Mega heatwave builds as millions currently under high-temperature alerts in the US West

Record-breaking summer heat is hitting the U.S. West.

By many forecast models, it is expected to bring sustained high temperatures through Labor Day weekend as millions are living under heat advisories caused by a growing mega heat wave.

The Weather Prediction Center said temperatures could rise above 110 Fahrenheit in some parts of the Southwest "where an excessive heat warning is currently in effect."

More than 55 million people are living under active high-temperature alerts in areas stretching from portions of the Northwest down to Southern California and in swaths of the United States that include at least 20 of the most populated cities in the country's West, according to meteorologists at CNN.

The National Weather Service in Los Angeles expects a "prolonged heat event" and urged residents in affected areas to "be prepared."

A weather model shared by the service on Twitter shows an excessive heat forecast expected to extend through Labor Day weekend for the entire Southern California coverage area.

Vulnerable populations are of particular worry for emergency services and meteorologists, as elderly individuals and those without proper air conditioning, or even workers who have to work outside, could experience serious heat illnesses in extended high temperatures.

Heatwaves have impacted multiple parts of the globe this summer. China has been experiencing its worst drought in decades amid a prolonged heatwave in many parts of the country that has caused safety issues for residents and halted agricultural production in many areas.

In the United Kingdom, England has seen multiple hottest summer days on record, according to official numbers out on Thursday, the BBC reported.

According to the country's Met Office, which tracks this data, four of the five hottest summers on record there have happened since 2003.