Although the worst of the heat has departed Seattle and Portland, temperatures have yet to peak in areas farther east in the interior of Oregon and Washington, as well as extensive parts of western Canada. The prolonged, exceptionally high temperatures pose a grave health risk for older adults and vulnerable populations without easy access to air conditioning.
The ongoing event affecting the northwestern United States and adjacent Canada is firmly within uncharted territory because of a combination of weather effects and climate-driven warming.
At its core, this event is being driven by an exceptionally strong heat dome. Heat domes, or sprawling ridges of high pressure, are a staple of summertime. They bring copious sunshine and sinking air that heats up as it is compressed.
This heat dome may have been pumped up by a tropical storm in the Pacific interacting with the jet stream last week, Oregon’s state climatologist, Larry O’Neill, told the Capital Weather Gang.
The timing of the heat dome has helped maximize its impact. Because it is occurring near the summer solstice, the added daylight is giving the heat dome extra time to increase temperatures, said Rebecca Muessle, a meteorologist at the National Weather Service office in Portland.
Wind directions have played a crucial role in the coverage and magnitude of the excessive heat. The high-pressure system has been centered near the international border, with clockwise flow around it bringing easterly winds for much of the Columbia River Basin and, broadly, Idaho, Washington and Oregon.
Between Saturday and Monday, the easterly winds helped push the coastal marine layer, or zone of ocean-chilled air, back over the water, permitting the heat to extend into the Interstate 5 corridor from Seattle south to Medford, Ore., and even to the coast in some areas.
This wind direction has also promoted “downsloping,” or the downhill movement of air, in this case descending from the Cascades through the I-5 corridor. When pockets of air move downhill, they are compressed by the increasing air pressure near the ground and subsequently warm up.
Between Saturday and Monday, the downslope flow was further enhanced by a zone of low pressure off the coast of Northern California. Muessle called this low a “big player” in the intensity of the heat west of the mountains by “bringing in the warmer air that you typically see east of the Cascades.”
But as the low-pressure area weakened and shifted to the north late Monday, it allowed onshore flow of winds off the ocean to return to coastal areas, which brought significant cooling.
Portland had its biggest overnight drop in temperature on record between Monday evening and Tuesday morning, as the mercury plummeted 52 degrees, from 116 to 64.
“It was very, very nice, a much-needed break,” Muessle said.
But this onshore wind won’t offer relief for interior locations that are predicted to bake under the heat dome for the next several days.
A heat wave of this magnitude required weather systems and winds to align, but it could not have been this extreme without human-caused climate change.
The role of climate change has been to substantially increase the likelihood of record-breaking temperatures.
Simple logic dictates a climate experiencing a background warming of several degrees will be more prone to hotter heat events. It’s like a slam dunk in basketball — if the floor rises, it becomes easier to score.
“Summers in the Pacific Northwest are about three degrees warmer today than 50 or 100 years ago,” said Zeke Hausfather, a climate scientist at the Breakthrough Institute. “All things equal, we would think a heat wave today would be about three degrees warmer.”
Such warming, Hausfather said, means exceptionally strong heat waves, such as this, become more frequent.
“Heat waves that used to occur as 1-in-1,000-year events are becoming 1-in-100-year events and 1-in-100-year events are becoming 1-in-20,” he said.
In addition, drought, which has connections to climate change, is playing a role as both a cause and effect of the ongoing heat. Fifty-five percent of the West is experiencing an extreme or exceptional drought — the two most severe categories — including about a quarter of Washington and nearly a third of Oregon. Dry air heats up considerably faster than humid air. That means the same input of heat can foster a higher temperature.
Research shows climate change has worsened the “megadrought” over much of the West, because warming temperatures dry out the land surface more quickly.
Looking ahead, observed temperatures associated with the heat wave are actually helping to dry out the area even further, while the impetus for the heat — high pressure — diverts precipitation and storm systems to the north. That will make high temperatures even tougher to shake.
Finally, it’s possible that climate warming has changed the jet stream to increase the strength of hot weather patterns such as what we’ve seen in the Pacific Northwest this week.
Michael Mann, a climate scientist at Pennsylvania State University, with colleagues, published a study in 2018 that connects summer weather extremes with a fundamental change in how the jet stream is behaving during the summer.
Hausfather said “robust debate” continues in the scientific community about the role of climate change on jet stream patterns.