A heat wave is expected to bake the U.S. next week. Here's a look at the forecast — and how to prepare.

Hundreds of millions of people across the U.S. are expected to face a heat wave this week, with temperatures hitting 90 degrees or higher — with some cities already seeing temperatures over 100.

It's rare to have such high temperatures so early in the year. This follows 2023's record-breaking heat waves, which experts say is a result of climate change.

🌡️ Where is extreme heat happening?

There was already a heat wave in the Southwest this week. Temperatures hit 113 degrees in Phoenix, Ariz., which saw people scrambling for relief. However, extreme heat is hardly done in the United States, with this heat heading to the Midwest and Northeast.

Temperatures are expected to hit the mid- to high- 90s in many areas, according to the Associated Press. The Ohio Valley and the Northeast (the latter of which just saw a bout of thunderstorms) may see daily records of extreme heat, while Detroit may see its worst heat wave in two decades — it's expected to hit 97 degrees on Monday, per weather reports.

Cities like Philadelphia and Boston may also see record highs — weather forecasts have temperatures reaching 97 degrees and 95 degrees next week, respectively — as well as extreme humidity.

Read more from Yahoo News: How to stay safe in the sun – and the heat illness symptoms to watch for.

🥵 Meteorologists say a ‘heat dome’ is to blame

Across the U.S. in general, record-breaking highs are likely, with temperatures soaring 15 to 25 degrees above average, especially from Tuesday through Friday. This means many areas will experience days in the 90s — significantly hotter than usual, affecting millions who aren't accustomed to such prolonged heat.

That's due to a "heat dome" — a weather phenomenon that occurs when a large area of high pressure in the atmosphere traps hot air underneath it, leading to hot weather for an extended period of time.

The National Weather Service has a heat map of the United States to indicate temperature patterns. As of Saturday, it places nearly all of the country (with the exception of parts of the Northwest and Northeast) in at least the "minor" level of concern, defined as a level of heat that "affects primarily those individuals extremely sensitive to heat, especially when outdoors without effective cooling and/or adequate hydration."

However, this changes over the course of the week as heat patterns shift — with more of the country receiving the “major” and “extreme” risks of heat-related impacts.

On Sunday, much of the midwest will be impacted by heat, with temperatures creeping into the extreme category by Monday. On Tuesday, the Chicago area will be primarily affected by extreme heat, with the extreme heat shifting toward the Northeast come Wednesday and Thursday.

☀️ Is this the start of a very hot summer?

The Weather Company and Atmospheric G2 believe that summer 2024 may be one of the hottest on record in the United States. Above-average temperatures will likely continue throughout July from New England to the Great Lakes, as well as in the central and northern Plains and Rockies, with Colorado and the High Plains seeing the highest temperatures when compared to what they typically see during the month.

In August, the northern Plains and upper Midwest are expected to experience temperatures well above average. Additionally, areas stretching from the Southwest to the Great Lakes, upstate New York and northern New England are also forecasted to have a warmer-than-usual August.

🧊 How to stay safe in extreme heat

It's important to know that extreme heat can be deadly: in 2023, more than 2,300 people died of conditions related to high temperatures, with some predicting that 2024 could see more deaths related to heat.

If you are experiencing extreme heat this year, make sure to follow guidelines to keep you and your family safe and healthy. Keep these tips in mind:

Know the symptoms of heat illness, such as muscle cramping, rash, headache, nausea and dizziness

Limit outdoor exposure during the hottest parts of the day

Drink plenty of water and replenish with electrolytes if needed

If suffering from heat illness, move to a shaded, cool place as soon as possible, take a cool bath and loosen or wet your clothes to reduce body heat

Seek out air-conditioned places to spend time during times of extreme heat