(CNN) — More than 25 million people in over a dozen states are under heat alerts Saturday. From the northern Plains to the Southeast, temperatures will be in the upper 90s, with heat index levels well into the triple digits.

The heat wave comes on the heels of another record-breaking heat wave over the last week, which impacted many of the same regions about to be affected by the second wave.

From Lincoln, Nebraska, to Fargo, North Dakota, temperatures will reach triple digits by the end of the weekend. The heat wave focused over the Northern Plains will be 20 to 25 degrees above normal.

There will be a reprieve from the heat, albeit brief, for areas of the Great Lakes and Mid-Atlantic where thousands are still without power from storms earlier in the week.

More than 320,000 customers were without power in at least a dozen states from Wisconsin through Georgia early Saturday, according to PowerOutage.us. More than 150,000 customers were without power in North Carolina and Virginia alone.

Temperatures will be 10 to 25 degrees below normal from Chicago to Portland, Maine, Saturday. But as quickly as the temperatures have dropped, they will rebound by the start of next week.

In all, more than 240 million people, around 75% of the Lower 48, will see temperatures of 90 degrees or above over the next seven days.

Another big heat wave next week

The heat dome currently situated over the northern Plains will move eastward to the Midwest and South, setting up another record-breaking week, and cutting short the reprieve from the heat many of these states will get the next few days.

St. Louis is forecast to have a high temperature of 84 degrees Saturday. By the time Tuesday rolls around, the high temperature will jump to 100 degrees.

Chicago’s high temperature on Saturday may not even reach 70 degrees, but by Monday, the high skyrockets to 95 degrees.

Raleigh, North Carolina, will go from a high of 83 degrees Sunday back up to a forecast high of 100 by Wednesday.

Heat is the number one cause of weather-related deaths in the United States, and providing guidance about the likelihood of heat-related ailments — including cramps, exhaustion, stroke and possibly death — helps protect the public in extreme heat.

However, sometimes hot overnight low temperatures are just as much to blame as the daytime high temperatures.

“Your body requires cooling off at night, and actually expects it while you’re sleeping,” said Jenn Varian, a meteorologist at the National Weather Service Las Vegas office. “When we have very warm overnight temperatures, your body is simply not able to cool off properly, which in (and) of itself can cause complications, but will set you up to be less prepared for the daytime heat as well.”

Temperatures need to drop to at least 80 degrees for recovery to begin. In fact, a person can lose up to two liters of fluid overnight through sweating if the temperature never drops below 85 degrees.

Dozens of cities could break records for the hottest morning low temperature over the next five days. More records are expected next week across the Midwest, Southeast and Mid-Atlantic.

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