Heat Dome to Bake Two-Thirds of the U.S. Through the July 4th Holiday

If you have July 4th plans that involve being outside — a barbecue, a parade, fireworks on the National Mall — this is the story you need to read before you go.

A massive heat dome is settling over the eastern two-thirds of the United States this week, and it's timed almost perfectly to ruin your holiday weekend. Here's what's happening and why it matters to you.

What's a heat dome?

Think of it like a giant lid on a pot. A heat dome is essentially an area of very strong high pressure that traps heat over a large region, resulting in weak winds and increased humidity — a stagnant weather pattern that can last for days to weeks, making heat feel insufferable because the human body can't cool off as effectively when sweat stays on the skin.

How bad is this one?

Pretty bad. A long and dangerous heat wave is set to blast a large swath of the United States this week, with temperatures rising ahead of the July Fourth holiday and feeling even hotter because of the high humidity arriving with it. New York City, Philadelphia, Baltimore and Washington, D.C. are all expected to see record highs into the 100s, according to CBS News meteorologist Nikki Nolan.

As temperatures climb, high humidity will push the heat index — that "feels like" number — to between 100 and 110 degrees F, and as high as 115 F in some spots, according to the National Weather Service. That's not just uncomfortable. That's genuinely dangerous.

More than 100 million people across the central and eastern United States are expected to experience Major (Level 3 of 4) or Extreme (Level 4 of 4) heat risk.

Who's in the crosshairs?

Extreme heat warnings are already in effect for major cities including Nashville, St. Louis, Cincinnati, Kansas City, Des Moines, Chicago, and Detroit. Extreme heat watches are also in effect for much of the Northeast, with temperatures over 100 degrees expected through Saturday. Southern cities including Dallas, Little Rock and Memphis will also see dangerously high temperatures.

The heat is moving like a slow wave. The oppressive heat and humidity will hover over the Midwest early this week, then move into the Mid-Atlantic and Northeast by Wednesday, with the heat dome lingering over the East — particularly the I-95 corridor — and peaking by Friday, just in time for the holiday weekend.

And don't count on the nights to bail you out. New York City may have three consecutive nights with temperatures not dropping below 80 degrees, which is very rare.

What about the July 4th angle specifically?

This is where the timing gets particularly rough. The Climate Prediction Center flagged the nation's 250th anniversary on July 4 as a key concern, noting that "staying hydrated and limiting exposure to very hot, humid weather is very important, especially for vulnerable populations including older adults." Washington, D.C. has a full day of celebrations planned on the National Mall — parade, concerts, and one of the nation's largest fireworks displays — all in what could be 100-plus-degree heat.

It's not just heat — wildfires too

In the West, the Four Corners area is under dangerous fire weather conditions, and three firefighters have already died when they were overcome by flames from fast-moving wildfires near the Colorado-Utah border — conditions the National Fire Information Center called "extremely critical" and rare for late June.

What should you actually do?

During extreme temperatures, people are advised to limit outdoor activity, stay hydrated, and ensure access to air-conditioning and other cooling areas, per the National Weather Service. The CDC says heat can impact anyone — warning signs include muscle cramping, dizziness, unusually heavy sweating, shortness of breath, headaches, weakness and nausea.

Bottom line: if you're heading outside this week, plan around the heat, not through it. Early morning and after sunset are your windows. The afternoon hours, especially Thursday and Friday, are when this thing peaks.