerly flow ahead of a weak disturbance. Southerly winds advect warmer, more humid air from the Gulf region, reducing the typical nocturnal cooling rate, while variable high and mid-level cloudiness modulates longwave radiation loss. Lake Michigan’s moderating influence keeps extremes in check compared with inland areas. Historical September climatology places average lows near 62°F, but the current pattern—following several days of highs in the 80s and 90s—favors a narrower diurnal range. Differing model solutions for the timing and coverage of Tuesday-night showers produce the observed market balance, with traders weighting the higher-probability bins that capture the most consistent guidance while assigning lower odds to outliers below 58°F or above 70°F. Updated NWS and model runs expected Monday evening will likely refine the range before resolution.