With the leading edge of a cool Canadian-source high pressure air mass nosing into the Midwest and Great Lakes, the first below-normal temperatures in 18 days will settle into the Chicago area the next couple days. The cold air aloft could trigger a few instability showers over and near lake Michigan, while farther inland overnight temperatures will drop into more fall-like upper 30s and lower 40s.
By Sunday winds will become more southerly, allowing the start of what looks to be a significant warm-up that could last through most of the coming workweek. A strong ridge of high pressure centered to our south and east will keep a persistent southwest flow here, and hold off a cold front slowly approaching from the northwest. The front may get close enough to spread showers/thunderstorms into our area by next Thursday.