**Recent National Weather Service guidance and ensemble model runs show substantial uncertainty in Chicago’s overnight minimum on August 22, centered near the long-term average low of ~65 °F.** Variable cloud cover, light winds, and Lake Michigan’s moderating influence are the dominant factors keeping the tightest market-implied odds clustered in the 64–67 °F range. Nocturnal radiative cooling will determine the low: clear skies and calm winds allow stronger surface heat loss and deeper inversions, while increased clouds or a weak frontal boundary can limit cooling. Lake Michigan’s thermal inertia typically prevents the lowest readings from falling far below climatology when onshore flow develops, but light southwesterly or variable winds may reduce this effect. Ensemble spreads reflect these variables, with some solutions dipping into the upper 50s under optimal cooling and others holding near 70 °F if clouds or mixing persist. The slight edge to 66–67 °F in trader sentiment aligns with guidance favoring near-normal conditions without a strong cool-air advection signal, while the broad distribution across adjacent bins underscores the inherent forecast uncertainty this close to the event. Updated model runs and the evening NWS forecast discussion will be the next key inputs.