A heat dome blanketing the southwest and Texas has caused widespread hazardous conditions for tens of millions of Americans as forecasters predicted record highs in the regions.

More than 23 million Americans were under some level of heat advisory or warning early Wednesday, according to Heat.gov. The National Weather Service is warning of "major" and "extreme" levels of heat risk in large swaths of the southwest and Texas as well as parts of the Gulf Coast.

"Hazardous heat continues across Texas and the Gulf Coast with temperatures in the triple digits," the weather service said Tuesday.

Sweltering conditions were expected to continue through the week with near record to record temperatures forecast for portions of Arizona, New Mexico, and Texas, according to the weather service.

The weather service office in Phoenix said temperatures in the city reached 112 degrees Tuesday afternoon — just one degree "shy of the record set in 2019." Meanwhile, forecasters in Houston estimated that the "hottest days of the year" will likely be on Tuesday and Wednesday with temperatures between 100 and 105 degrees and heat indices between 111 and 116 degrees, according to the weather service.

Forecasters said much of central and eastern New Mexico would face a moderate to major heat risk through Thursday, warning the potential of heat-related illnesses for vulnerable populations and those who work outside.

The ongoing heat wave comes amid a record-shattering hot summer. Nearly the whole nation has faced higher-than-usual temperatures in recent months, including a weekslong heat wave in July that was fueled in part by the warming climate.

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Heat, humidity bake Southern Plains

Dangerous heat and humidity were also plaguing the Southern Plains and high temperatures — between the upper 90s and triple digits — were expected to continue through much of the week, the weather service said. The heat is forecast to build northward into the Central Plains by Friday and Saturday.

"The combination of high heat and humidity will support daily maximum heat indices at or above 110F," the weather service said.

Although the heat wave is forecast to "gradually subside" over the weekend, the weather service said afternoon heat indices will remain in the upper 90s to mid 100s over the Southern Plains and Gulf Coast region into early next week.

Blazing temperatures in Texas pushes electric grid

Scorching temperatures in Texas are pushing the state's electric grid as power demand hit a record high on Tuesday, according to the Electric Reliability Council of Texas. After breaking multiple peak-demand records in April and May, the state's power demand reached a preliminary 85,558.98 megawatts on Tuesday, which topped the previous record of 85,508 MW set last August.

But the council, which operates most of the grid for 27 million customers, noted that its power network was operating normally and had enough supply for the current demand.

While the number of homes a megawatt can power varies by region and electricity usage in the area, according to the Northwest Power and Conservation Council, experts have estimated that a megawatt can power up to 800 homes on a normal day. In Texas, one megawatt can power about 200 homes and businesses during periods of peak demand, according to the Electric Reliability Council of Texas.

Temperatures in Houston, the state's biggest city, were expected to reach 103 degrees on Tuesday before easing to 100 degrees on Wednesday, according to AccuWeather meteorologists.

What are the dangers of heat-related illnesses?

Heat stress is the leading cause of weather-related deaths, according to the World Health Organization.

"Heatwaves and prolonged excess heat conditions are increasing in frequency, duration, intensity and magnitude due to climate change," the WHO said. "Even low and moderate intensity heat waves can impact the health and well-being of vulnerable populations."

Although heat-related deaths and illnesses are preventable, about 1,220 people in the United States are killed by extreme heat every year, according to the Centers for Disease Control and Prevention (CDC). Heat-related illnesses can become severe or even deadly if left unattended, the CDC warns.

Extreme heat makes it difficult for the body to keep its temperature in a normal range and can cause a number of conditions. Initial stages of heat-related illnesses are marked by excessive thirst and muscle cramps, according to the CDC. Severe stages can range from heat exhaustion to heatstrokes.

The CDC advises people to stay in cool environments and hydrate with water or a sports drink during hot days. The CDC recommends to loosen clothing; apply cool, wet cloths to the body; or take a cool bath to alleviate symptoms of severe heat-related illnesses, but noted that medical attention should be sought if symptoms persist or worsen.

What is a heat dome?

A heat dome occurs when a persistent region of high-pressure traps heat over an area, according to William Gallus, professor of atmospheric science at Iowa State University.

"The heat dome can stretch over several states and linger for days to weeks, leaving the people, crops and animals below to suffer through stagnant, hot air that can feel like an oven," Gallus said in an article in The Conversation.

Contributing: Doyle Rice and Janet Loehrke, USA TODAY; Trilce Estrada Olvera, Arizona Republic; Reuters