A diabolical mix of heat-producing ingredients came together from Saturday to Monday, June 26-28, to brew up the most spectacular – and dangerous – heat wave ever observed in the Pacific Northwest and southwest Canada. Millions of residents from Portland to Vancouver eastward suffered in widespread heat exceeding 100 degrees Fahrenheit (38 degrees Celsius), with temperatures well above 110°F in some places.

All-time records were toppled in dozens of towns and cities, in some cases by 4°F or more. That’s well above the roughly 2°F of warming observed regionally and globally over the past century. Such an outcome is in line with recent studies indicating that high-end heat extremes may increase more rapidly than overall temperature in some areas as human-produced global warming continues.

There are too many jaw-dropping records from this still-unfolding heat wave for a comprehensive listing right now. To cite just a few of the standouts:

•  Portland, Oregon, broke its longstanding all-time record high (107°F from 1965 and 1981) on three days in a row – a stunning feat for any all-time record –  with highs of 108°F on Saturday, June 26; 112°F on Sunday; and a preliminary 115°F on Monday. That 115°F is equal to the average daily high on June 28 at Death Valley, California. [Update: Just after 6 pm PDT Monday, Portland warmed further to a high of 116°F.]

• Oregon’s state capital of Salem hit a mind-boggling preliminary high of 117°F. If confirmed, that will tie the all-time state record high set on July 27, 1939, at Umatilla, and on August 4, 1998, at Pelton Dam. Likewise, the preliminary high of 118°F at Columbia Gorge Regional Airport ties the Washington state record set on August 5, 1961, at Burbank (Ice Harbor Dam).

•  Seattle, Washington, broke its official all-time high (103°F at Sea-Tac airport in 2009) with 104°F on Sunday, then topped that reading with a preliminary 107°F on Monday. [Update: Just after 6 pm PDT Monday, Seattle warmed further to a high of 108°F.]

•  Quillayute, Washington, broke its official all-time high by a truly astonishing 11°F, after hitting 110°F on Monday (old record: 99°F on August 9, 1981). Quillayute is located near the lush Hoh Rain Forest on the Olympic Peninsula, just three miles from the Pacific Ocean.

•  Canada broke its all-time national temperature record with a preliminary high of 46.6°C (116°F) in Lytton on Sunday, June 27, then beat the record again with a preliminary 47.9°C (118.2°F) on Monday. The old record was 45.0°C (113°F) from July 5, 1937. The high at Lytton is also the world’s highest temperature ever recorded north of latitude 50°N, according to international weather records expert Maximiliano Herrera.

Large numbers of people who lack air conditioning (AC) in this region make the heat far more dangerous than it would be in places like the southern U.S., where AC is near-ubiquitous. The U.S. Census Bureau’s American Housing Survey showed that about 49% of homes in Portland had central AC, with another 29% having at least one room AC. In Seattle, 44% of homes reported central AC and 22% had at least one room unit.  

Computer forecast models heralded the unprecedented heat close to a week in advance. The forecasts were so extreme that they were easy to dismiss at first, until multiple runs of multiple models converged on a take very close to what transpired.

Based on this guidance, NWS forecasters – who generally try to avoid extreme model solutions until an event draws closer, to avoid “windshield wiper” lurching in public forecasts – ended up predicting all-time record highs in Seattle and Portland several days in advance, a gutsy call highly unusual in itself.

The impressive model skill with this heat wave reminds us that we rely on model forecast guidance for a good reason. Collectively, the models give us a strong sense of what’s important in the weather to come, and we use that guidance to plan our days ahead. Likewise, climate models – which incorporate the physics, dynamics, and thermodynamics of the atmosphere, just as weather models do – tend to accurately capture the global and regional trends they’re designed to project when they simulate recent periods such as the 20th century.

How did this happen?

It’ll take post-event analyses to fully understand exactly how this historic heat wave developed. Several factors are clearly in the mix, though.

An off-the-charts “heat dome”. A series of weather events in the North Pacific, including a moist pocket of air and an unusually strong jet stream, helped set off a sequence of downstream kinks in the jet stream. The result was a dome of high pressure at upper levels of the atmosphere that was the strongest ever observed near its center in northern Washington and southern British Columbia.

As moisture-bearing winds were forced around the upper high, and as air gently sank within the heat dome itself, it left the core of the high virtually cloud-free. This, in turn, allowed for maximum heat-generating sunshine just days after the summer solstice.

Satellite image of Pacific Northwest region
Figure 1. Cloud-free skies prevailed over virtually all of the northwest U.S. and southwest Canada at 1730Z (10:30 a.m. PDT) Monday, June 28, 2021, as shown in this GOES-17 GeoColor satellite image. Much cooler marine air was hugging the cloudy areas along the coast from northern California to southwest Washington. (Image credit: RAMMB/CIRA/CSU)

Parched terrain. The ground was not exceptionally dry near the coast as the heat wave began. However, weeks of extreme drought over eastern Washington and Oregon have led to extremely low soil moisture. Drier landscapes allow summer sun to heat the near-surface air more efficiently, as little solar energy goes into evaporating moisture. The torrid air east of the Cascades likely helped the heat dome to intensify further.

Descending air and downslope winds. The record-strength upper high was positioned so far north that winds throughout the atmosphere were blowing from east to west across the coastal strip from Oregon to British Columbia. The east-to-west flow led to compressional warming of the upper-level air as it descended from the Cascades toward places like Eugene, Portland, Seattle, and Vancouver. At 5 a.m. PDT Monday, June 28, the temperature measured by weather balloon at Quillayute, Washington, just a few hundred feet above the surface was near 101°F.

