National Weather Service forecasts and ensemble model guidance indicate an overnight low in the mid-60s for Chicago on August 22 under mostly clear skies and light winds, positioning 66-67°F as the leading market outcome. This aligns with climatological norms near 65°F while reflecting current high-pressure dominance that limits cooling, with recent runs showing minimal spread between 64-67°F. Traders appear to weigh the narrow range of official guidance against typical model uncertainty in boundary-layer temperatures, with lower buckets like 62-63°F reflecting downside risks if radiational cooling strengthens overnight. Upcoming NWS updates and any late-model shifts in dew points or cloud cover will likely drive the next adjustments in implied probabilities.