A major heat wave will expand across the Midwest and much of the East during Fourth of July week, with blistering hot temperatures and oppressive humidity posing a danger for millions from the Plains to the East Coast.

Triple-digit heat indices are likely for many, and daily low temperatures will remain very warm, offering little relief at night.

This will be a big change for some who have recently been cooler than usual for June, particularly in parts of the Plains and Midwest.

Heat began building over the weekend for parts of the country and will spread east through the week. At least 230 million people in the U.S. — about two-thirds of the U.S. population — could be exposed to extreme heat that could lead to health issues.

(MAPS: 10-Day US forecast highs, lows)

Midwest highlights

The heat has already intensified in the Plains and Midwest this past weekend, and will lock in place in the Great Lakes and Ohio Valley for much of this week.

By Monday, highs in the 90s will stretch as far north as northern Michigan and northern Wisconsin, and 90s will be common in the Midwest through much of this week.

When you factor in the heat index, many across the Midwest will feel like the triple digits, making conditions outright dangerous to be exposed to for long periods. This includes places like Milwaukee, Chicago, St. Louis and Minneapolis. 

A few daily record highs are possible by the middle of the week, especially in the Ohio Valley, where some mid-upper 90s highs are possible.

East

In the East, the hotter air will begin arriving on Monday, and for many will last much of the week.

Highs in the mid-upper 90s will be common and some triple digit highs are possible in the mid-Atlantic for several days in a row. Dangerous heat indices from 100 to 110 degrees are also expected in areas such as Washington, D.C., Philadelphia and Richmond, Virginia.

A few daily record highs are possible in the first half of the week along and west of the I-95 corridor from the Washington, D.C. metro into parts of the Virginias and eastern Great Lakes before widespread record highs are expected from the Carolinas to New England by Thursday. Washington, D.C., New York City and Boston could all hit 100 degrees on Thursday, as the heat peaks.

And overnight lows may struggle to drop much below 80 degrees in the heart of Philly, Baltimore and Washington, D.C.

Parts of New England and upstate New York will be hot, at times, but will escape the worst of this long-lasting heat this week.

(MORE: Danger of nighttime heat)

South

While the South is used to hot summers, the heat moving in will be both persistent and oppressive.

Much of the region had highs in the 90s this weekend, and that torrid heat will continue to linger through all of this week. Some locations could even top 100 degrees in both the Southern Plains and in the Southeast from Virginia into the Carolinas.

Add in the southern humidity, and heat indices will be well over 100 from Texas to the Carolinas. 

A few daily high temperature records could fall in the South through this heat wave, including parts of Florida and Texas.

How long will the heat last?

Will this heat wave cook your Fourth of July weekend?

The latest forecast suggests the heat dome responsible for this heat wave will stick around for a few more days, keeping much of the country baking in the heat for the Fourth of July holiday, and even beyond.

This includes the Great Lakes, Mississippi and Ohio Valleys and mid-Atlantic.

(MORE: 5 things to know to stay safe in a heat wave)

Make sure you check back, as we will continue to update the forecast and keep a close eye on the holiday weekend. For a more detailed forecast for the Fourth of July, see our latest forecast article here.