Maps show heat wave forecast across U.S. Here’s what to know.

A heat wave is forecast across the eastern United States, and it’s expected to bring severe storms and several days of record high temperatures at the start of the weekend and into next week.

As a low-pressure system slowly shifts from the West, the Ohio Valley and the Midwest down into the lower Mississippi Valley face the threat of tornadoes, large hail and damaging winds, according to CBS News meteorologist Nikki Nolan. As the system moves eastward, showers and storms are also on deck for parts of the Northeast through Thursday. Behind the system are warm and above-average temperatures, or what the National Weather Service called “the first significant heat wave this season.”

Temperatures are expected to soar more than 25-35 degrees above average in the Plains on Friday, Nolan said. Potential daily record highs are also forecast for parts of the Rockies and Plains on Friday and Saturday.

Here’s what to know about the upcoming heat wave.

Heat wave forecast maps

“Summer has taken its time arriving in the Northeast, but it’s coming in HOT,” the National Weather Service wrote Wednesday in a social media post. “Triple-digit heat indices will be common through next week, with oppressive humidity.”

The heat index is what the actual temperature feels like when it’s combined with relative humidity.

These maps show daily highs across the country and their departures from average temperatures. 

How long will the heat wave last?

The heat is expected to shift into the Midwest and Ohio Valley by the end of the weekend and arrive in the Northeast and mid-Atlantic on Sunday through the first half of next week.

Parts of the Northeast face a “High Risk” — a greater than 60% chance — of extreme heat through the middle of next week, according to the National Weather Service.

What constitutes a heat wave?

A heat wave is a period of unusually hot weather that lasts more than two days, according to the National Weather Service. 

It often, but not always, covers a large area and exposes a large number of the population to hazardous heat, which can be extremely dangerous for vulnerable people, including children and the elderly.

How to stay safe during a heat wave

The simplest way to way to avoid heat-related illnesses is to stay out of the sun, experts say. 

If strenuous outdoor activities are unavoidable, you should dress for the heat and wear lightweight, loose-fitting and light-colored clothing, the National Weather Service recommended. You should also stay hydrated by drinking plenty of water, even if you’re not thirsty. 

It’s also important to be aware of any symptoms of overheating, which include muscle cramps, unusually heavy sweating, shortness of breath, dizziness, headaches, weakness or nausea, according to the Centers for Disease Control and Prevention. If you experience these symptoms, one major sign to seek care is if the symptoms don’t improve after getting into a cooler environment or hydrating, experts say.

Nikki Nolan

contributed to this report.