Latest National Weather Service ensemble guidance positions Chicago’s August 15 high near the upper 70s to low 80s, below the 82.8 °F seasonal normal, as persistent cloud cover, easterly onshore flow off Lake Michigan, and elevated boundary-layer moisture limit daytime heating. These conditions have tightened trader consensus around the 80–81 °F and 82–83 °F bins, which together command nearly 70 % implied probability. Minor shifts in cloud timing or weak frontal passage could nudge the maximum by a degree or two, while any unexpected clearing would be needed to reach the low-80s tail. Updated short-range model runs and NWS forecast discussions remain the key near-term catalysts.