A heat dome is peaking in the Northeast today. Here’s how extreme temperatures could get

By Mary Gilbert, CNN Meteorologist

(CNN) — Sauna-like conditions on Friday are threatening to break records in major Northeast cities during summer’s hottest month after dangerous heat peaked Thursday in the central United States.

More than 80 million people in the eastern half of the US are under at least a Level 3 of 4 “major” heat risk on Friday, which will be the East Coast’s hottest day of the heat dome. High temperatures in the middle to upper 90s are expected from Washington, DC, to Boston and low 90s will reach as far north as southern Maine.

These temperatures are up to 10 degrees higher than the typical hottest conditions for this time of year and could challenge the high temperature record for the day in Baltimore, Philadelphia, New York City and Portland, Maine.

The heat index — how hot it actually feels given the temperature and humidity — will reach or surpass 100 degrees for many: Washington, DC, Baltimore and Philadelphia could feel like 105 to 110 degrees Friday afternoon.

The stagnant, hot air has also prompted air quality alerts across New York, New Jersey and Connecticut.

On Thursday the heat index topped out in the 100s from the Gulf Coast to the Midwest while it soared above 110 degrees in parts of the Mississippi Valley. It reached a scorching 115 degrees in Greenville in far western Mississippi on Thursday afternoon.

Farther north, Chicago’s heat index eclipsed 100 degrees Thursday while Syracuse, New York, set a new record high temperature for the day of 94 degrees, breaking the old record of 93 degrees set in 1941. High temperatures reached into the 90s Thursday afternoon as far north as Burlington, Vermont – located about 30 miles from the Canadian border.

Heat remains the deadliest form of extreme weather in the US. Globally, heat waves are becoming more frequent, more severe and lasting longer as the world warms due to fossil fuel-driven climate change.

Heat this severe impacts anyone without access to effective cooling or proper hydration, not just vulnerable populations, according to the National Oceanic and Atmospheric Administration. And for those without air conditioning, staying indoors can become deadly during the hottest parts of the day, the agency warns.

Dallas County, Texas, reported its first heat-related death of the year on Thursday. The victim was a man in his 60s with “unknown pre-existing medical conditions,” according to the Dallas County Health and Human Services.

Humid conditions will also prevent temperatures from cooling significantly overnight. It could still be close to 80 degrees in Washington, DC, by sunrise Saturday — nearly 10 degrees higher than a typical low for late July.

When overnight temperatures don’t cool down enough to offer relief for overheated bodies, people are at greater risk for heat-related illnesses. Nighttime temperatures are taking the hardest hit from climate change, warming faster than daytime highs.

Climate change is making this week’s heat wave at least three times more likely for nearly 160 million people, almost half the US population, when compared with a world without fossil fuel emissions, according to an analysis from the climate research nonprofit Climate Central.

The heat dome will shrink Saturday, replaced by more typical, though still very toasty, late July conditions parts of the Northeast and Midwest. Sweltering heat will persist through the weekend, mainly in southern parts of the Ohio Valley and Mid-Atlantic on Saturday and in portions of the Southeast Sunday.

That general trend of a hotter-than-normal South is expected to continue through next week, according to the latest Climate Prediction Center forecasts.

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CNN Meteorologist Briana Waxman and CNN’s Luke Snyder and Emma Tucker contributed to this report.