Huge heat dome is building over US. Here's your forecast

Intense heat is on the way for more than half of the country, just in time for the second week of summer.

"A multiday heat wave is brewing across the middle of the nation ahead of the Fourth of July weekend, expanding from the Plains to the Midwest and even nosing into the East," said AccuWeather meteorologist Alex Sosnowski in an online forecast.

The upcoming heat will likely bring the most widespread 90-degree weather so far this year across parts of the central and eastern states, AccuWeather said.

How hot will it get?

NOAA's Weather Prediction Center said hot weather will be the story this coming weekend from the Plains to the Ohio Valley, with widespread above-average temperatures expected thanks to a strong upper-level ridge of high pressure (aka heat dome) developing.

Highs in the 90s are expected as far north as the Great Lakes and Minnesota, and 100s for much of Texas. Heat indices approaching 110 degrees are possible from the mid-South to the central Gulf Coast where the combination of heat and high dew points will be greatest.

Overnight lows will also be quite warm, with some record high minimums possible.

For much of next week, the core of the heat dome is likely to be centered on the Ohio Valley, the middle portion of the Mississippi Valley and the Tennessee Valley, according to AccuWeather. At times, the heat will expand outward and reach parts of the Plains, the Great Lakes, the East and the Gulf Coast.

"It will turn very hot and humid in the East by early next week (June 29-30)," AccuWeather senior meteorologist Chad Merrill said. "Multiple days with highs well into the 90s are likely along the Interstate 95 corridor in the mid-Atlantic."

What is a heat dome?

Heat domes, also called ridges of high pressure or death ridges, are large bulges of sinking warm air that can stretch up to 1,000 miles in summer, driving temperatures 30  degrees above normal and creating hazardous, drying conditions that often lead to deadly, multi‑day heat waves, Weather.com said.

According to climatecheck.com, the term describes the "oppressive" high-pressure atmospheric systems that cause warm air to be pushed to the Earth's surface and trapped there for long periods of time.

"The dome traps high-pressure air in one place, like the lid on a pot," the website said. "These large zones of hot air result in a combination of blistering temperatures, devastating wildfires, and drought conditions."

What causes a heat dome?

A heat dome occurs when a persistent region of high pressure traps heat, said William Gallus, professor of meteorology at Iowa State University.

"The heat dome can stretch over several states and linger for days to weeks, leaving the people, crops and animals below to suffer through stagnant, hot air that can feel like an oven," Gallus said in an article in The Conversation.

How long will it last?

While the heat dome in the central states is forecast to ease up during the second week in July, temperatures will still remain a few degrees above the historical average in much of the Plains and the Midwest, AccuWeather's Merrill said.

The core of the heat should then shift into the West toward the middle of July, he said.

What about thunderstorms?

Within the core of the heat dome, little to no thunderstorm activity is likely, Sosnowski said. However, rounds of thunderstorms will be possible leading up to and on the rim of the heat from portions of the Plains to the Upper Midwest and Northeast, AccuWeather said.

"A round or two of severe thunderstorms with damaging winds and flooding downpours from the Dakotas to the northern Mississippi Valley and Great Lakes is expected late this weekend into early next week," Merrill said.

Tips on keeping cool

• Drink water, more than usual, no matter how active you are. Don't wait until you're thirsty. Make sure your pets have plenty to drink.
• Avoid sugary, alcoholic or caffeinated liquids. They can increase body fluid loss.
• Stay in air-conditioned buildings as much as possible.
• Take cool showers or baths.
• Limit outdoor activity, especially in the middle of the day.
• Use drapes or shades on windows to keep out the sun.
• Limit oven and stove use to keep in-home temperatures lower. 
• Wear light-colored, loose-fitting clothing to improve perspiration evaporation.

What else can you do?

• Check on well-being of friends and neighbors – do they need help in keeping cool?
• Never leave people or pets in a closed car, even with partially open windows. The interior can rapidly reach dangerous temperatures.
• Seek immediate medical care for people with symptoms of heat illness.

Doyle Rice is a national correspondent for USA TODAY, with a focus on weather and climate.