A blast of sweltering heat will sweep across the United States over the next four days, and some places will see temperatures as much as 10 to 15 degrees Fahrenheit (5.6 to 8.3 degrees Celsius) above average for this time of year, according to the National Weather Service.

Hot weather in July is to be expected, of course — after all, it's the middle of summer — but a so-called heat dome is kicking these hot and humid temperatures up a notch.

A heat dome happens when a "dome" of high pressure traps hot air underneath it, said Mike Musher, a meteorologist at the NWS' Weather Prediction Center in College Park, Maryland. In the coming days, an enormous dome will envelope much of the Midwest before moving toward the East Coast over the weekend, he said. [Roasting? 7 Scientific Ways to Beat the Heat]

This dome formed largely because the jet stream passing over the U.S.-Canada boarder is preventing cooler air from pushing southward, Musher said. "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.

Much of the country will feel scorching temperatures over the next few days, according to weather prediction maps published by the National Oceanic and Atmospheric Administration (NOAA). In Minneapolis, for example, the average temperature on July 21 is 84 degrees F (29 degrees Celsius), Musher said. But this weekend, it will be in the mid- to high 90s Fahrenheit (about 35 to 37 degrees Celsius), he said.

By this weekend, as the heat dome moves eastward, temperatures in parts of the Midwest will drop to the 80s, Musher said. But the heat will continue to sizzle some areas. On Sunday, temperatures are expected to hit 100 degrees Fahrenheit (37 degrees Celsius) in several states, including parts of Kansas, Texas, South Carolina and Georgia, NOAA's weather prediction maps show.

Heat domes aren't rare, but this one appears to have produced the first sizable heat wave of the summer, Musher said.

Weather and government officials advised people to stay cool as the heat dome makes its way across the country. Even President Barack Obama tweeted, "This map says it all. Stay safe as it heats up: Drink water, stay out of the sun and check on your neighbors."

The White House issued a statement asking people to be watchful of heat exhaustion symptoms, including heavy sweating; skin that is cold and pale; nausea; or vomiting. Likewise, heat stroke symptoms include high body temperature; skin that is red, hot and dry; or even unconsciousness, according to the government.

Moreover, it's important to check on infants, young children and the elderly, who are less efficient at regulating internal body temperature than adults are, the statement said. But even adults should take care to wear light-colored and loose-fitting clothing, refrain from strenuous exercise, and drink plenty of water, the National Weather Service recommended.

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