Forecast models from the National Weather Service and private sources indicate Chicago's August 20 high will likely fall in the upper 70s to low 80s, driven by moderate west-to-east flow, variable cloud cover, and scattered afternoon convection that limits daytime heating near climatological norms of 82°F. Recent model runs show tight clustering around 78–81°F due to uncertainties in boundary-layer moisture and timing of any showers, with limited influence from the mid-month derecho's lingering effects. Traders assign near-equal weight to the 78–79°F and 80–81°F bins because small shifts in steering patterns or insolation could tip the official O'Hare observation across that threshold, while higher outcomes remain improbable absent stronger subsidence. Updated NWS guidance and ensemble spreads tomorrow morning will provide the next key data point for resolution.