"Historic" Northwest heat wave shatters records, poses health risks

A stifling "heat dome" is parked over the Pacific Northwest, bringing unprecedented heat to at least 25 million in the United States, and more in Canada. Temperatures are forecast to hit an apex Sunday and Monday in many areas.

Why it matters: Extreme heat threatens lives, ranking as the nation's top weather-related killer annually. In addition, extreme heat events such as this one are a clear manifestation of human-caused climate change, with numerous studies linking such events to the long-term increase in global average temperatures.

• Making the health threat more acute, this heat wave is a prolonged event with little overnight relief, as temperatures Sunday morning were near 80°F in Seattle and Portland.
• The heat is also affecting areas where less than half of residents have air conditioning, including Seattle.

Driving the news: A highly unusual weather pattern is in place over the Pacific Northwest, with a record-strong high-pressure area aloft — known as a "heat dome" — sitting over Washington State and British Columbia, weakening little through Monday and sliding only slowly eastward.

• This heat dome is yielding temperature departures from average of between 25°F to 50°F and above across multiple states and British Columbia.
• This heat, combined with a worsening drought, is raising the risk of wildfires across multiple Western states.
• It is also causing power demand to spike at a time when hydropower resources are lower than usual due to the drought conditions.

By the numbers: All of Oregon and Washington, plus portions of California, Idaho, Montana and Nevada, are under excessive heat watches and warnings.

• Numerous monthly and all-time temperature records were set and tied on Saturday across the Northwest, and these won't last long, since they will be broken Sunday and Monday.
• Portland, Ore., broke Saturday's record of 108°F with a new all-time high temperature of 113°F. 
• In Seattle, the temperature reached 104°F, which broke the existing benchmark set on Saturday. Seattle had never had back-to-back 100-degree days before this weekend.
• Monday looks even hotter in Seattle, with an astonishing high of 111°F.
• Canada is also seeing extreme heat, with the country's June high temperature record tied on Saturday and smashed on Sunday.
• Mountain areas were also extremely warm on Saturday, with all of Mount Rainier climbing above freezing. This means the snowpack, as well as area glaciers, are being affected. Warmer conditions are likely Sunday.

How it works: There are two main reasons the Pacific Northwest is so hot. The first is tied to the heat dome itself, which causes air to sink, or compress, warming as it does so. The second has to do with the location of the heat dome.

• The feature is parked to the north-northeast of the region, at the same time as an upper level low pressure area lurks offshore.

• Due to the clockwise flow of air around the high, easterly winds are blowing from high-to-low elevation areas in Idaho, Oregon and Washington.
• When air sinks, such as when it moves out of mountainous regions and into valleys, it compresses, increasing temperatures. These winds are why even coastal areas are so hot.

Context: Studies have shown that severe heat events such as this one are now on average about 3°F to 5°F hotter than they would without the many decades of emissions of greenhouse gases from fossil fuel burning, deforestation and other human activities.

• However, this may understate climate change's influence, since warming is also thought to be altering weather patterns in a way that makes strong heat domes more common and prolonged.
• Some recent studies using techniques known as extreme event attribution have shown that certain extreme heat events could not have occurred without the added boost from human-caused warming.

What they're saying: The NWS' Weather Prediction Center termed the heat "historic and dangerous" in an online forecast discussion on Sunday.

• The National Weather Service forecast office in Seattle warned people to seek cooling shelters in order to beat the heat. "Please continue to take this forecast very seriously and continue to follow all calls to action," forecasters wrote.

What's next: The heat will be most prolonged in inland areas of Washington and Oregon. There, temperatures are forecast to soar to between 100°F and 115°F through midweek. Coastal areas from Oregon to Washington will experience some cooling starting Tuesday.