An arm of hot and humid air from the South will extend across the Midwest into midweek.
Much of the South Central states have been rolling in a late-summer heat wave since last week, where the combination of high temperatures, excessive humidity, sunshine and other conditions resulted in AccuWeather RealFeel® temperatures above 110 F on multiple days.
The air began to surge northward this weekend over part of the upper Mississippi Valley.
In the Northern states, a heat wave is loosely defined as three consecutive days where temperatures reach 90 F or higher.
Much less-intense heat will expand farther to the east over a portion of the Great Lakes and much of the Ohio Valley. Most locations will have two- to three days with high temperatures ranging from 85 to 90 degrees.
The hottest day of the span around Chicago will be on Monday, when RealFeel temperatures will peak near 100 F for several hours.
The heat is hitting at a time when college and some high school and elementary school students are back in the classroom.
The pattern will offer a chance for those who have time off to enjoy some extended summer weather, of which has been rather limited this season.
Temperatures have averaged below normal from St. Louis and Chicago to Detroit, Cincinnati and Pittsburgh since June 1.
The temperature has approached 90 degrees on only a handful of days this summer, the majority of which have been scattered through the season. As of Aug. 24, Indianapolis and Pittsburgh have not hit 90 this summer.
A spell of very warm weather will also occur in the East at midweek. Temperatures in some neighborhoods from Washington, D.C. to New York City and Boston could reach the 90-degree mark.
While the region received a break from rainfall at the start of the week, the risk will increase from northwest to southeast during the middle part of this week.
Showers and thunderstorms associated with a press of cooler air will mark an end to the heat and humidity over much of the region Tuesday through Thursday.