Life-threatening heat dome to roast over 230 million ahead of America's 250th birthday

Temperatures in the 90s are spreading across the Midwest and will reach the East Coast by midweek, when it will feel like 100 to 110 degrees for tens of millions of people as a life-threatening heat dome expands eastward ahead of America's 250th birthday.

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More than 230 million people will see air temperatures surpass 90 degrees through the week, with more than 20 states forecast to surpass triple-digits in the days leading up to July 4, the FOX Forecast Center said.

Within a heat dome, a strong high-pressure system acts like a lid, trapping hot air near the ground and preventing cooler air from mixing in, allowing heat to build day after day.

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According to the FOX Forecast Center, to start the week, the heat will steadily get more unbearable across the Midwest, where temperatures will reach the mid to upper 90s in cities such as St. Louis and Detroit.

While air temperatures will peak in the Midwest on Tuesday, feel-like temperatures above 100 degrees could linger through midweek.

"With dew points rising into the mid to lower 70s, heat index values will approach 105 degrees or higher," the FOX Forecast Center said. "As a result, an extreme heat risk is likely across the Midwest, Ohio Valley and Tennessee Valley through Wednesday and Thursday."

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Parts of the central U.S. are already under an Extreme HeatRisk, the highest category on NOAA and the CDC's extreme heat scale, indicating a long-duration heat wave with little or no overnight relief that can affect anyone without access to cooling and adequate hydration.

As people flock outdoors to hang by the pool, at the beach, for barbecues and more, it is important to prepare for the conditions by wearing sun protection and drinking plenty of hydrating fluids.

By Wednesday, the core of the extreme heat will shift to the East Coast, where many of the country's most densely populated cities will experience days of potentially record-breaking temperatures and dangerously high heat index values.

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Washington D.C., Baltimore, Philadelphia, and New York City will see multiple days exceeding 100 degrees.

"When you add in the moisture in the air, it will feel like a stifling 110 to 115 during the hottest parts of the day," the FOX Forecast Center said.

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The last time either New York or Philadelphia had back-to-back triple-digit days was in 2011.

Raleigh, North Carolina, could see its warmest day on record, with a potential high of 106 degrees late this week.

Temperatures are expected to ease slightly across both the Mid-Atlantic and Northeast on July 4 itself.

FOX Weather Meteorologist Britta Merwin advised people visiting the nation's capital to take advantage of air-conditioned spaces, like museums.

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Heat safety experts also advise wearing loose, lightweight, light-colored clothing. Light clothing doesn’t absorb sunlight as easily as dark colors do.

Lightweight, breathable materials like linen, cotton and muslin are all great for hot temperatures.

Download the free FOX Weather app to stay up-to-date with the latest conditions and alerts to stay safe for the July 4th weekend.