Briefly cooler—then some real heat arrives

Somewhat cooler air dips into Chicago today and Tuesday, but it’s only a tease. In the atmospheric tug-of-war between today’s cooler air borne in by pleasant northeasterly winds and warm, humid air being pushed out, the warm air will yield briefly but ultimately triumph.
An atmospheric boundary (known to meteorologists as a front) that marks the transition zone between the cooler and warmer air masses will penetrate only to central Illinois by Tuesday morning, then stall and return north late Tuesday. Chicago’s daytime temperatures, cooling modestly only to the lower 80s today and Tuesday, climb well into the 80s midweek as warmer and more humid air returns. Then real heat and humidity arrive: 90º on Friday and lower or middle 90s over the weekend—and that might be only the beginning. The jury is still out, but computer models now suggest a lengthy period of hot weather, interrupted only by weak and modest cooling, lies ahead.
--By Richard Koeneman, WGN-TV Weather Center Meteorologist