WEATHER NEWS: U.S. heat wave: Record highs target Pacific Northwest; Northeast cools

Daily high temperatures about 10 to 20 degrees above average will persist through at least the end of the workweek, with elevated highs sticking around into the weekend. Several records will be set. Heat index values could reach dangerous levels.

London hit 104 degrees. How hot would that be where you live?

The episode coincides with the conclusion of a heat wave that brought highs in the upper 90s to near 100 in the Northeast and the Mid-Atlantic. Boston jumped to 100 degrees Sunday, something that happens only every couple of years on average. Newark hit the century mark for five consecutive days ending Sunday, the longest such streak on record, although there are questions about whether its temperature readings are reliable.

The Northeast faced one more steamy afternoon, on Monday, before a cold front slices through the region, triggering strong to severe thunderstorms and putting an end to the heat.

Excessive heat warnings and heat advisories from the National Weather Service affected about 65 million people in three regions of the Lower 48 on Monday: the coastal plain of the Northeast; portions of northern Texas and central and eastern Oklahoma through western Tennessee and northern Mississippi; and the Pacific Northwest, away from the coast.

In the days ahead, while the heat will ease in the Northeast, it will remain entrenched over the south-central United States and will become more intense in the Pacific Northwest.

The Weather Service wrote that the heat in the Pacific Northwest “will significantly increase the potential for heat related illnesses, particularly for those working or participating in outdoor activities.”

The agency emphasized particular concern for those without air conditioning — a significant percentage of residents in parts of Washington state, Oregon and Idaho. Only an estimated 44 percent of households in Seattle are air-conditioned, for example, and highs are expected to hit 90 degrees every day this week. The late July average in Seattle is 79 degrees.

Jacob DeFlitch, a meteorologist at the Weather Service in Seattle, said that although it’s not uncommon for Seattle to reach the 90s during the summer, the duration of this event will be unusual. He said Seattle could hit 90 on four straight days.

Seattle’s record is five 90-degree days in a row.

“Residents without air conditioners will experience a [buildup] of heat within their home through late in the week,” the Weather Service wrote.

Two processes are at work to drive this fledgling heat wave. First is the stagnation of a heat dome, or ridge of high pressure, to the west of British Columbia over the extreme northeast Pacific. Heat domes bring hot, sinking air and act as a force field that diverts the jet stream north into Canada. That deflects any storminess or inclement weather, fostering predominantly sunny skies. There will also be some downsloping, or air cascading from higher elevations to lower, which results in “adiabatic compression” — as air descends, it heats up and dries out.

That will make for extreme temperatures in Yakima, Wash., where the forecast calls for readings between 102 and 107 degrees through the midweek and approaching 110 degrees Thursday and Friday. Every day of the next five could tie or break a daily record.

Spokane will make it into the triple digits on Wednesday, Thursday and Friday, with highs falling back into the upper 90s for the weekend. Records may be tied or broken Thursday and Friday.

Especially hot will be the Columbia River Basin, where hot air pouring into the valley will experience even greater downsloping. Kennewick, Wash., southeast of Yakima, could spike to 113 degrees Thursday and Friday before a relative cool-down into the lower 100s by the weekend.

“It looks like they hit a max of 115 that occurred last year on June 27, but generally, yeah, that would definitely be in the subset of highest temperatures recorded there,” DeFlitch said.

Farther to the south, Portland, Ore., is expecting only one 100-degree day — Tuesday — but mid- to upper 90s are possible throughout the remainder of the week.

The heat wave, while intense, falls shy of the “thousand-year” heat event in June 2021 that brought a high of 108 degrees to Seattle and 116 degrees to Portland. Across the border in Canada, Lytton, B.C., broke the national temperature record three days in a row — and then burned down. Human-induced climate change is a catalyst in pushing otherwise hot weather into extreme and/or record territory.

Pacific Northwest heat wave was ‘virtually impossible’ without climate change, scientists find

“You hope there’s some adaptation, given the heat wave we had last year, but when you don’t have that much cooling overnight, it is hard to cool off,” DeFlitch said. “With only a subset of the population here having access to cooling, it can be a challenge in that regard.”

Jason Samenow contributed to this report.