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This story, originally published Wednesday afternoon, was updated Thursday morning.

It’s happening. Again. For the fourth time in five weeks, a punishing heat wave is set to bake the West and adjacent western Canada. This time, the most exceptional heat is expected to focus in the central and northern Rockies, developing this weekend and peaking around Monday.

The heat wave is forecast to bring triple-digit temperatures to at least 16 million people, challenging and breaking records into Canada. It’s also targeting an area where numerous wildfires have flared up and a smoky haze fills the skies. The arrival of even hotter, drier conditions could compound the situation.

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The blast of heat is set to arrive just days after <a href="https://www.washingtonpost.com/weather/2021/07/11/heat-wave-west-las-vegas/" target=_blank>Las Vegas soared to a record-tying 117 degrees</a> amid historically high temperatures from the Southwest into California’s Central Valley. Two weeks ago, a 1,000-year heat event brought <a href="https://www.washingtonpost.com/weather/2021/06/27/heat-records-pacific-northwest/" target=_blank>unprecedented temperatures to the Pacific Northwest and British Columbia</a>, producing highs of 108 degrees in Seattle, 116 in Portland and 121 in Lytton, Canada — <a href="https://www.washingtonpost.com/weather/2021/06/30/canada-record-heat-experts-react/" target=_blank>a new Canadian record</a>.

This event will not be as extreme as that heat wave in the Pacific Northwest, which scientists concluded would have been <a href="https://www.washingtonpost.com/weather/2021/07/07/pacific-northwest-heat-wave-climate/" target=_blank>“virtually impossible” without human-caused climate change</a>. But it will still be unusually hot.

What questions do you have about climate change? Ask The Post.

The instigating heat dome will take shape in the days ahead. Temperatures in the West should be near average through Friday before a ridge of high pressure parks itself over the Four Corners region. From there, the high-pressure “heat dome” will meander northwestward, building in intensity early next week.

By Monday, a key heat dome threshold could be reached — 600 dekameters — signifying a rare event, but one that has grown more common because of human-caused climate change. The threshold corresponds to the altitude that the lower atmosphere’s “halfway point” swells to vertically. The hotter the air, the more a column of atmosphere expands, and the higher that halfway level climbs. At 600 dekameters, or nearly 20,000 feet, that’s nearly two football fields higher than average.

The science of heat domes and how drought and climate change make them worse

At that point, the heat dome should be anchored over the central Rockies, dominating over most of the western United States and bringing hot and hazy conditions to parts of British Columbia, Alberta, Saskatchewan and Manitoba — some of the same areas roasted in the late June heat wave.

The heat will begin to manifest itself in abnormally high temperatures Saturday before becoming entrenched Sunday and reaching a crescendo Monday. While its intensity will gradually wane, the heat dome could persist over the northern Rockies through the workweek before dissipating next weekend. In other words, the northern Rockies and south-central and southwestern Canada can expect a long-duration excessive heat event.

Preliminary forecasts suggest highs in the northern United States and southern Canada could reach 20 to 30 degrees above average values for mid-July, which in many places is already historically the warmest time of year.

The most noteworthy departures from average will occur in northern areas such as the Columbia River Basin and northern Intermountain West. Spokane, Wash., may hit 100 degrees Sunday. Eureka, a town in extreme northwestern Montana and a stone’s throw from Canada, could hit 101 degrees Monday after days of being shrouded by wildfire smoke.

<a href="https://forecast.weather.gov/wwamap/wwatxtget.php?cwa=usa&wwa=Excessive%20Heat%20Watch" target=_blank>Excessive heat watches</a> are in effect in eastern Montana from Saturday afternoon through Wednesday, during which time temperatures could climb as high as 106 degrees with nighttime lows struggling to fall below 70. That’s a big deal there, since nighttime lows typically fall into the mid-50s in July. Average highs peak around 86 degrees. These watches may be expanded in the coming days and changed to warnings.

<a href="https://weather.gc.ca/warnings/index_e.html" target=_blank>Heat warnings</a> cover much of south central Canada in the provinces of Alberta, Saskatchewan and Manitoba.

Helena, Mont., will see three days in a row between Sunday and Tuesday near or at 100 degrees. It has never recorded more than three consecutive days at or above 100 degrees. Records there date back to 1880.

