Nearly all of the Chicago area will be under an Excessive Heat Warning this week as heat index values could rise as high as 115 degrees for some.
The National Weather Service preemptively issued the warning Wednesday afternoon for Cook, DeKalb, Kane, DuPage, Lasalle, Kendall, Grundy, Will and Kankakee counties in Illinois and Lake, LaPorte, Newton and Jasper counties in Indiana.
The warning was pushed back for most counties following a round of morning storms Thursday. The alert now begins at 3 p.m. Thursday for most of the counties, but at 10 a.m. Friday for Cook, DuPage and Kane counties. The warning continues through 7 p.m. Saturday.
Lake and McHenry counties in Illinois and Kenosha County in Wisconsin were also under a Heat Advisory from 10 a.m. Thursday through 11 p.m. Friday.
After a round of showers and storms in the morning, Thursday will become extremely hot and humid with highs from the upper 80s along the lakefront, to the low and mid 90s inland. But Heat Index readings will range between 100 and 108 degrees.
Air temperatures are forecast to rise into the upper-90s and drop only to around 80 degrees in the city Thursday and Friday nights, offering "little to no relief from the heat," according to the National Weather Service.
"The cumulative effects of temperatures and heat index values this high could lead to heat related illnesses with prolonged exposure," the watch states. "Those without air conditioning, elderly, small children, and pets are especially susceptible. Plan ahead."
Residents are being urged to find a cool place to shelter from the heat and to avoid outdoor activity during peak heating times of the day.
The National Weather Service also said "the urban heat island core of Chicago is most susceptible to night time temperatures that do not cool significantly, limiting the amount of relief that can occur at night."
ComEd announced Tuesday that it is increasing crew staffing and opening its Emergency Operations Center to "respond quickly if power outages occur."
Anyone who experiences an outage is urged to test "OUT" to 26633 ot call (800) 334-7661. Spanish-speaking customers can call (800) 955 8237.
Metra also said trains will need to operate at reduced speeds in the heat.
"When temperatures exceed 95 degrees, Metra is required to reduce train speed by 10 mph to compensate for heat related stress on the tracks," the rail agency tweeted Wednesday.
The city could see some relief Sunday, which looks to be partly sunny and not as hot with highs in the upper 80s. Still, humid conditions are expected with a chance of showers and storms.