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Record highs roast Northwest; heat wave looms for rest of Lower 48

Record-breaking highs over 110 degrees are predicted in interior Oregon and Washington state, while a sprawling heat dome may spread over the central and eastern U.S. next week

High temperatures on Friday, as predicted by the National Weather Service. (Pivotal Weather)
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Heat advisories and excessive heat warnings are blanketing the Pacific Northwest, where spiking temperatures are accompanying a stubborn heat dome. Highs are running some 10 to 15 degrees above average in spots, with readings as high as 115 degrees expected in spots.

Fifteen million people in the western United States and Pacific Northwest will see highs hit the century mark over the coming days. By the middle of next week, an even bigger heat wave may build in across the central and eastern Lower 48.

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Seattle and Portland have been setting daily records, and triple-digit temperatures are bleeding south into the highly populous Willamette Valley. Concern is growing for vulnerable populations too, since only 44 percent of homes in Seattle and 78 percent of residences in Portland have air conditioning.

Extreme heat in the Pacific Northwest

“Extreme heat will significantly increase the potential for heat-related illnesses, particularly for those working or participating in outdoor activities and for those that do not have access to air conditioning,” warned the National Weather Service on Thursday.

Also a factor is poor air quality stemming from ground-level ozone building as pollutants are baked by the intense sunshine. In other areas, smoke wafting north from explosive California wildfires is bringing hazy conditions and complicating high-temperature forecasts.

“We’re going to be experiencing a couple of days of really hot temperatures across parts of our inland areas,” said Scott Carroll, a meteorologist at the National Weather Service in Eureka, Calif. He explained that the northern reaches of the San Joaquin Valley are in line for brutal heat. “Some of the interior valleys could see temperatures getting up to 110 to 115 degrees this afternoon and tomorrow.”

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He noted that average late July high temperatures are “closer to 90 or the lower 90s,” but instead a few record highs may be in jeopardy. While Northern California has a comparatively sparse network of weather stations, readings 20 to 25 degrees above normal are at least “close to records,” Carroll said.

Similar temperatures will scorch the Columbia River Basin, with highs around Kennewick, Wash., peaking near or at 110 degrees each day through Saturday. Overnight lows will be in the upper 60s or lower 70s — exacerbating the heat risk.

Large parts of interior Oregon and Washington state will see highs above 100 degrees Thursday and Friday, challenging records; some locations could eclipse 110. AccuWeather predicts record highs Thursday of 109 degrees in Yakima, Wash.; 102 (tied) in Spokane, Wash.; and 112 in Medford, Ore. Temperatures will be comparably high on Friday before pulling back slightly over the weekend and more noticeably early next week.

Records falling in Seattle, Portland

Record temperatures have occurred in Seattle and Portland — the former hitting 94 degrees on Tuesday and the latter getting up to 102 degrees. Wednesday was a little bit cooler, but Thursday and Friday will be hot once again.

Seattle should peak around 90 degrees at the airport each day through Friday or Saturday; upper 80s are more likely at the University of Washington. Since Wednesday underachieved slightly and only hit 89, it’s unlikely Seattle will be able to string together three consecutive days at or above 90 degrees to nab an official heat wave.

In Portland, upper 90s to near 100 degrees are essentially a guarantee through Saturday. Considering Portland hit 99 on Monday, 102 on Tuesday and 96 on Wednesday, Portland may tie for its longest streak of 95-degree temperatures on record — six days. The forecast for Sunday his a high of 94 degrees, but it wouldn’t take much to get a seventh day and establish a record.

“It is unusual to have multiple days with hot temperatures, and by Portland’s standards, 90 is hot,” said Jon Bonk, a meteorologist at the Weather Service in Portland. “It’s rare that we spend a decent amount of time with temperatures above 90. Usually we’ll see two or three days in the 90s or higher, and then the afternoon sea breeze will start kicking in — we colloquially call it nature’s air conditioning. Generally that’s where we end our streak.”

That hasn’t been a case this time around, however. The cause of the heat is a ridge of high pressure — colloquially known as a “heat dome” — languishing in the extreme northeast Pacific just offshore of British Columbia. That’s yielding a broad region of subsidence, or sinking air. Air parcels warm up and dry out as they subside through a process known as “adiabatic compression,” yielding toasty temperatures and unrelenting sunshine. Heat domes also act as magic force fields of sorts, deflecting inclement weather and storminess to the north into Canada.

Bonk explained that this isn’t a classic setup for hot weather in Portland, which would ordinarily entail the heat dome parking directly over the Willamette Valley.

“To get to 100 or higher, we usually end up with a surface heat low, or a thermal low — an area where you get the intense heating from the [sinking] of the high pressure,” he explained. “That intense heat creates a low pressure area.”

That ordinarily invokes an easterly flow, causing air to subside and “downslope” down the Cascades, which results in compressional warming. But Bonk said that thermal low is instead straddling the Cascades. That means no downsloping, or extra warming — but the thermal low’s position also fends off the sea breeze. In other words, it could easily be hotter and it could easily be cooler; Portland is in the sweet spot to hover around 100 degrees.

Because of the lack of downsloping, the air won’t be as dry as it otherwise could be. That means slightly higher humidity, which will maintain overnight low temperatures in the upper 60s to lower 70s.

“That’s a major contributing factor to the heat risk,” Bonk said. “Even when we get hot temperatures, we usually get the sea breeze. [This time] we haven’t seen that much of it.”

It’s well established that the intensity and duration of extreme heat events is increasing in response to the effects of human-induced climate change. Seattle hit 108 degrees and Portland jumped to 109 on June 27 last year amid a thousand-year heat event.

An even bigger heat wave looms

While it’s too early to offer any specifics, there are signs that an exceptional heat dome — perhaps the most expansive and dominant of the summer — could take hold of the contiguous United States in a coast-to-coast clean sweep. It looks like the ridge of high pressure will begin to come together as the calendar flips toward August, and will peak in intensity by Aug. 6 or 7.

At the time range of eight to 10 days or more in advance, it’s impossible to provide exact estimates of city by city temperatures, but confidence is growing in highs 15 to 20 degrees above average. A few locations may see temperature departures from average up to 30 degrees, but pinpointing where is a challenge.

By the middle of next week, highs at or above 100 degrees could reach all the way to the Dakotas, the Corn Belt and the Upper Midwest. There are signs the heat will shift east thereafter.