Trader consensus on Polymarket implies a highest temperature around 65°F at Chicago O'Hare International Airport on April 18, with 66-67°F edging 64-65°F due to subtle differences in National Weather Service guidance and ensemble model outputs from GFS and ECMWF. Recent dense fog across the region on April 17 morning has delayed diurnal heating, while a moist southerly flow and lingering clouds from the wettest meteorological spring on record temper highs above the 60°F climatological normal. Key differentiators include timing of fog dissipation and partial clearing—earlier breaks favor 66-67°F via enhanced insolation, while persistent overcast or stronger mixing from gusty south winds cap at 64-65°F. New 12Z model runs and afternoon NWS updates could refine this tight spread before resolution.