Extreme heat will again scorch a large section of the US, including areas where thousands have lost power

By Travis Caldwell and Joe Sutton, CNN

Extreme heat is forecast again for a large swath of the US on Wednesday, testing power grids and leaving nearly 100 million people from Michigan to northern Florida under heat warnings and advisories.

Temperatures could soar as much as 20 degrees above average, pushing heat indexes well into the triple digits.

Fifty locations across the central and southern US may get close to or exceed high-temperature records 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 remain without power after storms across the Midwest on Monday. That includes more than 200,000 customers without power in Ohio Wednesday morning, according to Poweroutage.us, with residents in Indiana and West Virginia also affected.

The majority of Ohio has been under an excessive heat warning, according to NWS, and a spokesman for electric company AEP Ohio told CNN that some customers should prepare for outages to last until Thursday.

Columbus Mayor Andrew Ginther called on residents to use cooling centers and swimming pools on Wednesday to beat the heat, and to check on neighbors as power slowly comes back online.

“AEP is dealing with the damage from last night’s storms plus high demand because of excessive heat. They are working to restore power to everyone. I know it’s tough — I’m without power at my house as well,” the mayor said on social media.

Records have also been hit by some power-grid operators amid the surging demand for air-conditioning. Both the Tennessee Valley Authority and Texas operator ERCOT recorded highs for electricity usage.

Schools have been forced to alter schedules due to the excessive heat. In Wisconsin, where a heat index of 108 degrees Fahrenheit was recorded by NWS Tuesday at Milwaukee Mitchell International Airport, Milwaukee Public Schools dismissed students early and said it will do so again Wednesday.

“Young children and persons with certain health conditions are especially vulnerable to heat-related distress,” MPS announced on its website. “For the safety of all, the district has decided to shorten the school day.”

More than a dozen schools in Minnesota that are not fully air-conditioned moved to e-learning on Tuesday, according to Minneapolis Public Schools. Detroit Public Schools announced they will close all in-person schools three hours early through Friday due to the extreme heat.

Relief may soon arrive for the Midwest as the heat is forecast to break late Wednesday, with a cold front due to bring cooler temperatures, scattered showers and thunderstorms.

Temperatures will drop to more-average levels as the front moves through Minneapolis on Wednesday afternoon and Chicago by Wednesday overnight. The cold front is forecast to move through Ohio by Thursday.

Still, even with temperatures easing a few degrees, particularly throughout the Upper Midwest, temperatures in much of the region will remain above normal, with areas of record-setting temperatures persisting into the latter part of the week.

On Thursday, temperatures are forecast to climb well above normal for much of the Great Basin, the Rockies and Southwest. An excessive heat warning has also been issued for parts of interior southern California and Arizona, where daytime highs are expected to soar once again well into the triple digits.

Don’t rely on fans to keep you cool in extreme heat. Here’s how to stay safe.

Wildfires rage in Arizona, New Mexico

Meanwhile, states across the Southwest faced an elevated risk of wildfires due to windy conditions and relative humidity on Tuesday.

In Arizona, the Pipeline Fire just north of Flagstaff has scorched more than 20,000 acres as of Tuesday afternoon, Coconino National Forest officials said. The blaze was first spotted on Sunday, according to a Tuesday update from InciWeb.

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

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

There are 40 active, large wildfires in the US as of Tuesday that have burned almost 1.2 million acres in 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.

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