A heatwave baking the US Southwest for weeks is set to expand into central and eastern regions.

Beginning in the Midwest, the hot weather will extend east as far as the southern tip of Florida by Wednesday, say meteorologists.

Temperature records were surpassed in several major cities over the weekend, and some 59 million Americans began Monday under extreme heat advisories.

July is now expected to be the Earth's hottest month since records began.

On Sunday, the city of Phoenix, Arizona, extended its streak of temperatures above 43C (110F) into a 24th day, well past the previous record of 18 days set in 1974.

It is on course to be the first major US city to average over 100F (38C) for an entire month, according to NOAA statistics and a Washington Post analysis.

At least 18 heat-related deaths have occurred in surrounding Maricopa County since April, with 69 more deaths under investigation.

Meanwhile, in the border town of El Paso, Texas, residents experienced a 38th consecutive day at temperatures above 38C (100F).

The National Park Service has also reported at least four deaths among visitors to its parks in the southwest region.

Two female hikers were found dead in the Valley of Fire State Park in Nevada on Sunday, amid temperatures as high as 45C (114F). Police have not yet released their identities or a possible cause of death.

Extreme heat is the number one weather-related killer in the US.

Ocean temperatures in South Florida and the Keys could reach unprecedented highs as the heatwave extends east in the coming days.

According to BBC Weather, the heatwave has been caused by a "heat dome", a large area of high pressure.

Within this dome, air is heated from the surface and trapped in place by sinking hot air from above.

"Through this week, the heat dome will expand, bringing hotter weather and above average temperatures to pretty much the whole of continental US," said meteorologist Simon King of BBC Weather.

The National Weather Service's Climate Prediction Center says this latest heatwave will last another two weeks.

Media caption,

Watch: The heat index explained in 90 seconds

Data from the National Oceanic and Atmospheric Administration shows the US has this year set or tied more than 13,000 high temperature records, as well as 16,000 low temperature records.

Experts say heatwaves have become more frequent, more intense, and last longer because of human-induced climate change.

Washington state's Democratic governor Jay Inslee told ABC News Sunday the heatwaves reported around the world are evidence that "the Earth is screaming at us".

"The fuse has been burning for decades, and now the climate change bomb has gone off," he said.

"The scientists are telling us that this is the new age. This is the age of consequences." (BBC)

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