A "major heat wave" is expected to bake two-thirds of the nation through this weekend, with forecasters calling for temperatures to soar across much of the central and eastern United States, the National Weather Service said.

From the Plains to Chicago to New England, the hottest temperatures of the year are expected, thanks to a large dome of high pressure that will send temperatures climbing in the coming days, the weather service said.

On the East Coast, cities are already taking precautions. In New York City, where cooling stations were set up Wednesday and will remain in place through Sunday, temperatures are forecast to climb to "dangerously high levels by the weekend," reaching the mid- to upper-90s by Friday. The heat index, which is the measure of how hot it feels when humidity is factored in with air temperature, is forecast to reach close to reach 107 degrees Saturday, the city’s Office of Emergency Management and the Health Department said Tuesday.

The dangerous heat wave that’s going to roast two-thirds of the US through the weekend has begun.@ReporterBlayne has the latest on the hot temps from St. Louis. pic.twitter.com/lbpl180mkd — TODAY (@TODAYshow) July 17, 2019

Boston's mayor Tuesday warned residents to prepare for temperatures between 85 and 97 degrees, with the hottest forecast Saturday. With humidity, it could feel as hot as 105 degrees in Boston, the mayor's office said.

In swampy Washington, D.C., heat indices of up to 100 to 115 degrees were forecast for the afternoons of Friday through Sunday, according to the weather service.

And the Midwest is staring down similar scorching temperatures.Detroit is expected to see an afternoon heat index of up to 105 degrees Friday and Saturday; Chicago is forecast to get much of the same.

Very hot and humid conditions are expected across the Eastern US for the end of the week into the weekend. In addition to extremely hot daytime temperatures, overnight lows in most areas are not expected for fall below the mid-upper 70s, or even the lower 80s in some urban areas. pic.twitter.com/jtgWH3rnvt — NWS Eastern Region (@NWSEastern) July 16, 2019

The weather service is warning the Plains of a "major heat wave" that will grip the region starting Wednesday and into the weekend. Parts of Kansas, Nebraska, Iowa, Missouri, Oklahoma and Illinois are under an excessive heat warning, with highs each afternoon expected to be in the mid- to upper-90s and with a heat index as high as 113 degrees.

People shouldn't expect to catch a cool evening breeze, either.

"There also will be no relief at night, as low temperatures remain in the upper 70s and 80s," the weather service said.

As temperatures steadily climb, officials across the country are warning of the dangers of excessive heat.

"Hot weather is dangerous and can kill. People with chronic physical and mental health conditions should use air conditioning if they have it, and get to a cool, air-conditioned place if they don't," Dr. Oxiris Barbot, commissioner for the New York City Department of Health and Mental Hygiene, said in a statement.

The weather service is advising people to stay hydrated, try to find air conditioning if possible, limit strenuous activity, and to never leave kids or pets in unattended vehicles.

In Missouri and parts of Illinois, a group called Cool Down St. Louis is partnering with local volunteers and the fire department to install air conditioning units in the homes of elderly residents.

The organizer, Gentry Trotter, says the group is working overtime in anticipation of the heat wave.

“We are trying to get ahead of the storm before the storm gets here. And by the storm, I mean the heat!” Trotter said. So far, they've installed around 600 units this summer. As the heat climbs, the group's work will keep residents safe over the weekend.

An estimated 34 million people were already under heat advisories Tuesday evening, and another 21 million were under excessive heat warnings, according to the weather service.