The effect is similar to the “snow-eating” Chinook winds of winter that warm as they descend the eastern slopes of the Rockies. However, it’s bizarre to see it playing out in full flower during midsummer across the Pacific Northwest. Jet-stream contortions like this may be accentuating extreme weather patterns in summer as climate change unfolds.

All of these factors, and others, will be considered in attribution research on this historic heat wave. In a nutshell, attribution studies can simulate recent events in climate models and estimate how much climate change contributed to them. More than 100 such papers have been published in special annual issues of the Bulletin of the American Meteorological Society over the past decade. A project called World Weather Attribution is also organizing and publicizing such studies.

Regional heat waves in both the atmosphere and ocean are among the most straightforward to link to climate change. Several such events have been found to be essentially impossible without climate change in the mix. We will be surprised if attribution studies to come don’t find that climate change was a major if not essential player in the current heat wave.

Health impacts of the heat wave

The epic nature of this heat wave could exert a profound and long-lasting effect on the psyches of residents across the Pacific Northwest, where summer temperatures have long been an annoyance rather than an existential threat. “Existential” isn’t too strong a word here, because heat waves this intense – especially when they occur early in the warm season, and affect large populations without air conditioning – can lead to widespread fatalities and long-lasting health effects. Air pollution is an often-underrecognized part of this picture.

Daily air quality index for ozone, June 27, 2021
Figure 2. Peak ozone pollution levels for Sunday, June 27, 2021, as measured using the Air Quality Index (AQI). An AQI in the orange range is “Unhealthy for Sensitive Groups”, meaning members of sensitive groups like the elderly and those with lung disease may experience health effects, but the general public is not likely to be affected. An AQI with red colors is “Unhealthy”, meaning everyone may begin to experience health effects, and members of sensitive groups may experience more serious health effects. (Image credit: U.S. EPA)

The heat wave brought the worst ozone air pollution levels of the year to much of the Pacific Northwest and British Columbia on Sunday and Monday. Ground-level ozone, which is created from chemical reactions between volatile organic carbon (VOC) compounds and nitrogen oxides, is created more readily at warmer temperatures and when ultraviolet (UV) light is strong. UV light levels are at their maximum at the June 21 summer solstice, and thus have been very high during the current heat wave. The high heat and UV light levels helped the chemical reactions that make ozone occur faster, resulting in ozone pollution that topped out in the “Unhealthy for Sensitive Groups” range at several stations near Vancouver, British Columbia.

Fortunately, the high heat was accompanied by relatively strong winds of 10-20 mph across much of the region, which helped flush out the pollution from most of the cities. However, one station downwind from Seattle, Washington (Enumclaw) recorded 8-hour ozone levels in the red “Unhealthy” range on Sunday. This was the worst ozone pollution observed in the state of Washington since 2018, according to data from the EPA. The levels measured were about 50% higher than the federal standard for hazardous levels of ozone, and were capable of causing increased risk of stroke, heart attack, and breathing problems for sensitive groups.

As of 4 p.m. PDT Monday, one monitoring station in the Vancouver, Canada region and one in Washington state had measured ozone pollution in the red “Unhealthy” range; another station northeast of Vancouver measured ozone in the purple “Very Unhealthy” range.

Even people not unusually sensitive to air pollution could see adverse health effects from pollution levels in the “red” zone. According to the Health Effects Institute (a U.S. non-profit corporation funded by the EPA and the auto industry), there were approximately 12,800 U.S. deaths in 2019 that could be attributed to ozone pollution. 

The other main pollutant of concern, tiny particles known as PM2.5 (those that are less than 2.5 microns or 0.0001 inch in diameter) – which the Health Effects Institute estimated caused over 47,000 premature U.S. deaths in 2019 – was not as big of a concern on Sunday, since PM2.5 levels stayed below the federal standard at all but one station in the region.

Some of the health impacts of the heat wave may not be immediately obvious. It will take weeks to months for epidemiologists to obtain and sift through the data to determine the number of “excess deaths” –  the grim clinical term for the number of people who died during and after an extreme weather event compared to the total one would otherwise expect at a given time of year.

Is the heat wave ending? Not so fast

The crescendo of heat that slammed western Oregon, Washington, and British Columbia on Monday is expected to segue into less dangerous warmth by midweek. Readings will still remain well above average, though, and that may keep the interiors of non-air conditioned buildings from cooling off as much as they otherwise might. Seattle remained in an excessive heat warning through Tuesday night, and highs are predicted to reach the mid-80s from Wednesday, June 30, through at least Sunday, July 4. The average high for this time of year is around 74°F.

Further to the east, the heat “party” is just getting started. Close to the Idaho border, Spokane, Washington, is expecting an all-time high of 110°F on Tuesday, June 29, followed by 109°F on Wednesday (which itself would have broken the previous all-time high of 108°F set in 1928 and 1961, in records going back to 1881). By Saturday, July 3, Spokane will likely have seen its first-ever seven-day stretch of 100°F days.

In an online discussion on Monday morning, an NWS forecaster in Spokane captured the shroud of concern hanging over the region: “This heat we are about to experience will likely be one of the most extreme and prolonged heat waves in recorded history. This will threaten the health of all residents who are outdoors and especially those who are most vulnerable to heat.”

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Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...