Recent National Weather Service and model guidance point to a mild overnight period for Chicago on August 21, 2026, with minimum temperatures expected near the long-term normal low of 65°F. This produces closely matched market-implied probabilities clustered between 58–65°F because forecasters see modest variability from surface high pressure, light southerly flow, and possible scattered clouds or isolated showers that could enhance or limit radiational cooling. Dew points near 60°F and light winds reduce the potential for sharp drops below 58°F while preventing strong warm advection that would push readings above 65°F. Traders are therefore pricing the range of outcomes from ensemble spread in boundary-layer mixing and exact timing of any weak frontal passage rather than a single dominant driver.