ATLANTA (CNN) — With upwards of 300 record-high temperatures in jeopardy this week, more than an eighth of the U.S. population — over 40 million people — are on alert across the western U.S. for a long-lasting, potentially lethal heat wave.

"No easy way to say this, so we'll just cut straight to the chase: it's going to be *very* hot for a long time," tweeted the National Weather Service in Salt Lake City in the lead-up to this historic heat wave.

This heat wave and the exceptional drought in the Southwest are part of a damaging feedback loop enhanced by climate change, experts say. The hotter it gets, the drier it gets; the drier it gets, the hotter it gets.

"When it comes to extreme weather, climate change is loading the weather dice against us," Katharine Hayhoe, a climate researcher and the chief scientist for the Nature Conservancy, told CNN Weather in an email. "We always have a chance of extreme heat, particularly in the summer: but as the world warms, we see that summer heat waves are coming earlier, lasting longer, and are becoming hotter and more intense."

The Southwest is caught under a heat dome

On Tuesday, Salt Lake City recorded its third consecutive day of triple-digit heat, setting both daily and all-time records along the way. The city soared to a high of 107 degrees on Tuesday afternoon, tying its all-time record high, previously reached in the month of July.

For some perspective, records in Salt Lake City date back to 1874. In that time, there have been over 50,000 calendar days of temperatures observed. Tuesday marks only the third time the city has ever soared to 107 degrees, roughly a 1 in 50-year event.

The cause is a massive ridge of high pressure, commonly referred to as a heat dome, that is rapidly gaining strength over the western U.S. A combination of sinking air, clear skies and lengthy solar radiation will send temperatures as much as 10 to 25 degrees above seasonal values this week.

This ridge is also responsible for the unrelenting drought, as it directs rain away from the region.

"The hotter it gets, the stronger the ridge," said Hayhoe. "So while climate change may not be responsible for the ridge forming, it can make it last longer and be stronger than it would be otherwise, which makes the drought more intense and longer."

Amid a historic drought and the lowest water level on record in nearby Lake Mead, the state of Nevada will also be challenging its all-time record high this week, currently held by the town of Laughlin, which reached 125 degrees on June 29, 1994. Highs in Laughlin are forecast to be between 120 and 122 from Wednesday to Sunday; the average this time of year is 106 degrees.

Widespread triple-digit records are being observed as far north as Idaho and Montana.

On Tuesday, Billings, Montana, soared to 105 degrees, matching the hottest weather ever seen in June while obliterating the daily record of 98 degrees, which stood for over 30 years.

As the week progresses, so does the long-duration heat wave.

More records in jeopardy

On Wednesday afternoon, the city of Las Vegas will be knocking on the doorsteps of history as highs are forecast to reach 116 degrees, just 1 degree shy of the city's all-time record of 117 degrees. That's benchmark that has only been achieved four times since records began in 1937.

Check the forecast highs for these cities and yours

Not too far away in Phoenix, where residents are well accustomed to oppressive heat, the mercury is forecast to impress even by Phoenician standards. Highs this time of year typically settle in around 105 degrees. The average first 115-degree day generally arrives during the first week of July. However, with summer officially four days away, high temperatures in Phoenix soared to a record of 115 degrees on Tuesday.

In fact, forecast models indicate that they may reach or exceed 115 degrees every day from Wednesday through Friday. This would tie the all-time record for most consecutive 115 degree days in Phoenix at four days. Perhaps not surprisingly, in the summer of 2020, the city reached 115 degrees on four successive days on two separate occasions.

In the U.S., five of the 10 hottest years on record have all occurred since 2012, and the broader trend signals little change.

Related:

Salt Lake City ties all-time record; what the heat wave means for Utah's climate, drought and fire risk

At a sweltering 107 degrees, Salt Lake City on Tuesday tied the all-time hottest temperature ever recorded in city history as part of a mid-June heat wave that's already shattered scores of records since Sunday.

There have always been heat waves but they are getting worse

There have always been heat waves, droughts, wildfires and more, well before humans started changing climate. What scientists are increasingly beginning to say, explains Hayhoe, is how much worse climate change is making these events.

"Scientists are starting to be able to say 'a lot!' and even to answer this question with numbers for specific events," she said.

"For example, scientists have found that climate change made the 2019 European heatwave 10 times more likely, and the Siberian heatwave of June 2020 600 times more likely."

In the future, heat waves and drought will likely worsen, Hayhoe said. Particularly in areas already naturally at risk from drought.

Ironically, climate change will also make heavy rain events more frequent. "Which isn't good news either," explains Hayhoe. "It can be damaging, and it makes it hard to replenish soil water and groundwater depleted during a drought when rain falls in heavy downpours as most of it just runs off."

The future looks different than the records of the past.

"We can no longer rely on the past as a reliable predictor for future conditions, as we've been doing for hundreds and even thousands of years," Hayhoe said. "Instead, we must prepare for conditions that are hotter and droughts that are more damaging than we've seen before."

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