The extreme heat wave that has baked much of the southern US continues to spread to other regions of the country this week, with over 55 million people now under heat alerts.

The heat wave has scorched Texas, Oklahoma, New Mexico, among other southern parts over the past week, setting or challenging all-time records, Xinhua news agency reported.

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The life-threatening “oppressive” heat dome was producing “dangerous heat and humidity in Texas and spread into the lower Mississippi River Valley”, according to the US National Weather Service (NWS).

San Angelo in West Texas registered 114 degrees Fahrenheit twice in June, the highest ever recorded there. The border town of Del Rio hit 115 degrees Fahrenheit for the first time.

A reading of 119 degrees Fahrenheit on Friday in the Big Bend area of southwest Texas came within one degree of tying the state’s previous all-time high of 120 recorded in 1994, according to meteorologists for the NWS in Midland.

The heat wave is continuing to bake Texas, and will expand in the coming days across much of the southern Plains, the Deep South, the Lower Mississippi Valley and the Gulf Coast, according to the NWS.

More of the Gulf region will see dangerous heat on Tuesday, lasting in some places through the Fourth of July, according to weather forecasts.

Excessive heat warnings are plastered across much of Texas, parts of New Mexico and Arizona and along the Gulf Coasts of Louisiana, Mississippi and Alabama, while heat advisories stretch from northern Florida to southern New Mexico.

Meanwhile, with a high-pressure system building over Southern California, the first heat wave of the summer is expected to hit the western region later this week into the holiday weekend, focused across interior areas.

Triple-digit temperatures are expected across the hottest interior areas of Southern California, according to the NWS.

The oppressive heat has been caused by a stagnant upper-level ridge over the south-central portion of the country, according to the NWS. The dome of high pressure is expanding and will blast heat over much of the southern band of the country in the coming days.

As of Tuesday morning, more than 55 million people in the US were under some form of heat advisory, watch or warning, according to New York Times estimates using NWS advisories and LandScan population data.

A handful of heat-related deaths have been reported across the South.

Health officials urged people to take precautions during the heat wave, particularly those in what are considered vulnerable populations, such as children, the elderly and individuals with chronic illnesses.

Extreme heat has been the greatest weather-related cause of death in the United States for the past 30 years — more than hurricanes, tornadoes, flooding or extreme cold — killing over 700 people per year, according to the US National Oceanic and Atmospheric Administration.

In addition, the heat crisis does not affect people equally. Extreme heat mortality disproportionately affects Native American and Black communities, as well as those living in the urban core or very rural neighbourhoods, according to the US Centers for Disease Control and Prevention.

Studies have shown that climate change is making heat waves both more frequent and more intense, increasing the risks of heat-related illnesses and deaths, droughts and wildfires.