The extreme heat and humidity are returning for millions of Americans.

At least 17 US states across the northern Plains and parts of the Upper Midwest are set to be trapped under a blazing heat dome starting on Sunday.

AccuWeather meteorologists are expecting widespread highs of 95 to 105°F, with 'feels like' temperatures reaching 115°F into next week.

The heat dome could lock much of the Plains and Midwest into a prolonged stretch of 90°F to 105°F temperatures, with cities including Omaha, St Louis, Kansas City, Minneapolis, Des Moines and Denver expected to bear the brunt. 

Chicago is also forecast to see highs near 90°F on most days, although occasional lake breezes could provide some relief along the shoreline. 

The extreme heat is set to move east, bringing temperatures from the 90s to near 100°F for only a day or two, starting around July 14.

'As the heat builds across the northern Plains this weekend, the full extent of next week's eastward expansion into the East should become clearer,' according to AccuWeather. 'Rather than breaking down later next week, the heat dome is expected to restrengthen over the Plains and could persist through much of the remainder of the month.'

The upcoming heat dome comes just days after another high-pressure system parked over a large swath of the nation last week, with some East Coast cities' 'feels like' temperatures reaching 118°F.

A heat dome works like a lid of high pressure. It builds overhead, trapping hot air at the surface level, suppressing clouds and intensifying sunlight. 

It forms when the jet stream, the fast-flowing river of air high over the US, develops a bulge or ridge.

This ridge creates a barrier that blocks normal weather patterns, allowing warm air to sink and get trapped under the 'dome,' while keeping storms and cooler fronts away.

Meteorologist Jeff Berardelli posted on X: 'Whopper of a heat dome coming, and that’s no exaggeration! In all aspects: size, longevity, and especially intensity, this will be extreme. 

'The heat dome should shatter all-time records for upper-level pressure in the Northern Plains States. 

'Actual Temperatures (not heat index) in Montana, Wyoming, North Dakota and South Dakota could hit 110° in spots! The heatwave will peak [from] Sunday through Wednesday.'

The dangerous heat wave is set to intensify by the end of the week as a powerful heat dome builds over the western US. 

Temperatures will climb from seasonably warm on Thursday to dangerous levels by Friday as the sprawling system strengthens and pushes the jet stream, the ribbon of air that steers storms, north into Canada, allowing scorching conditions to take hold.

By Saturday, widespread highs in the upper 90s°F and low 100s°F are expected from the Rockies into eastern Oregon and parts of California, although communities along the immediate West Coast should escape the worst of the heat.

Conditions will become even more extreme on Sunday, with temperatures nearing 105°F across parts of Utah, Montana, Wyoming and the Dakotas.

Billings, Montana, is forecast to hit around 104°F on Saturday before climbing close to its all-time record of 108°F on Sunday. 

Salt Lake City could reach 103°F on Saturday and 105°F on Sunday, just shy of its record of 107°F, while Grand Junction, Colorado, is expected to see highs around 104°F on both days, only a few degrees below its all-time record of 107°F.

AccuWeather warned that high heat index values can lead to heat-related illnesses such as heat exhaustion or heat stroke within minutes of exposure.

Since humid air is already full of moisture, sweat evaporates much more slowly. That means your body cannot cool down well and it feels hotter than the actual thermometer says.

Elderly adults, children, those with chronic health issues, and those without air conditioning are expected to be at the greatest risk during the widespread humidity-induced heatwave.

Heat exhaustion often starts with heavy sweating, fatigue, developing cool, pale, clammy skin and a fast, weak pulse, muscle cramps, dizziness, nausea and headaches.

As for heat stroke, which is far more deadly and can develop within 15 minutes in extreme conditions, the first symptoms are often a throbbing headache, confusion, slurred speech, nausea, dizziness, and a fever above 103°F.

'Organizers of outdoor festivals should consider cooling stations and readily available drinking water,' AccuWeather said.

'Young children, older adults and those with chronic medical conditions are especially vulnerable to heat-related illnesses.'