The Pacific north-west braced for more sweltering temperatures on Tuesday as authorities in Oregon reported multiple heat-related deaths, with forecasters warning that the dangerous weather wasn’t over yet.

Officials in Multnomah county, which includes Portland, have said they are investigating four suspected deaths tied to the heatwave, which has cooked the region in triple-digit temperatures for days on end.

The county medical examiner was investigating at least three heat-related deaths reported on Friday and Sunday, officials said, involving county residents who were 64, 75 and 84 years old. Heat also was suspected in the death of a 33-year-old man, who was transported to a Portland hospital.

Dozens of locations in the west and Pacific north-west tied or broke previous heat records over the weekend and are expected to keep doing so into the week. Portland saw record daily temperatures on Friday, Saturday and Sunday and was on track to do so for a fourth consecutive day with a forecasted high of 102F (38.9C) on Monday, said National Weather Service (NWS) meteorologist Hannah Chandler-Cooley. Salem also set a new record, hitting 103F (39.4C) on Sunday and topping the 99F (37.2C) mark set in 1960.

An NWS forecast predicted the “most dangerous portion of the heatwave” would last through Tuesday evening. “We are looking at the potential for breaking more records,” said Chandler-Cooley. Multnomah county has been operating several daytime cooling centers to provide relief for the people more vulnerable to heat-related illness.

The US north-west, better known for its rain and cooler climates, has been rocked by brutal heatwaves in recent years, including a 2021 heat event that was blamed for hundreds of deaths.

The temperatures aren’t expected to reach as high as they did during that 2021 heatwave, which killed an estimated 600 people across Oregon, Washington and western Canada. But the duration could be problematic because many homes in the region lack air conditioning. Round-the-clock hot weather keeps people from cooling off sufficiently at night, and the issue is compounded in urban areas where concrete and pavement store heat.

Heat illness and injury are cumulative and can build over the course of a day or days, officials have warned. In San Jose, California, a man who was unhoused died last week from apparent heat-related causes, the city’s mayor, Matt Mahan, said on X/Twitter, calling it “an avoidable tragedy”.

In Death Valley national park, where temperatures reached 128F (53.3 C), a motorcyclist died from heat exposure over the weekend while another motorcyclist was hospitalized with “severe heat illness”, park officials said.

Meanwhile, officials in Phoenix, Arizona, recently reported that heat deaths last month – the hottest June ever experienced in the city – have almost doubled compared with the same period last year.

The heatwave has sizzled states across the US west over the past week. While parts of California will begin to see some relief from the brutally hot conditions, the heatwave is forecast to shift north into Oregon and Washington and move east, covering parts of the Great Basin and Arizona, where more records will likely be broken.

“The multi-day length and record warm overnight temperatures will continue to cause heat stress in people without adequate cooling and hydration,” NWS meteorologists wrote in a forecast published on Monday.

The weather has also fueled the outbreak of new wildfires as hot, dry conditions meet flammable grasses that dried out after a wet winter. A wildfire in the mountains of Santa Barbara county grew to more than 26,000 acres by Tuesday morning, forcing evacuations of the area including at the former Neverland Ranch once owned by Michael Jackson.

Further north, the Shelly fire, which erupted in California’s Marble Mountain Wilderness, has grown to more than 6,000 acres and is 0% contained. And a small but smoky blaze, dubbed the Royal fire, has burned through more than 150 acres (60 hectares) of forest west of Lake Tahoe and sent ash raining down on the tourist town of Truckee, California.

The US heatwave came as the global temperature in June was record warm for the 13th straight month and it marked the 12th straight month that the world was 1.5C (2.7F) warmer than pre-industrial times, the European climate service Copernicus said.