This article, first published on Friday, was updated Saturday morning.

As relentless and punishing heat roasts the West, Boise, Idaho, has reached at least 95 degrees for a record 20 straight days. Now temperatures are about to be turned up even more, as a major heat dome builds from the central Rockies into southern Canada.

This new heat wave, set to peak between Sunday and Tuesday, will bring above-average temperatures, from interior Oregon and Washington state and across northern Utah, Idaho, Wyoming, Montana and into the Dakotas. South-central Canada will also be smothered by the heat. Montana will be at the epicenter, with parts of the state 20 to 25 degrees or more above historical norms.

It’s the fourth intense heat event to strike western parts of the Lower 48 in just the past five weeks. Last week, Las Vegas soared to 117 degrees, reaching its warmest temperature ever recorded.

This heat wave will not be quite as intense as the previous three, but it threatens to exacerbate the region’s historic drought and wildfire situation. According to the National Interagency Fire Center, 70 large fires are burning across a dozen states. On Thursday, the center moved Oregon and Washington state to “Preparedness Level 5,” the highest, based on the amount of wildfire activity and need for firefighting resources.

Meanwhile, “a huge mass of smoke” attributed to the fire activity covers large parts of the United States and Canada, according to the National Oceanic and Atmospheric Administration, contributing to degraded air quality in some areas.

A major concern on Sunday and Monday is the prospect of dry thunderstorms, from the Sierra Nevada mountain range northward through much of northern Nevada, eastern Idaho and central Montana. These storms could unleash cloud-to-ground lightning that ignites new blazes.

The setup

The incoming heat wave was merely in its formative stages on Saturday, but by late this weekend it will be bringing highs topping 100 degrees to more than 15 million people. The heat dome will take shape over the Four Corners region before intensifying and moving northwest.

The intensity of heat domes can be judged by the extent to which they expand the atmosphere vertically due to heating. In extreme northern Montana and southern Alberta and Saskatchewan, that key level will peak to the equivalent of four football fields higher in altitude than average.

The heat dome will stay in place for much of the workweek, meaning a prolonged period of abnormally high temperatures in the northern Rockies and southern Canada. Its intensity will decrease some as the week wears on, and it is forecast to migrate south toward the southern Plains by the weekend.

The impacts

The heat dome will generate dangerously hot surface temperatures in places that aren’t necessarily accustomed to such heat. Excessive heat warnings and heat advisories are up across central and eastern Montana and in parts of southern Idaho, as well.

“Extreme heat will significantly increase the potential for heat related illnesses, particularly for those working or participating in outdoor activities,” warned the National Weather Service office in Glasgow, Mont. The city is expected to get hit by five consecutive days at or above the century mark through Wednesday, with a high of 106 degrees on Monday. That will still fall a bit shy of the record 107-degree reading on that date in 1941.

Little relief is in sight until the end of next week; the Weather Service in Billings wrote that “[r]ecord highs are expected or possible at least Sun. through Tuesday.”

Billings is slated to hit 103 degrees on Sunday and 106 on Monday. During the peak of the heat wave into early next week, it may not drop below 70 degrees at night. That doesn’t sound like a big deal, but in that part of the world, it is — overnight lows, even in July, usually dip into the upper 50s at night, meaning many residents don’t have air conditioning.

On Monday, Billings could be as hot or hotter than Phoenix.

The hot weather will also impact areas such as Great Falls, which could see triple-digit highs on Monday. “[M]odel guidance continues to indicate a prolonged period of hot afternoon temperatures over several days, combined with limited relief during the overnight hours,” wrote the Weather Service office in Great Falls.

To the west and south, Boise and Salt Lake City will also peak in the lower 100s on Sunday and could stay near 100 degrees for most of next week. Salt Lake City has already hit 100 degrees 15 times this year, and this heat wave is expected to bring it “within striking distance” of its record of 21 occurrences, according to the Weather Service.

“No matter how you look at it, it’s going to be hot and it’s important to start messaging this now,” wrote the Weather Service in Pocatello.

Heat warnings also cover much of south-central Canada from southern Alberta to southwest Ontario.

Coast-to-coast wildfire smoke

Meanwhile, copious amounts of wildfire smoke are clogging the skies over the West, streaming north and gathering over the Hudson Bay and even wafting east and bringing a thin veil of overcast to the Mid-Atlantic.

Rampant wildfire activity, some extreme, continues in the West, including in south-central Oregon, where the Bootleg Fire has already torched nearly 275,000 acres. It exploded in size last weekend. Seven of the blazes in the West have burned at least 50,000 acres.

Fires are made larger and more intense by the excessive heat, which helps dry vegetation on the ground and makes it easier for wildfires to spread. Meanwhile, 64 percent of the West is experiencing an extreme or exceptional drought — the two most severe categories. This dry weather fosters hotter temperatures, which reinforces the seemingly unbreakable cycle. Human-caused climate change intensifies both hot weather and drought, which in turn increases the risks of wildfires.

Western states endured a fourth day of scorching heat on July 12 as temperatures again threatened to shatter records. (Reuters)

Air quality alerts are in effect in south-central Oregon, near the Bootleg Fire, where smoke could “irritate the eyes, lungs and worsen some medical conditions,” according to the Weather Service. Similar alerts, driven by high concentrations of fine particulate matter lofted by the fires, blanketed much of the Columbia River Basin in northern Idaho and western Montana. Even northern Minnesota was under an air quality alert as wildfire smoke drifted down from Canada.

Residents in Alberta reported smoky skies and enough ground-level pollutants to cause one’s eyes to sting. Air quality advisories cover much of south-central and southwest Canada.