Brutal heatwave bakes large swaths of the US

About 130 million people in the United States are under threat over the weekend and into next week from a long-running heatwave that has broken or tied records with dangerously high temperatures.

Ukiah, north of San Francisco, hit 47C on Saturday, breaking the city's record for the date and tying its all-time high. Livermore, east of San Francisco, hit 43.8C, broke the daily maximum temperature record of 42.7C set more than a century ago in 1905.

Las Vegas tied the record of 46C, last reached in 2007, and Phoenix topped out at 45.5C, just shy of the record of 46.7C dating to 1942.

The National Weather Service said it was extending the excessive heat warning for much of the southwest into next week.

"A dangerous and historic heatwave is just getting started across the area, with temperatures expected to peak during the Sunday-Wednesday time frame," the agency said in an updated forecast.

In more humid areas of the US, temperatures could spike above 38C in parts of the Pacific Northwest, the Mid-Atlantic and the Northeast, said Jacob Asherman, a weather service meteorologist.

Meteorologists predicted that temperatures would be near daily records in the regions through most, if not all, of the coming week, with lower desert highs reaching 46.1C to 48.8C.

Rare heat advisories were extended even into higher elevations including around Lake Tahoe, on the border of California and Nevada, with the National Weather Service in Reno, Nevada, warning of "major heat risk impacts, even in the mountains."

"How hot are we talking? Well, high temperatures across (western Nevada and northeastern California) won't get below 37.8C until next weekend," the service posted online. "And unfortunately, there won't be much relief overnight either."

The hottest temperature ever officially recorded on Earth was 56.67C in July 1913 in Death Valley, eastern California, though some experts dispute that measurement and say the real record was 54.4C, recorded there in July 2021.

The worst is yet to come across the West and Mid-Atlantic. Triple-digit temperatures are likely in the West, between eight and 16C higher than average into next week, the National Weather Service said.

Firefighters dispatched aircraft and helicopters to drop water or retardant on a series of wildfires in California over the weekend.

In Santa Barbara County, northwest of Los Angeles, the Lake Fire has scorched more than 49 square kilometres of grass, brush and timber.

Firefighters said the blaze was displaying "extreme fire behaviour" and had the "potential for large growth" with high temperatures and low humidity.