Millions of people will have to brace themselves for an exceptionally hot July Fourth week as a large swath of the eastern United States prepares for a heat wave — all as deadly wildfires continue to burn in the West.

Limited time: Save 25% on NBC News subscription

Get exclusive reporting, live Q&As and ad-free reading.

Around 145 million people are under heat alerts from the Midwest to Florida and across the Northeast and New England. The heat is expected to ramp up through the week, with temperatures soaring into the upper 90s and 100s. Heat index values — or what the heat actually feels like to the human body — could reach 105 to 115 degrees Fahrenheit.

Cities in the risk zone include Boston, New York City, Philadelphia, Baltimore, Washington, D.C., and Chicago.

On Tuesday, major to extreme heat risk is expected, with warm overnight temperatures offering little relief and increasing the risk of heat-related illness.

The heat will peak Thursday through Saturday for the Midwest, Great Lakes, mid-Atlantic and Northeast. By July Fourth, close to 100 new daily record highs will be set.

New York City is expected to record temperatures as high as 95 Friday, with a heat index of 106. Washington and Raleigh will have highs of 103 later on this week with heat indexes of 108 and 107, respectively.

Chicago Mayor Brandon Johnson issued a warning to residents during a news briefing Monday, urging them to prepare for the extreme heat which “can be very dangerous.” The city is under an extreme heat warning until Wednesday, according to the National Weather Service Warning Coordination Meteorologist Mike Bardou, who also spoke at the briefing.

“If you can, stay inside with air conditioning on,” Johnson said. “If you do not have air conditioning, keep your blinds closed, but windows slightly open.”

Johnson also shared that there will be six community cooling centers open around the city by the Department of Family & Support Services. He encouraged Chicagoans to stay hydrated and check on their more vulnerable neighbors.

Bardou, who spoke about a 1995 heat wave that killed hundreds in Chicago, warned that heat-related illnesses can take victims by surprise.

“Heat is kind of one of those sneaky things, in a sense, relative to a thunderstorm coming or blizzard, you don’t necessarily realize the effects until it’s too late,” Bardou said. “And the effects of heat are cumulative, so day by day that goes by that you don’t have that ability to recover, cool off, the effects are worsening.”

Western wildfires

Over the weekend, numerous new wildfires erupted across the interior West, especially across Utah and Colorado.

Three firefighters were killed while tackling fires on the Colorado-Utah border this weekend. They were identified as 27-year-old Sydney Watson, 27-year-old Nick Hutcherson and 38-year-old Emily Barker, according to the U.S. Forest Service.

“We mourn the loss of three firefighters who answered the call to protect others and made the ultimate sacrifice in service to their fellow citizens,” U.S. Wildland Fire Service Chief Brian Fennessy said.

“Our thoughts are with their families, loved ones, friends and crewmates as they face an unimaginable loss,” he said in a statement. “These firefighters embodied the courage, professionalism and selflessness that define the wildland fire service.”

Two more firefighters were injured in the same incident and are receiving medical care, according to the Forest Service.

The largest of the fires, the Cottonwood Fire in southwest Utah, is now more than 93,000 acres and remains zero percent contained, according to WatchDuty.com, which has been tracking the blaze.

On Monday, fire alerts remain in effect for the Four Corner states — Arizona, Colorado, New Mexico and Utah — as well as parts of Wyoming, South Dakota and Nebraska.

There is an elevated risk for wildfires across the region and the Southwest, with the risk Tuesday increasing to critical once again for parts of Utah, Colorado, Arizona and New Mexico.