A few months ago, when Chicago finished its 10th warmest winter (followed by its warmest spring ever), I predicted a warm summer. Actually, the state climatologist predicted a warm summer, and I repeated this prediction.
Regardless, the mechanics are simple. Warm winters and springs keep Lake Michigan warm, which means come summer the lake can't absorb as much heat on hot days. This means, all things equal, a warm spring leads to a warm summer. (Oddly, though, warm summers have no effect on winter temperatures.)
How accurate was the prediction? Well, so far, this summer is worse than 1988:
The brutally hot and often bone-dry summer of 1988, serves as a benchmark for hot summers in the Chicago area. That year produced more 32°C and 38°C temperatures than any other on the record books here—47 and 7 respectively.
By June 19, the 1988 season had logged 10 days of 32°C temperatures. The long-term average of 90s [Fahrenheit] by June 19 has been just three. That means this year has been producing 90-degree days faster than one of the most prolific heat-generating summers in the Chicago area's history.
Someday I'll have a summer house in northern Saskatchewan. For the next three months, though, I expect to be uncomfortable.