Warm southerly flow ahead of an approaching cold front is driving National Weather Service forecast models toward a Chicago high temperature near 79°F on May 18, underpinning the market’s 92% implied probability for 78°F or higher. Ensemble guidance shows dew points climbing into the upper 60s with limited cloud cover and only modest mixing, conditions that historically support rapid daytime warming this time of year. Normal May 18 highs average 71°F, so the current setup represents a clear positive anomaly. Traders are weighting the consensus of latest GFS and ECMWF runs heavily, though any faster frontal passage or widespread showers could trim the peak by several degrees. Updated short-range guidance expected this evening will provide the next key test of that trajectory.