Extreme heat will once again scorch much of the United States, including areas where thousands of people have lost power.

Heat wave would affect 85 million people in the US 1:49

(CNN Spanish) --

New extreme heat forecasts for this Wednesday in a large swath of the Midwest and southeastern United States, testing power grids and leaving more than 88 million people under extreme heat warnings or advisories .

Temperatures could rise to as much as six degrees Celsius above average, pushing heat indices to over 37 degrees.

Fifty places in the central and southern United States may approach or exceed high temperature records on Wednesday, according to the National Weather Service's Weather Prediction Center.

Multiple cities, including St. Louis, Nashville and Charlotte, North Carolina, have already broken daily records this week.

Adding to the misery, many homes are still without power after storms hit the Midwest on Monday.

More than 275,000 homes lost power in Ohio early Wednesday, according to Poweroutage.us, with residents of Illinois, Indiana, Michigan and West Virginia also affected.

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Most of Ohio has been under an excessive heat warning, according to the NWS, and a spokesperson for power company AEP Ohio told CNN that some customers should prepare for blackouts that will last into Thursday.

Columbus Mayor Andrew Ginther urged residents to use cooling centers and pools Wednesday to beat the heat and keep an eye on neighbors as power slowly comes back online.

"AEP is dealing with the damage caused by last night's storms plus the high demand due to excessive heat. They are working to restore power for everyone. I know it is difficult, I am also without power in my house," the mayor said in social networks.

Some power grid operators have also broken records amid rising demand for air conditioning.

Both the Tennessee Valley Authority and Texas operator ERCOT posted peaks in electricity use.

• A "dangerous and deadly heat wave" is approaching the southwestern US, warns the National Weather Service

Fire alert in the southern and central US 0:45

The effects on the communities due to the extreme heat wave

Schools have been forced to modify schedules due to excessive heat.

In Wisconsin, where the NWS recorded a heat index of 42.2 degrees Celsius on Tuesday at Milwaukee's Mitchell International Airport, Milwaukee Public Schools (MPS) dismissed students early and said they will do so again on Wednesday.

"Young children and people with certain health conditions are especially vulnerable to heat-related distress," MPS announced on its website.

"For everyone's safety, the district has decided to shorten the school day."

More than a dozen schools in Minnesota that are not fully air-conditioned switched to e-learning on Tuesday, according to Minneapolis Public Schools.

Detroit Public Schools announced that it will close all in-person schools three hours early through Friday due to extreme heat.

Relief may be coming soon for the Midwest as the heat is forecast to end Wednesday night, with a cold front bringing cooler temperatures, scattered showers and thunderstorms.

Temperatures will drop to more average levels as the front moves through Minneapolis on Wednesday afternoon and Chicago on Wednesday night.

The cold front is forecast to move across Ohio by Thursday.

• See here how to stay safe from heat waves.

Wildfires rage in Arizona, New Mexico

Meanwhile, southwestern states faced elevated wildfire risk due to windy conditions and relative humidity on Tuesday.

In Arizona, the Pipeline Fire just north of Flagstaff has burned more than 20,000 acres as of Tuesday afternoon, Coconino National Forest officials said.

The fire was first detected on Sunday, according to a Tuesday update from InciWeb.

A second fire a few miles away, the Haywire Fire, has burned approximately 4,000 acres, according to InciWeb, and both fires remain 0% contained.

New Mexico firefighters are battling the two largest fires in state history, the Hermits Peak/Calf Canyon Fire just northeast of Santa Fe and the Black Fire in the Gila National Forest, which have burned more than 600,000 acres. total.

There are 40 major wildfires active in the United States as of Tuesday that have burned nearly 1.2 million acres across six states: Alaska, Arizona, New Mexico, California, Texas and Utah, according to the National Interagency Fire Center.

"More than 6,200 wildland firefighters and support personnel are assigned to incidents," the NIFC said Tuesday.

-- CNN's Robert Shackelford, Judson Jones, Theresa Waldrop, Andy Rose and Dave Alsup contributed to this report.

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