Originally Published: 21 JUL 22 02:48 ET

Updated: 21 JUL 22 13:16 ET

By Aya Elamroussi and Dakin Andone, CNN

(CNN) -- More than 85% of Americans are bracing for temperatures above 90 degrees Fahrenheit through the weekend, with millions in the south-central US expected to experience readings in the triple digits.

More than 100 million people are under various heat alerts Thursday in more than two dozen states from parts of the American West to New England, a suffocating cocoon that experts believe will become increasingly common due to the effects of climate change.

"Widespread high temperatures in the mid-to-upper 90s and low 100s will encompass a majority of the country on Thursday and Friday," the National Weather Service warned Wednesday.

The areas at the highest risk for the dangerously hot temperatures span the Southwest, central and south-central US along with the coastal mid-Atlantic region and the Northeast, the weather service noted.

The distressing heat wave -- which has exacerbated a flash drought in the southern and central Plains -- has pushed state and local leaders to issue heat emergencies and offer resources to residents to mitigate the high temperatures.

The mayor of Washington, DC, on Thursday announced a heat emergency -- triggered when the district sees a temperature of 95 degrees or higher -- that will last until Monday morning.

"Stay hydrated, limit sun exposure, and check on seniors, neighbors & pets," Mayor Muriel Bowser said on Twitter.

Philadelphia declared a heat health emergency for Thursday due to the expected oppressive heat, activating emergency programs likes special field teams that conduct home visits and outreach for people experiencing homelessness, the department of health said in a news release.

Similarly in New York, residents are encouraged to stay indoors in the upcoming days as the heat continues to sweep across the state to avoid "dangerous conditions that can lead to heat stress and illness," according to Jackie Bray, the commissioner of the state's homeland security and emergency services division.

Temperatures over 90 degrees are expected to remain in New York City, Philadelphia and Boston through the weekend -- if not longer.

The excessive heat across the US has been matched by deadly conditions in Europe, where records have been smashed and the European Forest Fire Information System put 19 European countries on "extreme danger" alerts for wildfires.

Heat index values indicate danger

Meanwhile, triple-digit heat will continue to bake parts of California, Texas, Arkansas, Oklahoma, Kansas, Missouri and Tennessee on Thursday -- meaning 1 in 5 Americans will endure dangerous conditions after what has already been a historic week in terms of topping heat records, said CNN meteorologist Robert Shackelford.

The heat is expected to persist through the weekend in many places, and more than 85% of the population -- or 275 million Americans -- could see high temperatures above 90 degrees over the next week. More than 60 million people could see high temperatures at or above 100 degrees over the next seven days.

Heat index values -- the temperature it feels like when heat is combined with humidity -- could top 100 degrees in a number of states through this weekend, particularly in the Midwest, the Southeast and on the East Coast.

Large swaths of the South -- including parts of eastern Texas, Louisiana, Oklahoma, Arkansas, Mississippi and Alabama -- and the central East Coast from South Carolina to New Jersey will see some of the most pronounced danger (seen in these maps in dark orange) from the heat on Thursday.

That danger becomes more apparent in parts of the Midwest this weekend, in parts of southern Illinois, Missouri and Iowa, before shifting back to the East Coast on Sunday.

Forecast heat index values indicate much of the rest of the US should exercise extreme caution.

Triple-digit heat records across multiple states

While much of the western US has been gripped by an extended, unrelenting drought, the recent heat and lack of adequate rain has catalyzed a "flash drought" in other parts of the country.

The US Drought Monitor last week announced states including Texas, Oklahoma, Missouri and Massachusetts were experiencing a flash drought, which the National Oceanic and Atmospheric Administration defines as the "rapid onset or intensification of drought."

It grew even worse this week in the central and southern Plains, the Drought Monitor said in its weekly update Thursday: More than 84% of Texas is in severe or worse drought conditions, the highest percentage in over a decade, while the area of Oklahoma experiencing drought doubled in size.

Arkansas went from less than 1% of the state seeing severe drought to more than a quarter of the state. Missouri similarly went from 2% to a third of the state experiencing severe drought.

Meanwhile, triple-digit records were set Tuesday and Wednesday in multiple locations across Texas, Arkansas and Oklahoma, where Tulsa EMS reported responding to nearly 250 heat-related emergency calls so far this year.

"Those numbers are what we would expect to see in mid- to late-August," Adam Paluka, spokesperson for the Emergency Medical Services Authority, said Wednesday. "So we're four to six weeks ahead of where we would normally see those mid-200 call numbers."

"It's very concerning," he added, "especially because the amount of patients that are being transported indicates that some of those calls are heatstroke, which can be deadly."

In Abilene, Texas, temperatures on Wednesday reached 110 degrees, breaking a 1936 record on that date. Another record of 104 degrees was set in San Antonio, Texas, surpassing the 101 degrees last experienced in 1996.

And as of Tuesday, the Austin area reached at least 100 degrees on 38 out of the last 44 days, according to the weather service.

"We're asking people to conserve power so that the systems continue to operate," Austin Mayor Steve Adler said Wednesday. "We're asking everybody to do that so that we can get through this together."

The Electric Reliability Council of Texas, which operates about 90% of Texas' power grid, said it set another record Wednesday for power demand -- surpassing a record set a day prior.

Also, Wednesday, a record high of 103 degrees in Fayetteville, Arkansas, topped the 102 degrees seen on that date in 2012.

Another Arkansas city, Mountain Home, saw 107 degrees Wednesday afternoon, according to the National Weather Service.

"This would shatter the old record high of 102 degrees for this date set back in 2012. Official record reports are not sent out until midnight but it sure looks like a new record high," the weather service wrote Wednesday evening.

Confronting the heat

To help residents brace through the heat, Boston Mayor Michelle Wu announced that at least 12 community centers will open to anyone who wants to cool off. Additionally, more than 50 splash pads will be available at city parks and playgrounds, she said, as she declared a heat emergency through Thursday.

Meanwhile, some local officials have taken the step to hire chief heat officers to help navigate the response to the extreme heat.

Jane Gilbert, chief heat officer for Miami-Dade County, told CNN's Don Lemon on Tuesday that Miami now has nearly double the days with a heat index -- what the air feels like -- over 90 degrees than it did in the 1970s.

"That is not only concerning to people's health but their pocketbooks. Our outdoor workers can't work as long, they lose work time. People can't afford this AC, the higher electricity cost. It's both a health and an economic crisis."

David Hondula, director of the Office of Heat Response and Mitigation for Phoenix, echoed that sentiment, saying, "The heat can affect everyone, we're all at risk."

High temperatures are one of the top weather-related causes of death in the US, according to Kimberly McMahon, public weather services program manager with the National Weather Service.