Another summer week, another spot in the U.S. is sweltering. In June it was the Southwest, with planes unable to take off in Phoenix; then it was Miami, which just had its hottest month on record; now, it’s the Pacific Northwest’s turn.

A high-pressure system will send temperatures in Oregon and Washington to rare heights. Seattle could see only its fourth ever 100°F (38°C) temperature reading on Thursday, while Portland has a chance to break its all-time heat record.

While the rarity of such extremes make it hard to pick out any clear trends in such exceptional temperatures, in general, rising average temperatures fueled by greenhouse gases make such extremes more likely.

The sizzling weather comes courtesy of a high-pressure system that is moving up from the Southwest. Temperatures will peak on Tuesday across Northern California, on Wednesday across southern Oregon and on Thursday from Portland northward.
 


Days that hit 100°F are rare in Seattle — there are only three on record in the past 130 years, according to the National Weather Service. On July 16, 1941, the temperature at the federal building in downtown Seattle hit 100°F; it reached 100°F again at SeaTac airport (where official measurements have moved) on July 20, 1994, and then, on July 29, 2009, the highest temperature in Seattle’s history, 103°F (39°C), was recorded at SeaTac.

Temperatures this week are unlikely to topple that all-time record, but they could give Seattle its fourth 100-degree day on Thursday, when the heat is expected to peak there. The current forecast is for 99°F (37°C), but it could hit 100°F if offshore winds prevail, Mike McFarland, a meteorologist at the Seattle NWS office, said.

Days that reach into the 90s aren’t a given for Seattle summers. “We would normally get a handful of hot days up in the 90s,” MacFarland said. “Some years we get more, some we skip it entirely.”

The heat will be worse down in Oregon, though, which typically sees more hot summer days than Seattle, which has the cooling influence of the nearby ocean.

“It’s more common for us to hit 100 than Seattle,” Kathie Dello, deputy director of the Oregon Climate Service at Oregon State University in Corvallis, said.

Pacific Northwest temperature forecast

Forecast temperatures around the Pacific Northwest for Thursday, Aug. 3, 2017.

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But while it’s more common, it’s still not the norm for the region. Speaking around 11:30 a.m. local time on Tuesday, Dello said the temperature in Corvallis had already reached 83°F (28°C), “which is huge. That’s normally our average high.”

“We can go an entire summer without seeing 100,” she said, and streaks of days above 100°F are “very, very rare.”

The last time Portland saw temperatures over 100°F was during a heat wave in late June when the temperature reached 101°F on June 25. The last time there were two days in a row of 100-plus temperatures was on July 28 and 29 in 2009, according to the NWS. And the last streak of three or more 100-degree days was in 1981, when Portland recorded its all-time high temperature of 107°F.

It is possible that the city could meet or break that record during this week’s heat wave, David Bishop, a meteorologist with the Portland NWS office, said. Right now the forecast for Wednesday and Thursday is for a high of 106°F (42°C).

The area could also see nine or 10 days of 90°F (32°C) or higher, Dello said.
 


While such heat extremes have happened before, they become more and more likely as average temperatures in the Pacific Northwest — and the world — rise as greenhouse gases in the atmosphere trap more and more heat. Any heat wave that rolls through is coming on top of already higher temperatures than in the past.

Summer temperatures in Portland and Seattle have risen by about 2°F (1°C) since 1970, according to a Climate Central analysis of local temperature records.

The heat raises concerns over wildfires, as humidity is also low. Oregon has six large fires burning and Washington three. The hot, dry weather is perfect for allowing accidental fires to spread rapidly.

The heat could also draw down on the region’s water supplies, making things tight if rains fail to return in the fall, Dello said.

Forecasters and local officials are urging people in the region, where air conditioning isn’t as prevalent as in other parts of the country, to stay hydrated and limit the amount of time they spend outside. They’re also urging residents to check on neighbors, especially the elderly, who are particularly vulnerable to heat, and pets.