South winds gusting over 40 mph brought unseasonably mild air across the metro area on Monday. The 65-degree high recorded at O’Hare airport was just 1 degree shy of the record for the date, set in 1998. The warmest December temperatures on record were 71-degree highs observed in 1970, and again in 1982. Thunderstorms, another rare occurrence this time of year, marked the arrival of polar air Monday evening. One-day temperature falls of 30 degrees or more are expected as much colder conditions settle across the region. The rest of the work week is expected to be dry until a reinforcing push of cold air sweeps in Friday night. Computer guidance suggests a pocket of stronger jet stream winds may help produce a period of light snow that could whiten the ground by Saturday morning.