Recent model runs from the National Weather Service and global ensembles show notable spread in the timing and strength of a weak cold front approaching the Midwest, keeping implied probabilities for Chicago’s August 31 minimum closely bunched between the 65 °F or below and 84 °F or higher bins. Day-to-day variability in boundary-layer moisture, cloud cover, and nocturnal radiative cooling under late-summer high pressure creates the primary uncertainty, with ensemble members differing by 8–12 °F on the overnight low. Historical climatology places typical late-August minima near 62–68 °F, yet the current pattern’s weak steering flow allows small shifts in frontal speed or upstream ridging to swing the outcome several degrees. Updated short-range guidance and the 00Z model cycle on August 30 will provide the next high-impact data point for traders.