(CNN)A dangerous and potentially deadly heatwave threatens much of the continental US, with dozens of record high temperatures both in the daytime and in the evening expected this weekend.
More than 150 million people in nearly 30 states were under a heat watch, warning or advisory on Thursday morning, according to CNN meteorologist Dave Hennen.
Over the next few days, more than 85 percent of the lower 48's population will see temperatures above 90 degrees Fahrenheit, Hennen said, and more than half will see temperatures in excess of 95 degrees.
Much of the heat expected for Thursday was forecast to descend on the Midwest and Mississippi, before making its way to the Northeast on Friday and Saturday, Hennen said.
On Thursday, cities under excessive heat warnings included Chicago, Oklahoma City, Omaha, St. Louis, Indianapolis, Detroit and Philadelphia.
Minneapolis, Little Rock, Memphis, Nashville and Raleigh were all under heat advisories.
Cleveland, Pittsburgh, Washington, Baltimore, New York and Boston were all under excessive heat watches and expected to see high temperatures this weekend.
Nighttime cooldown? Not much
The extreme temperatures overnight are an important and dangerous threat, because it means peoples' bodies and homes will not have the opportunity to cool off.
"Even after the sun goes down, the temperatures will not drop much below 80 degrees," said Rich Guidice, executive director of Chicago's Office of Emergency Management, "offering little to no relief."
The heat wave has prompted the city of Chicago to open cooling centers throughout the city for anyone looking for relief.
Chicago officials warned residents to look out for signs of heat exhaustion and heat stroke, including cramps, feeling weak and nausea.
Residents should not hesitate to call 911 if they believe they are suffering from heat stroke, said Dr. Allison Arwady, the acting commissioner of the Chicago Department of Public Health.
According to meteorologists, the heat wave will encompass much of the eastern half of the country into the weekend, before a cold front should bring relief late Sunday and into early next week.
Heat wave made worse by climate crisis
Experts say the heat wave is only made worse by the ongoing threat of climate change. According to last year's National Climate Assessment, the number of hot days in the US is increasing.
Heat waves have also increased in frequency, rising from an average of two per year to six per year in the last five decades. The threat is especially pronounced in the Northeast, where "the frequency, intensity, and duration of heat waves is expected to increase" due to climate change.
By 2050, the Northeast can expect approximately 650 more deaths each year because of extreme heat, the assessment found.
