It will be 90 degrees or warmer for nearly 90% of Americans this week

• Over 104 million Americans live where some level of heat alert is in effect.
• Several cities are likely to see their highest temperatures of the summer so far.
• Dangerously high temperatures and humidity could quickly cause heat stress or heat stroke.

Tens of millions of Americans will swelter through the hottest weather of the summer over the next few days as a record-breaking heat wave builds across much of the central and eastern U.S.

"A dangerous and widespread summer heat wave is expected through this upcoming weekend across much of the central and eastern U.S.," the National Weather Service said. "A large dome of high pressure will allow high temperatures to surge into the 90s and 100s in many locations, while heat indices will top 100 and approach 110 degrees or higher."

In all, more than 115 million Americans live where some level of heat alert is in effect, according to the Weather Service. And a total of 290 million will see high temperatures of at least 90 degrees at some point in the next week, BAM Weather meteorologist Ryan Maue tweeted.  

Several cities are likely to see their highest temperatures of the summer so far, including Chicago, Detroit, New York City and Washington, D.C., according to the Weather Channel.

No AC?:How to stay safe during a heat wave

Nighttime will offer little relief as temperatures dip only into the mid- to upper-70s in many locations, which can be a health threat: "At those levels, nighttime provides little relief for people that may not have additional or adequate cooling resources and the body has to work harder to cool itself," University of Georgia meteorologist Marshall Shepherd wrote in Forbes.

The Weather Service predicts that dozens of records for warm nights will be set across the country this week.

'Breaking' the heat index?:US heat waves to skyrocket as globe warms, study suggests

Lights out?:Triple-digit temperatures could bring blackouts in New York City, DC and Chicago

Some record highs are also forecast to be broken this week, including in Philadelphia; Newark, New Jersey; Manchester, New Hampshire; and Atlantic City, New Jersey, according to AccuWeather.

Factoring in the heat index, temperatures will be even more brutal: "Friday will be suffocating across much of the eastern U.S., with the heat index 100 degrees to 110 degrees from the Plains to the Great Lakes and the Middle-Atlantic to NYC,"  Maue tweeted. 

The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. 

The National Weather Service has issued excessive heat watches and heat advisories in the Plains, Midwest, Ohio Valley and parts of the East to warn residents of the "dangerously hot temperatures ahead," the Weather Channel said. 

"Dangerously high temperatures and humidity could quickly cause heat stress or heat stroke if precautions are not taken," the Weather Service in St. Louis warned. "The very young, the elderly, those without air conditioning and those participating in strenuous outdoor activities will be the most susceptible."

For folks in the South cleaning up from Hurricane Barry, the high heat will add to the misery.

"While the heat is not uncommon for this part of the nation during the middle of July and in the wake of a tropical storm, it can add to the difficulty of cleanup operations for those dealing with mud, fallen trees and power outages in portions of Louisiana and Mississippi," AccuWeather senior meteorologist Alex Sosnowski said.

Some relief is in sight for next week: AccuWeather meteorologist Paul Pastelok said that "temperatures are forecast to throttle back to seasonable levels or even slightly below average for a time next week in the East as heat shifts to the western U.S. There should also be a substantial drop in humidity in the Northeast next week."

However, the high heat in the West, combined with lightning strikes, will lead to increased wildfire danger.