Sizzling temps to scorch 230M Americans as life-threatening heat dome lingers through nation's 250th birthday

A life-threatening heat dome is building over the central and eastern U.S. through the July 4 holiday weekend, bringing days of extreme and record-breaking temperatures to more than 230 million people.

The dangerous heat comes as millions are expected to attend outdoor events marking America's 250th birthday, with feels-like temperatures already climbing to between 100 and 110 degrees from Chicago to the Gulf Coast and east to Washington on Wednesday.

WHAT IS A HEAT DOME AND HOW CAN IT AFFECT YOU DURING THE SUMMER MONTHS?

More than 65 million people across 20 states are forecast to reach air temperatures of 100 degrees this week and more than 150 million people are currently under Heat Alerts.

Dozens of cities across the Mid-Atlantic and Northeast could see one of their top five hottest days on record Thursday and Friday.

According to NOAA, heat-related illnesses kill more Americans annually than any other type of extreme weather.

CITIES ACROSS MIDWEST AND EAST COAST ENACT EMERGENCY PLANS, ALTER JULY 4 CELEBRATIONS AS EXTREME HEAT BUILDS

Local leaders throughout the eastern two-thirds of the country are advising people to be very cautious outdoors in what is the first major heat wave of the summer. 

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.

HOW HIGH PRESSURE AND LOW PRESSURE DRIVE THE WEATHER

While air temperatures are expected to peak by Wednesday across parts of the Midwest, including St. Louis, Chicago, and Detroit, feel-like temperatures above 100 degrees could linger through Thursday.

"An extreme heat risk is likely across the Midwest, Ohio Valley and Tennessee Valley through Wednesday and Thursday," the FOX Forecast Center said.

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.

EXTREME HEAT SAFETY: RECOGNIZING SIGNS OF HEAT ILLNESS AND HOW TO TREAT IT

Mid-Atlantic, Northeast could see some of their hottest days on record

By Thursday, the core of the extreme heat will be anchored over 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.

EXTREME HEAT SAFETY: HOW TO PREPARE AND STAY COOL DURING A HEAT WAVE

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.

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.

JULY 4TH FORECAST OUTLOOK: WHAT WILL THE WEATHER LOOK LIKE ACROSS THE US FOR AMERICA'S 250TH BIRTHDAY?

Heat safety experts also advise wearing loose, lightweight, light-colored clothing and frequently applying adequate sunscreen. 

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.