Trader consensus on Polymarket reflects tight forecast clustering around 78-83°F for Chicago O'Hare's highest temperature on April 17, driven by a persistent upper-level ridge over the Midwest sustaining warm air advection well above the 60°F climatological normal. Latest guidance from models like GFS and ECMWF, alongside Weather Underground's 80°F call under partly cloudy skies with light winds, supports this range, but subtle differences in morning cloud cover timing and boundary-layer mixing create uncertainty differentiating 78-79°F (24.5% implied probability) from 80-81°F (27.0%). Recent days' 80°F highs underscore the pattern's stability, with no disruptive fronts imminent; watch this afternoon's observations and evening model runs for final sharpening ahead of resolution based on official NHC O'Hare measurements.