For the fourth time in the past month and a half, a strong heat wave is roasting parts of the western U.S., as wildfires run amok. High-temperature records could fall in parts of the northern Rockies on Monday, where the most exceptional lobe of warmth is concentrated.

There are signs that the seemingly unrelenting heat that has proved a staple of summer 2021 won’t be going anywhere anytime soon, with prolonged hot, dry conditions likely for weeks over large areas of the western Lower 48.

The heat is contributing to increased wildfire danger as dozens of blazes rage across 12 states in the West, including in south-central Oregon, where the Bootleg Fire already has charred more than 300,000 aces. Six additional wildfires have already swollen to 50,000 acres in size or larger — and roughly two months remain until the peak of wildfire season.

Red flag warnings, connoting the potential for “extreme” fire behavior, blanket most of Montana and Idaho, western Wyoming, eastern Oregon as well as southeast Washington. That’s where the combination of hot temperatures and low humidity will dry the landscape and its vegetation enough to make for a breeding ground for fires.

The out-of-control fires are pouring large quantities of smoke into the atmosphere, which have surfed the jet stream eastward. Skies were noticeably paler beneath the smoke even in the Northeast.

Exceptional heat baking Northern Tier

Monday was set to mark the crescendo of this latest heat wave, which was most pronounced in the interior Pacific Northwest, the Columbia River Basin, Idaho and especially Montana. Billings, Mont. was expected to climb to 107 degrees Monday afternoon, beating out the record of 105 degrees set in 1960. In fact, Billings has only reached 107 degrees or higher three times before dating to 1934; its all-time high of 108 degrees was attained on July 14, 2002.

Its average high this time of year is in the upper 80s, meaning temperatures across the region are running 15 to 20 degrees above average.

Most of eastern Montana is under an excessive heat warning, the National Weather Service cautioning of temperatures spiking to near 110 degrees.

“Today is very likely the hottest day of this heat wave,” according to the Weather Service in Billings. “We are looking at temperatures at near record to record values. Overnight lows will struggle to drop below 70 degrees providing only limited relief.”

They warned that “extreme heat will significantly increase the potential for heat-related illnesses.”

Elsewhere, Bismarck, N.D., will probably crest above 100 degrees Monday afternoon, too, while Boise, Idaho, and Salt Lake City should remain in the mid-90s. Boise has reached at least 95 degrees for a record 23 straight days. Salt Lake City has already hit 100 degrees 17 times this season, including Sunday, not far from its annual record of 21 occurrences.

Triple-digit heat will also spill across the Canadian border into southeastern Alberta and Saskatchewan on Monday. It comes after Lytton, British Columbia, spiked to 121 degrees after setting a Canadian national record three days in a row amid a 1,000-year heat wave that crushed all-time records in the Pacific Northwest by staggeringly high margins. Shortly thereafter, Lytton burned to the ground as wildfires raged in southwest Canada.

The southwest U.S., meanwhile, was enjoying some relief from the heat because of clouds and rain from the southwest monsoon.

Drought and fire concerns

The exceptional heat will amplify ongoing wildfire risk across the northern part of the West. The hot temperatures, made possible in large part by drought exacerbated by climate change, are responsible for extracting humidity from the ground and drying it out even more. Roughly two-thirds of the West is encapsulated within “extreme” or “exceptional” drought, the two most severe categories highlighted by the U.S. Drought Monitor.

Apropos to impacts, the Drought Monitor notes that, under these conditions, “fish rescue and relocation begins; pine beetle infestation occurs; forest mortality is high; wetlands dry up; survival of native plants and animals is low [and] wildlife death is widespread.”

Pine beetles proliferate rapidly in dry weather, and infestations can kill millions of trees — littering the ground with more fuel for the next wildfire.

Worsening matters, “dry thunderstorms,” or thunderstorms that produce little to no rainfall, are expected as far north as Montana. That’s because of monsoonal moisture streaming northward at the mid-levels of the atmosphere, proving sufficient to initiate storms but not overcome the atmosphere’s dry lower levels.

“Still expecting development of scattered dry thunderstorms in [areas west of Billings],” wrote the Weather Service.

That risk for a few dry thunderstorms wraps southwestward into northern California, where sporadic lightning strikes could ignite blazes. However, dry thunderstorms in California were not as widespread as anticipated through early Monday. Because of the rather limited coverage of the dry storms, most of the red flag warnings in California were peeled back except in far northeast California.

But as hot and dry conditions are likely to persist for the foreseeable future, fire concerns will remain high. The deck already appears stacked for a dangerous peak of wildfire season into the early fall when winds begin to increase.

Smoke and pollution concerns

The scores of wildfires littered across the map in the West have resulted in plumes of smoke billowing at times 40,000 feet high. In south-central Oregon, where the Bootleg Fire was continuing to burn, an air quality alert was in effect because of a “high concentration of smoke particulates.”

“Smoke can irritate the eyes, lungs and worsen some medical conditions,” warned the Weather Service. “Stay inside if possible. Keep windows and doors closed.”

In Calgary, Alberta, residents reported stinging of the eyes last week, while “obscene smoke” was reported in Manitoba over the weekend.

Wildfire smoke tends to reduce air quality primarily close to its source where it hovers close to the ground. At greater distances, where it is lofted higher into the atmosphere, it has little or no effect on air quality.

Entrained by strong jet stream winds, the smoke was drifting all the way to the East Coast on Monday, resulting in a milky-white tinge to the sky and producing vibrant fiery sunrises and sunsets.

“Smoke can be seen across the majority of North America,” wrote the National Oceanic and Atmospheric Administration on Sunday night.

Jason Samenow contributed to this report.