An exceptional outbreak of extreme heat is set to engulf the West for much of the upcoming week, toppling records and bringing conditions that are extremely dangerous for some. Highs in spots could flirt with 120 degrees as sweltering temperatures exacerbate existing drought concerns and bolster the odds of a dangerous and deadly fire season.

Nearly 40 million Americans could see highs in the triple digits, with hundred-degree heat extending all the way north to the Canadian border.

The heat could last for much of the week, its effects especially acute in the Southwest, before a pattern change this weekend.

The record-setting temperatures predicted are made more likely in a world beset by human-induced climate change.

“A prolonged and record-breaking heat wave is underway across the western U.S.,” the National Weather Service declared Sunday.

On Sunday, Phoenix made it to 113 degrees, following the city’s first 110-degree reading of the year on Saturday. Salt Lake City hit 102, beating out a century-old record of 100 degrees set in 1918. Las Vegas reached 108.

That’s just the start of a string of days with records likely to topple across the West, and the Weather Service is urging hot-weather preparedness. Heat is responsible for more deaths than any other weather hazard in the United States in an average year.

Multiple Weather Service offices in the West have issued excessive-heat warnings for dangerously hot conditions, affecting more than 50 million people.

Triple-digit heat as far north as Montana on Monday

To start off the week, some of the most exceptional temperatures are in store for the northern portions of the Intermountain West.

On Monday, Billings, Mont., is forecast to hit 102 degrees, crushing the previous record for this day by four degrees. Billings is usually in the mid-70s at this time of year.

The city has seen temperatures so warm this early in the year only once, on June 5, 1988. Excessive-heat warnings blanket most of eastern Montana. And highs will only get higher on Tuesday, when Billings is slated to hit 107 degrees — one degree shy of its record for any day of the year: 108, observed on July 14, 2002.

“Get ready for extreme HEAT today and Tuesday,” wrote the Weather Service in Billings. “Record highs will certainly be broken, as could some June records tomorrow. Billings and Sheridan could even come close to setting all-time records.”

Sheridan, in north-central Wyoming east of the Bighorn Mountains, could hit 107 degrees, too, tying its record.

“Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors,” the Weather Service in Glasgow urged in its excessive-heat warning.

On Tuesday, Casper, Wyo., could hit 102 degrees, while Salt Lake City eyes 103. Those would be records in both locations, as well as the hottest either city has gotten so early in the season. Denver, meanwhile, will peak around 100.

The heat in the Intermountain West will get shunted south by midweek.

Exceptional heat in the Southwest on Tuesday through Saturday

By Tuesday, the heat will expand as far west as downtown Los Angeles, where 100 degrees is in reach. But the most extreme heat will target the desert areas to the east, where it will also persist for days.

All of Southern California away from the immediate coastline, as well as southern Nevada, most of Utah and southern and western Arizona, is blanketed beneath excessive-heat warnings.

After a high near 115 on Monday in Phoenix, the forecast is for 116 to 118 degrees Tuesday through Friday, which would break daily records. Palm Springs, Calif., is predicted to be similarly hot during this span.

By midweek, it’s possible that overnight lows in parts of Arizona, including Phoenix, do not drop below 90 degrees — a grave danger for those without adequate cooling options. Overnight temperatures are often more dangerous than daytime highs, because cooling shelters are usually closed and the body becomes deprived of its nocturnal cool-down period. A number of hydration stations and cooling shelters have been opened across the greater Phoenix metro area.

The heat also presents a meteorological danger to residents who work outdoors or cannot easily escape it.

On Wednesday and Thursday, Death Valley, Calif., home to some of the world’s most extreme heat, is forecast to reach 127 degrees. That’s just two degrees shy of the highest temperature ever recorded in the United States during June, set there on June 30, 2013.

The Weather Service in Las Vegas tweeted that the city has a chance to challenge its highest temperature ever recorded of 117 degrees this week. Its forecast high is at least 114 degrees Tuesday through Saturday.

In a lengthy discussion, the Weather Service in Las Vegas stressed that the exceptionally high temperatures over such a long duration present “substantial risk for heat related illnesses and impacts.” It warned of “rapid onset of dehydration, hyperthermia, heat cramps, heat stroke & heat exhaustion,” as well as “possible power outages and/or failing air conditioning units.”

“The last time we experienced heat of this magnitude and duration was late June & early July 2013,” it wrote. “During that event, southern Nevada saw nearly 30 fatalities and over 350 heat related injuries as well as temporary power outages.”

The Las Vegas office wrote that it was describing the heat dangers in such detail “to bust the myth that this is typical for the Desert Southwest.”

Escalating fire danger

The alerts for excessive heat overlap with red-flag warnings for extreme fire danger in many areas, including parts of California, southern Nevada, much of central and western Utah, and central Montana.

The elevated fire danger comes amid the near-record heat and relative humidity values as low as 4 percent. The Telegraph Fire, burning about 60 miles east of Phoenix, has already torched nearly 90,000 acres and is at 74 percent containment. State Route 77 near Globe, Ariz., remains closed; smoke was visible upward of four miles high, a thin veil of it drifting over downtown Phoenix.

Behind the heat and its connection to the Western drought

The extreme temperatures are thanks to that dome of high pressure, which has brought clear skies, sinking air and an absence of weather systems to the region. It has also diverted the jet stream north, deflecting storms and disturbances and allowing sunshine to pour down from above and bake the bone-dry environment.

In heating the atmosphere, it reinforces the ongoing megadrought that has swallowed most of the West, more efficiently sapping moisture from the soil and drying out vegetation. In addition to depleting water resources, which has major, cascading societal and economic effects, the exacerbated drying also means more dead or parched vegetation, producing an environment ripe for wildfires as the winds kick up in late summer and early fall.

More than 50 percent of the West is currently experiencing an “extreme” or “exceptional” drought, according to the U.S. Drought Monitor, the two most severe tiers.

Furthermore, dry air heats up faster, meaning the drought is working with the effects of human-induced climate change to spur even hotter temperatures, which, in turn cements the drought. It’s a serious and problematic cycle that is unlikely to relent in the near future.

The heat won’t quite relent, but it will decrease in severity by the weekend as the instigating heat dome shifts south and east and weakens.

correction

A previous version of this article incorrectly stated the year during which Billings, Mont., observed its hottest temperature on record. This version has been corrected.