PORTLAND, Oregon (REUTERS) - The north-western United States is enduring an extreme heat wave for the second straight summer, with this year perhaps less deadly but more prolonged.

The National Weather Service issued an excessive heat warning for north-west Oregon on Thursday (July 28), with forecasts in some inland areas running as high as 110 degrees Fahrenheit (43 degrees Celsius).

Portland is likely to experience its eighth straight day of temperatures above 32 degrees Celsius this weekend, a standard that has been reached only four times on record, with three of those in the past 13 years, the Oregonian reported on Thursday.

"AC (air-conditioning) is definitely a must now," said Jack Hogan, 28, a Portland man who has had to give up or cut short his daily sun bathing. "Once it gets over 90 (32 degrees Celsius), my body just starts to overheat after a little while in the sun."

The June 2021 heat wave killed more than 100 people in Oregon, in addition to 619 in the western Canadian province of British Columbia.

Officials in the region said many victims lacked air-conditioning, which is less common here than in places with historically hotter climates.

Much of the United States has experienced extreme heat in recent days, which scientists say is exacerbated by climate change.

About 100 people took refuge in one Portland cooling centre on Wednesday night, said Mark Meininger, part of the volunteer emergency team.

Meininger said the prolonged, lower-level heat this year might be more difficult for emergency managers and the public to contend with than last year's extreme levels, when dozens of cities smashed record temperatures.

"Last year we just had record-setting, blasting heat,"Meininger said. "What we have this year is an odd, cumulative consequence of hot weather that slowly gets people to realise they need to get out of the heat."