A heat alert was issued in the U.S., as much of the nation will be under a "heat dome" over the next few days.

Heat alerts had been issued in over a dozen states, from Louisiana to Minnesota, and advisories were extended to cover parts of Michigan, Indiana and northwest Ohio, and Kentucky, NBCNews reports.

Excessive warnings, however, were announced in Iowa, Nebraska, North and South Dakota and Illinois.

The so-called heat dome is said to intensify the already scorching July summer weather in the country. According to Mike Musher, a meteorologist at the NWS' Weather Prediction Center in College Park, Maryland, a heat dome is an atmospheric phenomenon where a "dome" of high pressure traps hot air beneath it.

"In the coming days, an enormous dome will envelope much of the Midwest before moving toward the East Coast over the weekend," Musher told Live Science.

According to Musher, the dome formed because the jet stream passing over the U.S.-Canada border is preventing cooler air from pushing southward.

"During the summer months, with the jet typically so far north and not much cold air to dig into the United States, it's natural for these large high pressure systems to develop," he said.

Temperatures are forecasted to soar over the weekend, from mid- to high 90s. Towards the end of the week, temperatures are expected to shoot up to 100 degrees in some states, which include parts of Kansas, Texas, South Carolina and Georgia, the National Oceanic and Atmospheric Administration (NOAA) prediction maps indicated.

According to Weather.com, Midwest cities will experience temperatures near 100 degrees with heat indices above 100, while the Southeast will experience mid-90s to 100 degrees temperature.

About 200 million people will experience temperatures of 90 degrees or higher, and about 130 million will experience heat indices of 100 degrees or higher, meteorologists said.

According to Musher, this reported heat dome has produced the first massive heat wave of the summer.

The White House issued a statement warning people to be watchful of symptoms of heat exhaustion, such as heavy sweating, coldness and paleness in the skin, nausea or vomiting, including heat stroke symptoms like high body temperature, redness on the skin or even unconsciousness.