National Weather Service United States Department of Commerce

Temperatures will yet again peak 90°+ for many locations away from the Lake Michigan shore. Heat index readings (combination of heat and moisture) will peak near to a few degrees above the temperature. A lake breeze and associated lake cooling will bring some heat relief to shore areas with primarily low 80s immediately lakeside and areas just inland this afternoon. For more on heat safety, see weather.gov/heat.
The unusually warm late-September pattern will continue through Tuesday before a cold front moves through and ushers in more typical temperatures for this time of year. There is a slight chance of showers and thunderstorms Tuesday associated with the passage of the cold front.
An Air Quality Alert (Air Pollution Action Day) has been declared by the Illinois EPA for the Chicago metropolitan area through Monday, September 25th. The air quality index is expected to be in the unhealthy for sensitive groups range. Active children and adults, especially people with pulmonary or respiratory disease such as asthma, should limit prolonged outdoor activity. More information can be found at www.airnow.gov.