Relatively cool Friday, but heat is on the way

Chicago area temperatures on Friday will reflect proximity to Lake Michigan. Highs, confined to the 70s near the lake because of light onshore winds, will rise to the lower and middle 80s inland. On Saturday, however, and for a few days beyond that, southwesterly winds will prevail right to the shore of the lake, and no lake-induced cooling should occur. The season’s first real heat wave will begin and, present indications suggest, last through Wednesday. Modest cooling arrives after that.
As often occurs when hot weather prevails across the central and eastern U.S., it becomes cool in the West because weather systems are often about 1,200 miles in east-west extent. Temperatures have been 15 to 25 degrees above normal from the Rocky Mountains westward, but readings there will cool to near or below-normal in coming days.