Two-thirds of the continental United States is expected to fry under an intense heat wave over the weekend in a phenomenon caused by a large dome of high pressure lingering over the affected areas. The unusual heat wave has already begun in some areas such as St. Louis.

The National Weather Service (NWS) said the areas to be struck by temperatures ranging from 90°F (32°C) to more than 100°F (38°C) stretched from Texas to much of the Plains.

NWS is warning the Plains of a "major heat wave" starting Wednesday and into the weekend. Parts of Kansas, Nebraska, Iowa, Missouri, Oklahoma and Illinois are now under an excessive heat warning. Highs each afternoon in these states are expected to be in the mid- to upper-90s and with a heat index as high as 113°F (45°C).

Heat advisories have also been issued from South Carolina to New Jersey on the East Coast.

The heat index is a measure of how hot it feels when humidity is factored in. Forecast heat indices are far higher and might reach 115°F (46°C) in some areas.

The NWS Weather Prediction Center (NWS WPC) tweeted: “The heat is on! Millions will endure a heat wave later this week. Warm overnight lows will provide no relief.

“Extreme heat is dangerous. Please visit https://www.weather.gov/safety/heat for information on how to protect yourself and others.”

In all, NWS placed 34 million people under heat advisories and another 21 million were under excessive heat warnings. The combined total represents a sixth of the United States’ total population.

This heat wave is expected to bring the hottest temperatures so far this year to New York City, Chicago, Detroit, and Washington, D.C., according to the Weather Channel.

Detroit is expected to see an afternoon heat index of up to 105°F (40°C) on Friday and Saturday. Chicago is forecast to also swelter under 105°F heat while the Washington, D.C. area is in store for heat indices of up to 115°F (46°C) during the afternoons of Friday through Sunday, said the NWS.

Forecasters in St. Louis said a "major heat wave" will bombard region starting Wednesday and into the weekend. The St. Louis area came under an excessive heat warning from 1:00 p.m. Wednesday to 8:00 p.m. Saturday. Highs each afternoon expected to be in the mid- to upper-90°F and with a heat index as high as 113°F (45°C).

The NWS Eastern Region tweeted: “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.”

Day 4 heatwave forecast for the USA July 2019 Day 4 heatwave forecast for the USA July 2019 Photo: NWS

There will only be scant respite in the evening from the dangerous heat.

"There also will be no relief at night, as low temperatures remain in the upper 70s and 80s," according to the NWS

Experts warn hot weather is dangerous and can kill. People with chronic physical and mental health conditions should use air conditioning if they have it. They should try to get to a cool, air-conditioned place if they don't.

New York City set-up cooling stations Wednesday as a precaution. Temperatures in the city are forecast to climb to "dangerously high levels by the weekend," reaching the mid- to upper-90s by Friday.

The heat index is forecast to reach close to reach 107°F (42°C) Saturday, said the city’s Office of Emergency Management and the Health Department on Tuesday.

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