Latest National Weather Service guidance and ensemble model runs point to a daytime high near 79°F at Chicago O’Hare on May 18, driven by southerly flow ahead of an approaching warm front that lifts readings several degrees above the climatological normal of 71°F. This setup, reinforced by consistent short-range model consensus and a slight severe-weather risk noted by the Storm Prediction Center, underpins the 77.5% market-implied probability for 78°F or higher. Minor deviations could arise from unforecast cold-air intrusions or rapid model shifts, though the short forecast horizon and stable steering patterns limit such scenarios. Updated NWS briefings and fresh model runs over the next 24 hours will further refine the expected maximum before resolution.