A stifling heat wave enveloping the Pacific Northwest and western Canada is expected to expand this week, threatening more temperature records.

The big picture: The weather map looks "strongly reminiscent" of late June 2021, which produced a deadly excessive heat event in the Pacific Northwest, experts say.

• However, there are some key exceptions, UCLA climate scientist Daniel Swain said in a climate video briefing Monday.
• Swain adds that this time, the center of the heat dome — an area of unusually strong high pressure aloft — is located over southwestern Canada, instead of over the northwestern U.S.

Context: This heatwave is occurring during mid-May rather than late-June, which makes the background temperatures cooler and the records lower.

• That's fortunate for the affected regions, since temperatures have been about 20 to 30°F above average for this time of year, and will continue to be about 15°F to 25°F this week.

Zoom in: Still, the heat wave has already broken numerous daily and monthly high temperature records in Oregon and Washington, and this heat is forecast to spread east across the Great Basin this week, the National Weather Service (NWS) noted Tuesday in a forecast discussion.

• On Sunday, Hoquiam, Washington hit a monthly record high at 91°F, while Quillayute's 92°F tied its monthly record.
• On Monday, Seattle had a fourth day in a row with a record high, per the NWS.

Thought bubble from Axios Seattle's Melissa Santos: This isn't the hottest weather Seattle has seen in recent summers. But it is definitely hot for May, and it comes at a time when some of the resources families use to stay cool — such as spray parks and wading pools — aren't yet open for the season, making the heat more difficult to manage for many.

• Santos noted that only about half of Seattle's homes have air conditioning.

By the numbers: Some of the Canadian records have been especially noteworthy. For example, based on preliminary data, the small town of Arviat, Nunavut, which sits along the western edge of chilly Hudson Bay, recorded its all-time May high temperature record on May 13, when the temperature climbed to 21.2°C (70.2°F).

• This was about 7°C (12.6°F) above the previous May record, an unusually large margin for a monthly record.
• Numerous records have been broken in British Columbia, where the heat is likely to continue through midweek.
• A monthly record was set in Lytton on Sunday, where the temperature climbed to 35.9°C (96.6°F). Normally cool Tofino, located on a Peninsula on Vancouver Island, also set a monthly high Sunday, at 28.3°C (83°F).

Threat level: In Canada, the heat has exacerbated the already above average start to the wildfire season, with large blazes burning out of control in multiple provinces. About 90 wildfires were burning in Alberta alone as of Sunday.

• Other wildfires are burning out of control in British Columbia and Saskatchewan.
• Some Canadian oil production has been curtailed due to the fires in Alberta, and smoke has moved south of the border, prompting air quality alerts in Minnesota.

What they're saying: Michael Wehner, a climate scientist at Lawrence Berkeley National Laboratory, told Axios that it is unusual to have another significant, record-breaking heat event so soon, in virtually the same region, as the June 2021 event.

• "I realize that the two events are completely independent, but it is still a surprise," he said via email.
• He cited published research showing that human-caused climate change has made the high temperatures during this and many other heat waves at least 3°F to 5°F hotter.
• A slew of other studies show climate change is causing heat waves to be more intense, more frequent and longer lasting.
• A climate attribution study on the 2021 Pacific Northwest heat wave found that it would have been "virtually impossible" to have occurred without climate change.

What we're watching: The possibility that record warm global ocean temperatures, tied in part to a developing El Niño in the tropical Pacific Ocean, will translate to more extreme heat events on land in the coming months.

Go deeper:

Record early season heat wave envelopes Pacific Northwest

El Niño forming quickly and could be "significant" event, NOAA finds