Millions of people are facing dangerous and potentially record-breaking heat as a heat wave grips much the central and eastern United States through the Fourth of July weekend.

Forecasters said temperatures are expected to soar into the 90s and low 100s across large parts of the country, with many places expected to challenge or break daily records as the heat intensifies and spreads east.

Michael Muccilli, a meteorologist with the National Weather Service, called the forecast “the hottest temperatures so far this season” on Monday afternoon, and he warned the danger would be “further heightened” because of the number of people expected to spend time outdoors in the lead-up to the long holiday weekend.

High humidity will make conditions feel even more oppressive, forecasters warned. Heat index values — a measure of what the temperature feels like to the human body, when humidity is considered with the air temperature — could reach as high as 110 to 115 degrees.

And there will be little relief overnight, as temperatures are expected to remain in the 70s across many of the areas facing high temperatures. In some major cities nighttime lows could stay in the 80s.

Here’s what to expect and when:

Monday:

The heat began building over parts of the Plains on Sunday, and by Monday nearly 40 million people were under extreme heat warnings. Areas of Minnesota, Wisconsin, Iowa, Missouri, Kansas, Illinois, Indiana, Michigan, Kentucky and Tennessee are expected to endure several days of temperatures in the 90s through at least Thursday, forecasters said.

The temperature at Chicago’s Midway Airport was expected to climb to 92 degrees on Monday, said Casey Sullivan, a meteorologist with the Weather Service, and could be even higher in the days following.

People in the affected areas should take action to protect their health, he added. “If you can stay in air conditioning, that’s probably the number one thing to do. If you have to be outside, limit your time outside. If you have to work outside, take frequent breaks, drink plenty of water, try to take breaks in an air-conditioned environment.”

Tuesday:

By Tuesday, the heat will become more intense around the Great Lakes, especially, and it will begin to spread farther east across Ohio and into western Pennsylvania and New York.

Overnight temperatures in the Chicago area would be in the high 70s through much of the week, Mr. Sullivan said.

These conditions do not “really allow for any cooling,” Mr. Sullivan said. “The next day of the heat wave, it allows it just to be that much more oppressive. So it’s a cumulative effect on people.”

Wednesday:

Once again, places that were already hot will become hotter, and higher temperatures will spread farther east. By Wednesday, most of the East Coast will be at some risk of dangerous temperatures, according to the Weather Service, but the most extreme temperatures will remain in the states around the Great Lakes.

Most of Wisconsin, Ohio, Michigan and Illinois will be under the Weather Service’s most extreme and rare warning level, reserved for long-lasting extreme heat that offers little to no relief at night.

Washington, D.C., is forecast to be around or above 100 degrees for four consecutive days between Wednesday and Saturday. Mr. Muccilli said if that does happen, it would tie a record for the city.

Thursday:

The worst of the heat is forecast to reach much of the East Coast by Thursday. But it will not abate in the areas around the Great Lakes, either.

Some 250 million people in the eastern half of the United States, from the Dakotas to Boston and Chicago to Miami, live in areas that are expected to experience some level of dangerous heat on Thursday, according to the Weather Service.

Cities including Washington, New York, Philadelphia and Boston could see temperatures climbs into the low 100s, with several daily records under threat. Forecasters said Philadelphia could also tie its all-time June record high of 104 degrees.

In any given year, the average number of extreme high risk heat days is around one or fewer along the I-95 corridor from Boston to Washington, DC. Some locations in this heat wave may experience up to 4 consecutive days, Mr. Muccilli said.

Periods of prolonged, persistent heat are among the deadliest weather hazards in the United States because the effects build over time. The consecutive hot days and warm nights will prevent the body from fully recovering.

Friday:

Friday looks a lot like Thursday, which means little relief for some places that will have had four or more days of extreme temperatures and in some areas like New York City may be the hottest day of the week.

There is some chance of afternoon and evening thunderstorms on Friday and Saturday across the Mid-Atlantic and Northeast, which could provide some temporary relief. However, most locations will likely stay hot and dry.

July 4 weekend:

The hot and humid conditions are expected to persist into the holiday weekend, before a sprawling high pressure system driving the heat, often referred to as a heat dome, gradually shifts westward back toward the Plains, allowing for cooler conditions in the East.

The heat is likely to peak on the East Coast by Saturday, but is still expected to stick around into early next week before really subsiding, Mr. Muccilli said.