The Chicago area woke up to a muggy morning Thursday, bracing for the hottest and most humid day of the week.

An Air Quality Alert was issued by the National Weather Service for the entire metro area and many northwest Indiana counties due to the hot and stagnant air. The alert will remain in effect until 3 p.m. Thursday.

Residents, especially those with respiratory problems, are urged to limit their time outdoors. The Illinois counties under the alert include: Cook, DuPage, Grundy, Kane, Kendall, Lake, McHenry and Will. Those in Indiana under the alert are La Porte and Lake.

As the heat kicks in throughout the day, temperatures are expected to climb into the high 80s to low 90s with high humidity pushing heat index readings up to the 100-degree mark.

Hottest day this week! Upper 80s - low 90s

Heat Index: 95º - 100º

Storms after Midnight

Cooler Weekend@NBCChicago pic.twitter.com/kf2EGtOPJO — Alicia Roman (@AliciaRomanNBC) August 4, 2016

DAY PLANNER: Hot & humid. Upper 80s - low 90s (cooler lake) Heat Index: 95º-100º. Mostly sunny skies@NBCChicago pic.twitter.com/XAr3EYRyw8 — Alicia Roman (@AliciaRomanNBC) August 4, 2016

Temperatures drop into the low 70s Thursday evening, with a chance of storms developing overnight.

The chance of stormy weather lingers into the morning hours Friday before turning partly sunny and cooler.

A relief from the heat stays through Sunday, with sunny skies excepted during the weekend with daily highs in the low to mid 80s.