Official NWS forecasts and supporting model guidance project a high near 89–90°F for Chicago on August 31 amid southwesterly flow that advects warmer, humid air ahead of a late-summer pattern. Morning scattered showers and thunderstorms could reduce insolation and cap the peak slightly below model maxima, while afternoon clearing would allow temperatures to reach the low 90s. This uncertainty between modest convective suppression and stronger surface heating explains the near-even market split between the 88–89°F and 90–91°F bins. The outlook sits well above the 1991–2020 average high of 81°F, with resolution depending on the precise timing and coverage of any remaining convection before peak heating.