WA baked through the start of a heatwave on Saturday as the temperature peaked at 35C - but that was just a warm-up for the week ahead with an expected 40C on Sunday, 42C on Monday and Tuesday, 41C on Wednesday and 39C on Thursday.

That's equal to the longest run on record of days above 39C, set in 1933 and again in 1965.

A leading climate scientist has warned Perth will be hit by longer, hotter heatwaves capable of sending the mercury "into the 50s", unless action is taken.

Sunday's expected high is hot enough to roast a chicken in a car parked in the sun, where the temperature can top 70C - which celebrity chef Heston Blumenthal says in one of his recipes will roast a chicken in three to four hours.

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Fremantle Docker Lachie Neale showed just how quickly a car can heat up, during a demonstration with the RSPCA to highlight the dangers of leaving pets locked in cars.

He described the heat as blistering after sitting inside the locked car with a digital thermometer that reached more than 50C in just 10 minutes.

RSPCA chief executive David van Ooran said it took as little as six minutes for a dog to overheat and die in a car.

Docker Lachie Neale with Mayo, urging dog owners not to leave pets in their cars. Photo: Sean Middleton

Docker Lachie Neale with Mayo, urging dog owners not to leave pets in their cars. Photo: Sean MiddletonSource:News Corp Australia

West Australians flocked to the beach for a dip and to shopping malls to cool off in airconditioning on Saturday.

The forecast has sparked fire and public health warnings, particularly for children and the elderly, and there will be little relief at night with forecast minimums as high as 26.

But spare a thought for the residents of Pilbara town Marble Bar, where the temperature is tipped to exceed 44 for six consecutive days.

Weather Bureau spokesman Neil Bennett said a trough of low pressure near the west coast, combined with a high pressure system in the Great Australian Bight, was pulling hot easterly winds towards the west coast.

The State Emergency Management Plan warns heatwaves, defined as three or more consecutive days of high day- and night-time temperatures. are among the deadliest natural phenomena to face WA.

A report by the Climate Council said heatwaves that used to occur every three years are now happening every 200 days because of climate change.

Professor Will Steffen, a climate researcher at the Australian National University, said this week's savage weather was an example of how heatwaves are becoming "hotter and longer". "Heatwaves will be even worse than they are now," he said.

"What happens after mid-century depends on how we get emissions under control but if we keep burning fossil fuels ... that means really, really excessive heat during extreme weather - into the 50s."

- News.com.au