On Tuesday, temperatures are expected to reach the low 90's for just the third time this year and the first time this month.
The warmer weather and high humidity will make conditions oppressive.
The heat index is expected to be 95 to 100 degrees, making this the hottest day of the year.
On average, Chicago sees about 14 90-degree days during a year.
The National Weather Service says heat safety precautions should be taken for those at risk to heat-related illnesses or if you will be spending prolonged periods of time outdoors.
Cooler weather is expected to move in later Tuesday evening, possibly triggering some severe storms, with high winds and heavy downpours.