In Chicago, forecasters said Thursday's temperature could break the all-time record of 105 degrees set July 24, 1934, and the city could have record-breaking temperatures three straight days by Friday, the Chicago Tribune reported.
The National Weather Service predicted the heat index in Chicago -- what it feels like outside -- would reach 118 Thursday and Friday. An extreme heat warning has been issued for the area until 10 p.m. Friday.
Temperatures from the high 90s into the triple digits were forecast in the mid-Atlantic and Midwest Thursday.
Meanwhile, power was restored to most customers in the Baltimore and Washington areas.
Baltimore Gas and Electric Co. said outages fell to 67,000 by 7 p.m. Wednesday, down from a high of more than 675,000 after Friday's storms, The Baltimore Sun reported.
In the Washington, D.C., area, 38,000 homes and businesses remained without electricity amid the scorching heat, The Washington Post reported.
Pepco said it expected to have restored power to more than 90 percent of the utility's customers by Wednesday night -- days earlier than Pepco had predicted.
But some of those without electricity expressed frustration and anger.
A roadside sign in the Washington area said, "Pepco: very warm humans feeling forgotten."
In the Washington area, several Independence Day celebrations were canceled so police and fire personnel would be available to respond to emergencies.
The number of deaths attributed to the weather rose to 12 in Virginia after a person struck by a falling tree on Monday died Tuesday, the Post reported. Seven deaths have been attributed to the weather in Maryland.
In West Virginia, where Gov. Earl Ray Tomlin said 238,685 customers remained without power Wednesday evening, governments and charities provided food and water to thousands of people, CNN reported.
The American Red Cross in West Virginia was prepared to hold it first mass feedings in four regions, Becky Howard, regional chief development officer, said
The Red Cross expected to provide up to 25,000 meals a day starting Thursday, Howard told CNN.