A large swathe of the southwestern United States was on alert Thursday for a potentially deadly heatwave that could push temperatures as high as 47 degrees celsius (117 Fahrenheit) over the coming days.

Millions of people in California, Nevada and Arizona were warned to expect dangerous conditions for at least some of the weekend, with the National Weather Service advising residents to stay out of the sun.

While the region usually heats up at this time of year, forecasters warned it would be considerably hotter than average.

Inland and desert areas of California will be particularly hot on Friday and Saturday, with the tourist city of Palm Springs expected to hit 45 degrees, while nearby Ocotillo Wells could reach 47 degrees.

"We've had some prior heat waves this year, but not as intense as this one or as long duration," San Diego weather service meteorologist Alex Tardy said.

Prolonged exposure to high temperatures can be exceedingly dangerous for humans.

The World Health Organization says excessive heat stresses the body, and increases the risk of respiratory and cardiovascular disease.

"Heatwaves can acutely impact large populations for short periods of time, often trigger public health emergencies, and result in excess mortality, and cascading socioeconomic impacts," the WHO says on its website.

Heatwaves and temperature variations are a natural part of the climate, but scientists say human-caused global warming is creating a greater number of extreme events, sometimes with devastating consequences.

In June last year a "heat dome" sat over the western United States and Canada.

The intense temperatures and worst-in-a-millennium drought gripping the region led to numerous fires.

In the village of Lytton, northeast of Vancouver, temperatures reached 49.6 degrees in the days before a destructive fire swept through.

California, along with much of the American West, is on high alert for wildfires.

Years of below-average rainfall has left huge tracts of countryside tinder-dry, and almost the entire state is classed as suffering from severe drought or worse.

In 2020 and 2021, a total of almost seven million acres (2.8 million hectares) were burned in California alone, and forecasters are warning there could be another grim year ahead.

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© 2022 AFP