“Some forecast models have temperatures warming to between 105 and 110 in a few locations, so extreme heat remains possible,” wrote the National Weather Service office in Great Falls.

Billings, Mont., is predicted to hit 101 degrees Sunday, with 104 degrees in Boise and 101 in Salt Lake City. All three locations could rival or match those numbers Monday and Tuesday.

Last Tuesday, Salt Lake City topped 100 degrees for<a href="https://twitter.com/NWSSaltLakeCity/status/1415047800279027712" target=_blank> the 15th time this year</a>, and the upcoming heat wave is expected to bring it “within striking distance” of its record of 21 occurrences, according to the Weather Service. That same day, Salt Lake City set Utah’s <a href="https://twitter.com/NWSSaltLakeCity/status/1415323045959372801" target=_blank>highest low temperature on record</a>, dropping only to 82 degrees.

Even farther from the heat dome’s immediate influence, the threat of hazardous heat will remain. In California’s Central Valley, many cities are already hovering near 100 degrees, as if clinging to remnant heat left by the last bout of serious warmth. Temperatures will tick upward in the coming days, reaching 103 degrees in Hanford on Monday and 100 in Redding.

Death Valley, Calif., will crest near 120 degrees for most days in the extended forecast.

Death Valley had planet’s hottest 24 hours on record Sunday amid punishing heat wave

The ridge of high pressure will begin to break down mid- to late next week as a low pressure system brings unsettled weather to the central Lower 48 on Wednesday.

In addition to the hot temperatures, the sultry air mass will spur additional wildfire growth and ignition across the West, where dozens of fires are already raging. Extreme fire behavior contributed to fire tornadoes and fire-born thunderstorms.

<a href="https://inciweb.nwcg.gov/accessible-view/" target=_blank>Seven active blazes</a> have torched more than 50,000 acres or more, including the <a href="https://www.washingtonpost.com/climate-environment/2021/07/12/bootleg-fire-oregon-disaster-environment/" target=_blank>Bootleg Fire in south-central Oregon</a>, which has consumed over 212,000 acres. The Bootleg Fire exhibited extreme behavior on Wednesday, generating blaze-induced storm clouds <a href="https://twitter.com/weatherprophet/status/1415519520337260551" target=_blank>surging 40,000 feet into the atmosphere</a>.

The combination of high heat and low humidity will further desiccate the landscape, sucking moisture out of the ground and making it ripe to rapidly burn.

The dry landscape makes it easier for temperatures to skyrocket — which in turn spells more drying. As it stands, 57 percent of the West is experiencing a severe or exceptional drought, the two highest categories outlined by the federal government’s <a href="https://droughtmonitor.unl.edu/" target=_blank>U.S. Drought Monitor</a>.

“The hot and dry conditions on top of <a href="https://forecast.weather.gov/glossary.php?word=drought">drought</a> and receptive fuels will keep fire weather potential high,” warned the Weather Service in Glasgow, Mont.

Wildfire smoke is already veiling much of the Lower 48, pooling over the Plains and even reaching as far north as Hudson Bay. On Thursday morning, it was arriving along the Eastern Seaboard, knocking back temperatures a degree or two and making for vibrant sunrises behind a milky-white veil of subtle overcast.

Wildfire smoke is clogging skies in the Western U.S., Canada

For some, the smoke could even become trapped near the ground, degrading air quality and posing health concerns.

“Friday could see smoke closer to the surface and could begin to impact air quality,” wrote the Weather Service in Glasgow. “The impact of the smoke could increase into next week.”

Even northern Minnesota was at risk of dangerous levels of near-surface pollutants induced by fires. The Minnesota Pollution Control Agency wrote that “fine particle levels are expected to be in the red [air quality index] category, a level considered unhealthy for all individuals.”

Those fires were located in Ontario and Manitoba. In Calgary, Canada, the smoke was enough to cause a stinging of the eyes and wash city skylines into a monochromatic haze. Smoke across the Northern Tier of the United States is expected to last through at least Friday.

Harvard University, B.A. Atmospheric Sciences

Matthew Cappucci is a meteorologist for Capital Weather Gang. He earned a B.A. in atmospheric sciences from Harvard University in 2019, and has contributed to The Washington Post since he was 18. He is an avid storm chaser and adventurer, and covers all types of weather, climate science, and astronomy.

Matthew Cappucci

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