Forecast uncertainty across National Weather Service and NOAA model runs drives the tight clustering of market-implied odds around 58–65 °F bins for Chicago’s August 21 low, with no single outcome exceeding 30 %. Ensemble guidance shows variability tied to overnight cloud cover, wind speeds, and the precise timing of a weak frontal passage, producing a several-degree spread in projected minimums near historical late-August normals. Traders weigh recent surface observations and short-range model consensus against climatological baselines, reflecting genuine scientific dispersion rather than a dominant trend. Updated NWS briefings and afternoon model releases remain the key near-term catalysts that could shift probabilities before resolution.