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Weather Service warns of ‘dangerous’ and ‘historic’ heat wave in Pacific Northwest

Seattle, Portland and Spokane could see temperatures near and above 100 degrees Sunday into early next week.

Forecast highs Sunday in the West from the National Weather Service. (WeatherBell)

This story has been updated.

Confidence is increasing that an episode of historic heat will torch much of the Pacific Northwest late this week into early next, with temperatures 30 degrees or more above average set to topple monthly and all-time records.

“Record-Breaking and Dangerous Heatwave coming to the West,” tweeted the National Weather Service Weather Prediction Center Wednesday. “Over 80 sites are forecast to break daily high temperature records starting this weekend.”

Highs on Sunday could approach 100 degrees in Seattle and 107 in Portland, Ore., cities where a significant portion of the population lacks air conditioning. Seattle has only reached the century mark twice in more than 75 years of records. If Portland hits 107, it would match its highest temperature ever recorded.

“Three words will describe weather for inland areas this weekend into early next week … HOT, VERY HOT,” the National Weather Service office in Portland wrote in a discussion.

In Spokane, Wash., the mercury could reach 110 degrees by Monday. The Weather Service in the Spokane office is referring to the episode as a “historic and dangerous heat wave,” predicting triple-digit heat Saturday to Tuesday. “We currently anticipate that Tuesday will be the hottest day with highs between 105 and 115 for most of the inhabited area,” it wrote.

The Weather Service has issued excessive-heat watches from extreme Northern California through large parts of Oregon and Washington, warning that heat-related illnesses are likely for some, with concern for residents without adequate air conditioning — “i.e. a lot of people,” the Spokane office wrote.

How the heat wave will evolve

Temperatures were already toasty Tuesday across the Columbia Basin of eastern Washington state and extreme northeast Oregon, where highs climbed into the upper 90s. That marks a mere taste of what’s to come. Wednesday and Thursday will feature similar temperatures before the mercury spikes Friday, with a few spots hitting 100.

Widespread highs in the range of 100 to 110 degrees are expected Saturday, both within the coastal valleys and well inland. Temperatures may top 110 degrees in some spots by Sunday, with signals that the heat may stick around until the middle of next week.

The skyrocketing temperatures can be traced to a sprawling ridge of high pressure colloquially known as a “heat dome.” That high deflects inclement weather to the north, bringing copious sunshine, sinking air and clear skies. Weak high pressure is in control across the northwestern Lower 48 but will be replaced by a robust high-pressure system slated to move ashore and into southern British Columbia during the day Saturday.

How a heat dome is pushing extreme temperatures to new heights in the West

In downtown Seattle, a high near 96 is forecast Saturday, 97 Sunday and 94 on Monday. Average highs this time of year tend toward 74.

The airport has hit 100 degrees or higher only twice — once on July 20, 1994, and again July 29, 2009, when highs spiked to 103. While that’s not in the forecast now, temperature predictions have been trending hotter, and both the American GFS and European models hint that it’s possible, illustrating just how rare heat of this magnitude will be.

It has been nearly three years since Seattle logged an official heat wave, entailing three consecutive days at or above 90 degrees.

Farther inland, temperatures will be markedly warmer, as in Spokane. The city of nearly 220,000 could see highs topping 90 degrees for upward of a week in the current weather pattern. Both weekend days are expected to top 100 (102 Saturday and 106 Sunday), with Monday and Tuesday’s high anticipated to reach around 110. That would break the city’s June record of 105 and all-time (any month) record of 108 degrees.

“Multiple days in a row of these temperatures will make heat illness all the more likely if precautions aren’t taken because the heat will continue to stress the body each day,” wrote the Weather Service office in Spokane.

In Yakima, Wash., the heat will also be unbearable, with highs near 110 degrees Sunday through Tuesday. In nearly 80 years of bookkeeping, Yakima has measured only two stretches of three or more days hitting 105 degrees. Yakima has also hit 110 degrees only once on record — on Aug. 10, 1971. It’s not out of the question that Monday ties or breaks a record.

Other cities in the Pacific Northwest will also be excessively hot: A high of 107 is expected in Boise, Idaho, on Monday. Medford, Ore., could hit 107, too.

The heat will be dangerous both because of its extreme magnitude and extended duration. With nighttime lows falling to only around 70 in most places, there will be little opportunity for the air inside homes to cool off. Two-thirds of Seattle-area and nearly one-third of Portland homes lack air conditioning. That means the elderly and vulnerable populations will be especially at risk for heat-related illnesses, since mild nighttime lows will offer little relief for the body.

Conditions will also be problematic from a wildfire and drought standpoint. Red-flag warnings for wildfire danger are posted from the Four Corners region through interior Oregon, and more alerts will be issued in the days ahead. A few “dry thunderstorms” — or lightning-producing thunderstorms with little to no rain — are possible, which could ignite new blazes.

As of Wednesday, large fires were active in 11 states. So far this year, more than 29,000 fires, the most in a decade, have burned more than 1.1 million acres, according to the National Interagency Fire Center.

Equally disconcerting will be the exacerbation of ongoing extreme to exceptional drought in the West. The forthcoming outbreak of heat, made worse by human-induced climate change, will further dry soils and reinforce drought conditions. The drought, in turn, will make future extreme heat events more likely.

“The heat will unfortunately be bad news for farmers, too,” wrote the National Weather Service in Spokane. “Low relative humidities and no precipitation will worsen the already bad drought. Plus, fuel models suggest record dryness this early in the season, so wildfires are a concern.”

More than 55 percent of the West is experiencing an extreme or exceptional drought — the two most severe categories — as scant precipitation and above-average temperatures wreak havoc on agriculture and water resources. Looking ahead, continued anomalously hot and dry conditions are expected there for the remainder of the summer.

This next blast of heat comes on the heels of a historic heat wave that set nearly 4,000 records in the West last week. Phoenix hit at least 115 degrees for a record six days in a row, and Tucson topped 110 for a record eight straight days. The heat wave expanded into Mexico, where a location just south of the U.S. border soared to 124.5 degrees, the highest temperature ever recorded in the country during June.

The intensity of last week’s heat wave, like the one forthcoming, is connected to human-caused climate change and the buildup of heat-trapping greenhouse gases in the atmosphere.

Even after the worst of the heat wave eases by the middle of next week, temperatures in the Pacific Northwest are expected to remain warmer than normal. Computer models project above-average temperatures through the July Fourth weekend.