During the day, the humidity levels will combine with air temperatures near the century mark to drive heat index values - which is how hot it feels to the human body - as high as 110 to 115 degrees (43C to 46C), particularly along the East Coast, according to Weather Service forecasts.
To complicate things, the Midwest is bracing for potential tornadoes amid the scorching temperatures.
"A number of record highs and overnight warm minimum temperatures are likely".
Boston, New York, Philadelphia, Baltimore and Washington will be under excessive heat warnings on Friday. That kind of warning is issued when the combination of heat and humidity is expected to make it feel like it is at least 105 degrees for a prolonged period.
"Extreme heat is unsafe, period", New York City Mayor Bill de Blasio said in a recent statement.
In addition, de Blasio ordered office buildings 100 feet or taller to raise building temperatures to 78 degrees Fahrenheit Friday to conserve energy.
In this case, head to a local cooling center if your city has one (New York has this handy map, but official state government sites should point you in the right direction).
"We are about to enter a heat emergency, and must do all we can to keep New Yorkers safe", de Blasio said. The organization says it's not safe for the athletes or the spectators. So far, around 500 cooling centers have been set up around the city for public use. More than 130 million Americans are will be under a heat watch, warning or advisory on Friday and Saturday.
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On Saturday, the forecast called for 99F (37C) in NY and Philadelphia and 100F (38C) in Washington.
"The health and safety of our horses and jockeys is our highest priority", said Maryland Jockey Club President and General Manager Sal Sinatra.
Relief from the heat is expected into early next week.
Detroit officials are urging residents to drink plenty of water, reduce outdoor activities, eat light and check on family and neighbors.
June of this year was the hottest June on record for the world, according to the National Oceanic and Atmospheric Administration. It isn't clear yet how July will fair in terms of breaking records, but temperatures for this week are high. Extreme heat events are becoming more common and severe - and they are lasting longer - due to global climate change.
Although it's been hot over the last few days, the hottest temperatures are expected on Saturday and Sunday, AccuWeather reports.
According to the release, older adults are at increased risk for heat-related illness and complications.
In addition, the National Climate Assessment, published by the Donald Trump administration previous year found heat waves are on the increase in the United States and have been since the 1960, though the 1930s still stand out as having the most extreme heat events on record in the USA, due to weather variability and land use practices at the time.