Summer will make a dramatic entrance in the U.S. this week with a heat dome that will bring stifling heat and uncomfortable humidity to millions.

Major cities across the United States are preparing for extreme temperatures caused by a heat dome that hit the U.S. Plains on Friday and is expected to expand to much of the rest of the country over the coming days.

Weather forecasters have warned of extreme and dangerous temperatures as a major heat wave sweeps across the central and eastern U.S. states.

Temperatures could reach a record-breaking 100 degrees in the metro and up to 103 on the plains, forecasters said.

Several days of record-breaking heat are expected in Denver, and the National Weather Service has issued a heat advisory that will go into effect on Friday. The advisory extends into Saturday, and it's for the Denver metro area as well as parts of Colorado's Front Range to the north and south and for much of eastern Colorado as well.

The head index is expected to reach around 105 and an extreme heat warning is in effect until Monday, according to the National Weather Service.

Chicagoans can expect intense heat compounded by muggy humidity for the next several days. The National Weather Service has declared an extreme heat warning in Cook County from

Scorching temperatures and high humidity are a bad combo, especially when that's the forecast for three days in a row, making heat stroke a real possibility.

North Texas is sweltering on the first day of summer, with temperatures in the mid-90s and heat index values near or above 100 degrees. Summer officially begins at 9:42 PM, bringing over 14 hours of daylight.

Triple-digit heat index values means heat with more humidity. But San Antonio won’t see actual temperatures of 100 degrees for at least another week.