Heat wave expected to bake two-thirds of nation through weekend

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 U.S., the National Weather Service said.

Many in those areas could see the hottest temperatures of the year, thanks to a large dome of high pressure that will send temperatures climbing in the coming days, the weather service said.

Already on Tuesday night, an estimated 34 million people were under heat advisories and another 21 million were under excessive heat warnings, according to the weather service. The affected areas stretched from Texas to much of the Plains. The East Coast is feeling the heat, too, with advisories issued from South Carolina to New Jersey.

Although he didn't declare a heat emergency, Boston's mayor on Tuesday warned residents to prepare for the heat and take precautions for temperatures expected to be between 85 and 97 degrees, with the hottest forecast on Saturday. With the humidity, it could feel as hot as 105 degrees, the mayor's office said.

In New York City, temperatures and heat indices are forecast to reach "dangerously high levels by the weekend" — with temperatures in the mid- to upper-90s by Friday, with heat indices of close to 107 degrees on Saturday, the city's office of emergency management and health department said Tuesday.

The heat index is a measure of how hot it feels when humidity is factored in with air temperature.

Cooling centers are set to be open in New York City from Wednesday to Sunday.

"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.

This heat wave is expected to bring the hottest temperatures so far this year in Chicago, Detroit, New York City and Washington, D.C., The Weather Channel reported. Widespread record-high temperatures are not expected because mid-July is typically the hottest time of the year.

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; and the Washington, D.C. area could see heat indices of up 100 to 115 degrees during the afternoons of Friday through Sunday, according to the weather service.

Forecasters in St. Louis warned of a "major heat wave" that will grip the region starting Wednesday and into the weekend. The area was under an excessive heat warning from 1 p.m. Wednesday to 8 p.m. Saturday, with highs each afternoon expected to be in the mid- to upper-90s and with a heat index as high as 113 degrees, according to the weather service.