The heat dome that settled over the Pacific Northwest over the weekend, shattering records in Portland and Salem, Ore., will linger for at least another day, with temperatures expected to reach the upper 90s by Tuesday afternoon, forecasters said.

The National Weather Service issued another excessive-heat warning on Tuesday for much of Washington State and Oregon that will remain in effect until Sunday. Forecasters predicted that temperatures would remain unseasonably hot into next week.

But a wave of cool ocean air provided a measure of relief overnight, with temperatures falling into the 60s in Portland and Seattle early Tuesday morning.

In Portland, the temperature fell to 64 from a record high of 116 on Monday afternoon, a difference of 52 degrees and a record overnight drop for the city, the National Weather Service said. In Salem, one of the cities hardest hit by the heat wave, temperatures dropped 56 degrees, to 61 from 117. The average overnight temperature drop for Portland is 20 to 30 degrees, the Weather Service said.

But this was only a break from the heat, not long-term relief, forecasters said.

In Portland, temperatures will rebound into the upper 90s on Tuesday and highs will reach the mid-80s later in the week, said Clinton Rockey, a meteorologist with the National Weather Service in Portland. Temperatures will still be 10 to 20 degrees above average at least until next Tuesday.

“That said, it’s a heck of a lot better than being 30 to 40 degrees above normal,” Mr. Rockey said.

While tying a single heat wave to climate change requires extensive attribution analysis, heat waves around the world are growing more frequent, longer lasting and more dangerous. The 2018 National Climate Assessment, a major scientific report issued by 13 federal agencies, notes that the number of hot days is increasing. And the frequency of heat waves in the United States jumped from an average of two per year in the 1960s to six per year by the 2010s. Also, the season for heat waves has stretched to be 45 days longer than it was in the 1960s, according to the report.

It is all part of an overall warming trend: The seven warmest years in the history of accurate worldwide record-keeping have been the last seven years, and 19 of the 20 warmest years have occurred since 2000; worldwide, June 2019 was the hottest June ever recorded, and June 2020 essentially tied it.

Last year tied with 2016 as the hottest year on record, according to one analysis.

The oppressive heat that settled in the Pacific Northwest was the result of a wide and deep mass of high-pressure air that, because of a wavy jet stream, parked itself over much of the region. Known as a heat dome, such an enormous high-pressure zone acts like a lid, trapping heat so it accumulates.

Matthew Cullen, a National Weather Service meteorologist in Seattle, said that Tuesday should be the last day of extreme heat in that city, with temperatures expected to hit 95 degrees, 20 degrees hotter than normal.

“The much bigger thing for us is that low temperatures are dropping well into the 60s,” Mr. Cullen said. “While that is slightly above normal, it allows people to open windows, clear out the hot air and cool down their homes overnight. When we don’t have those cooler overnight temperatures, that’s when things become really dangerous for a lot of folks out here.”

Heat Wave Hits North America

As suffocating heat hits much of Western North America, experts are concerned about human safety and power failures.

• • Western Canada: Canada broke a national heat record on June 27, when the temperature in a small town in British Columbia reached almost 116 degrees Fahrenheit, breaking an 84-year-old record by nearly 3 degrees, with dangerously hot weather expected to continue for several more days.
  • Pacific Northwest U.S.: A heat dome has enveloped the region driving temperatures to extreme levels — with temperatures well above 100 degrees — and creating dangerous conditions in a part of the country unaccustomed to oppressive summer weather or air-conditioning.
  • Severe Drought: Much of the Western half of the United States is in the grip of a severe drought of historic proportions. Conditions are especially bad in California and the Southwest, but the drought extends into the Pacific Northwest, much of the Intermountain West, and even the Northern Plains. The extreme heat is exacerbating the dry conditions.
  • Growing Energy Shortages: Power failures have increased by more than 60 percent since 2015, even as climate change has made heat waves worse, according to new research published in the journal Environmental Science & Technology.
  • Baseline Temperatures Are Rising: New baseline data for temperature, rain, snow and other weather events reveal how the climate has changed in the United States. One key takeaway, the country is getting hotter.

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