Nearly 130 million Americans are under extreme-heat alerts this week. Relief is coming, but the summer heat could still be above normal for weeks.

• A heat wave from the US Midwest to the Gulf Coast caused unusually high temperatures this week.
• Relief is coming, but temps will still be above average.
• Rising global temperatures are making summers longer and hotter.

This summer has been hot. July was the hottest month in recorded history. And an end-of-August heat wave caused soaring temperatures of 100 degrees Fahrenheit or greater in Chicago, Kansas City, Oklahoma City, and Des Moines on Thursday.

Adding to the misery, when the heat is accompanied by humidity, it can feel much hotter — as high as 120 degrees Fahrenheit, the National Weather Service said.

The sweltering temps caused weather experts to issue extreme-heat alerts in 19 states, affecting about 130 million people, the Washington Post reported on Wednesday.

"It is quite a heat wave for late in the summer," Bryan Jackson, a forecaster with the National Weather Service, told Insider. "It's been extreme in a lot of different ways," he said, not only in the high temperatures but in how many states it affected.

Schools in several states, including Illinois, Iowa, and Kansas, either pushed back the start of the school year or sent students home early because of the heat, The Weather Channel reported.

ERCOT, the agency responsible for supplying much of Texas's power, asked people to reduce their electricity use during peak hours, though it was still over 100 degrees Fahrenheit.

Nearly two dozen cattle died at a University of Nebraska-Lincoln farm during the heat wave on Tuesday, CNN reported. Last summer, a similar heat dome killed roughly 2,000 cows.

As a low-pressure system from the Canadian prairies moves into the Great Lakes and New England this weekend, temperatures are expected to start to cool, Jackson said.

A cold front in the Midwest may help break the heat in Nebraska, Iowa, northern Indiana, and other states over the next couple of days. For example, Chicago's forecast is for temperatures in the high-70s and low-80s by Sunday and Monday.

But it's forecasted to remain hot in southern Missouri and Kansas and throughout the South and Gulf Coast this weekend.

"Relief for there doesn't really come until Monday, Tuesday," Jackson said.

With the relief comes the possibility of thunderstorms over the next few days in some Midwestern states and the central Mid-Atlantic coast.

Storms can sometimes develop on the edges of heat masses, and Northeast Ohio experienced power outages and flooding due to heavy rains this week.

But even with cooler weather, the NWS still forecasts hotter-than-average temperatures over the next week centered over the South Central US, as there has been for the past several months.

"There have been a few breaks, but it's been a very hot summer over the southern plains and the Gulf Coast," Jackson said.