**Forecast consensus points to an overnight low in the mid-70s for Chicago on August 31, 2026, aligning with the tightly bunched market probabilities around 72–77°F.** Official guidance from sources such as the National Weather Service and private models (WeatherBug, WillyWeather) projects humid conditions with lows in the mid-70s under partly cloudy skies and light southwest winds, reflecting the absence of a strong cold front or significant radiational cooling. Key differentiating factors include persistent southerly flow advecting warm, moist Gulf air (elevated dew points near 70°F), which limits nocturnal temperature drops, combined with Chicago’s urban heat island effect that retains warmth in the city core. Recent August 2026 observations show overnight lows trending warmer than the long-term August average of ~64°F, with recent nights in the upper 60s to low 70s amid similar patterns. Model agreement is strong on the mid-70s range, though slight variations in cloud cover or wind speed could nudge the minimum by 1–2°F. Traders appear to weigh the high-probability mid-70s outcome against minor upside risk from any forecast revision or downside from unexpected clear-sky cooling, producing the observed clustering of implied probabilities. Resolution hinges on the official daily minimum recorded at Chicago-area stations.