‘Dangerously hot’ weather is brewing again in the Pacific Northwest

It’s been a summer marred by record-shattering, deadly heat in the Pacific Northwest and yet another significant heat wave is on the way. Triple digit temperatures will roast cities like Portland, Medford and Spokane, while the risk of wildfires ramps up over the region.

The forecast coincides with the release of Intergovernmental Panel on Climate Change’s latest assessment review of climate science, which notes a firm link between human-caused global warming and top-tier heat events, like the late June episode that brought a high of 108 degrees to Seattle and 116 degrees in Portland, Oregon.

This event will not be as intense as the late June heat wave, but Seattle, Tacoma, Portland, and Eugene, Ore. are all under excessive heat watches with temperatures peaking between Wednesday and Friday before relenting some over the weekend.

The area around Medford, Ore., has been upgraded to an excessive heat warning for “dangerously hot conditions” set to begin Tuesday and not ease until Saturday. By Wednesday, Medford could approach 107 or 108 degrees.

At least 25 million Americans are slated to experience highs eclipsing 100 degrees over the upcoming week, including many in California’s Central Valley, the Willamette Valley in Oregon and the Washington state interior east of the Cascades.

On Monday, the ingredients for the upcoming sizzler were beginning to come together. Surface high pressure banked well offshore over the northeast Pacific will bring northerly winds over the water paralleling the coast, keeping the breeze over land light and variable. At the same time, a “heat dome” at the mid-levels of the lower atmosphere will build in from the northwest, inducing a northwesterly continental flow from British Columbia.

The heat dome will be significant enough in magnitude to boost the mid-levels of the atmosphere vertically, since warm air expands. The atmosphere’s “halfway mark” of mass will surge the equivalent of two football fields higher. The high pressure will also deflect any inclement weather, allowing sunshine to pour across the landscape unimpeded.

Dew points in the 40s inland and 50s near the coast, representing dry air, will make it easier for the air to heat up too, in some cases to dangerous levels.

“Another round of possible record breaking high temperatures expected Thursday and Friday,” wrote the National Weather Service in Seattle in their morning briefing on Monday. “This event will not be as hot as the late June event,” they noted.

Seattle could hit 91 on Thursday and 93 on Friday, the latter set to beat out a daily record that’s stood since 1992. Offshore flow will help push temperatures in the 90s all the way to the coast.

In the Columbia River Basin to the east, readings will climb even higher. Spokane is projected to hit 97 on Wednesday, 98 Thursday and 102 degrees on Friday. That should also set a record on Friday. The current number to beat is 100 degrees, observed in 2015. Spokane’s been dealing with a case of weather whiplash as of late, reporting a high of only 68 degrees on Sunday.

“[The heat] will likely allow regional wildfires to become increasingly active with renewed haze and smoke,” warned the Weather Service in Spokane. Smoke-filled skies have dominated in the Pacific Northwest during much of the summer as barrages of wildfires torch millions of acres. Lytton, British Columbia - the town that recorded an astounding 121 degree temperature in late June - burned to the ground after establishing a new Canadian nationwide record temperature three days in a row.

In south central Oregon, firefighters have managed to get the upper hand on the Bootleg Fire, which was 96% contained on Monday morning. It’s claimed 413,000 acres.

Portland is expecting a high of 99 on Wednesday, with 104 degrees in reach Thursday and Friday. That would tie a record Thursday and break one on Friday. Portland’s average high this time of year is 83 degrees.

“Most areas within the Willamette Valley [have] around an 80 to 90 percent chance of daytime high temperatures reaching or exceeding 100 degrees,” wrote the Weather Service in Portland. A few spots could nick 110 degrees.

Much of area in the vicinity of the Dixie Fire could climb to 100 to 105 degrees Wednesday and Thursday, the heat worsening conditions for firefighters. It’s the second largest fire in California state history, standing at 489,287 acres around midday Monday, and only 21% contained.

The Weather Service office in Medford expressed concern about the fire danger between Tuesday and Thursday in northern California and southern Oregon due to “[e]xcessively hot, very unstable and dry air.” The Predictive Services of the National Interagency Fire Center highlighted the potential for dry lightning that could ignite new blazes as the week wears on.

An abrupt end to the heat is likely into next weekend as the pattern reverses and a trough, or dip in the jet stream, unleashes cool and dry air. That will bring a welcome reduction in temperatures, but the arrival of a significantly less humid air mass, along with gusty and changeable winds, could spell problems for the spread of fires.