Recent National Weather Service and model guidance indicate Chicago's August 20 high will likely fall in the upper 70s to low 80s, below the 82°F climatological normal, as a cooler, drier air mass follows recent frontal passages. Variable cloud cover, potential scattered showers, and modest southerly flow will determine whether the maximum reaches 80–81°F or stays at 78–79°F, with ensemble spreads reflecting modest uncertainty in daytime heating and boundary-layer mixing. Traders assign nearly equal weight to these bins because small shifts in timing or intensity of any lingering moisture can alter peak readings by a degree or two, while historical August variability and the narrow resolution thresholds keep both outcomes competitive.