The mercury was expected to hit 107F (41.6C) into the weekend, smashing through previous high temperatures logged in March.

An extreme heat advisory was in place for swathes of southern California and Arizona, with forecasters warning of dangerous effects on local populations.

"We are seeing temperatures that are much, much warmer than normal," National Weather Service meteorologist Rose Schoenfeld told AFP.

"It's also the first major heatwave of this year... occurring after periods of cooler temperatures. So the population has not gotten used to these temperatures yet."

Schoenfeld said anyone spending time outside was at risk of heat stroke, with experts warning people to seek out air conditioned rooms, and to drink plenty of fluids.

The last 11 years have been the hottest in recorded history around the planet.

Scientists say there is overwhelming evidence that heat waves such as the one pummeling the western United States are a clear marker of global warming, a process driven chiefly by humanity's unchecked burning of fossil fuels.

Schoenfeld said temperatures in parts of southern California were up to 35F (20C) higher than normal for this time of year.

"We are likely to continue to break daily calendar day records, as well as some areas all-time records," she said.

With the northern hemisphere still technically in winter until March 20 -- the first day of astronomical spring -- the soaring temperatures were wreaking havoc on wildlife.

Many plants and trees are already blooming, and vegetation is growing at a fantastic clip, fuelled by heavy rains in December and January.