One of the most extreme heat waves ever observed in the western United States this early in the season is near its climax. The punishing blast of heat, which began Sunday, has set hundreds of records while simultaneously worsening a historically severe drought, intensifying fires and degrading air quality.

About 40 million Americans have endured triple-digit heat and more than 50 million have been under excessive-heat warnings this week.

After focusing in the northern and central Rockies earlier in the week, the core of the heat has shifted into the Desert Southwest and California’s Central Valley, where scores of additional records are predicted to fall through Saturday.

California’s Independent System Operator, which manages the state’s flow of electricity, is encouraging residents to reduce their energy use Thursday evening.

While it’s just mid-June and the hottest time of the year is historically still weeks away, temperatures have matched their highest ever observed levels in parts of Utah, Wyoming and Montana. Salt Lake City; Sheridan and Laramie, Wyo.; and Billings, Mont.; all made history Tuesday, soaring to 107, 107, 94 and 108 degrees, respectively.

California's extreme drought has forced farm manager Salvador Parra to fallow 2,000 of his acres and dig deep for water to save the crops already planted. (Reuters)

On Wednesday, the mercury in Las Vegas swelled to 116, just one degree shy of its highest temperature ever recorded. Death Valley, Calif., famous for holding the world record for heat, hit 125 degrees, the highest temperature reached this year in the United States. Denver hit the century mark for a second-straight day Wednesday, the earliest in the season it has reached 100 twice in a row.

“What we are seeing in the Western U.S. this week — I’d be comfortable calling it a mega-heat wave because it is breaking 100-plus-year records, and it is affecting a wide region,” said Mojtaba Sadegh, a professor at Boise State University who specializes in climate extremes.

The “mega-heat wave” is being supercharged by climate change, scientists say. “Currently, climate change has caused rare heat waves to be 3 to 5 degrees warmer over most of the United States,” Michael Wehner, a climate scientist at Lawrence Berkeley National Laboratory, wrote in an analysis published Tuesday.

Forecasters fear that the longevity and intensity of the heat, which is the leading cause of weather-related deaths in a typical year in the United States, will take a toll on people’s health. They continue to issue dire warnings: “This magnitude and duration of heat is dangerous! Limit outdoor exposure and drink plenty of water!” the National Weather Service Las Vegas posted on its website Thursday.

The interaction of the blistering heat and a toxic soup of air pollutants has resulted in pockets of poor air quality, the worst in years in some locations. Parts of Phoenix saw the worst air quality since at least 1980 on Tuesday, tweeted Ryan Stauffer, an air quality expert at NASA. On Wednesday, several areas east of Los Angeles also experienced unhealthy air.

In a vicious, self-reinforcing cycle, the heat has intensified the historic drought plaguing the West, which has, in turn, exacerbated the heat. Nearly 55 percent of the West is experiencing an “extreme” or “exceptional” drought, according to the U.S. Drought Monitor. Soil moisture is at or near the lowest levels seen in more than 120 years in many areas, so energy that would normally go into evaporation is directly heating the air and surfaces instead.

Meanwhile, the hot, dry air has created tinderbox conditions, and blazes have erupted in several states. The fire risk on Thursday is particularly worrisome because of the prediction for dry lightning in parts of the West, which is a major ignition source.

The forecast

Excessive heat warnings Thursday blanket much of California outside coastal and mountainous areas, southern Nevada, western and southern Arizona, and southern Utah. In many of these areas, high temperatures are forecast to exceed 100 and even 120 degrees in desert areas of Southern California.

  • In Phoenix, the high the next three days is expected to reach 116 or 117 degrees, probably breaking daily records on Thursday and Friday.
  • In Las Vegas, highs are predicted to reach 113 or 114 through Saturday, near daily records.
  • Tucson is forecast to reach 114 or 115 degrees through Saturday and about 110 Sunday. It has already hit 110 on five consecutive days and threatens to extend the streak to eight or nine days, breaking the record for its longest such streak of six days.
  • The Weather Service in Las Vegas says both Arizona and Nevada have a chance to challenge state records for their highest temperatures ever observed of 128 and 125 degrees, respectively.
  • Death Valley, Calif., is predicted to have forecast highs between 125 and 126 degrees through Saturday, just a few degrees shy of its highest June temperature on record of 129 degrees, set in 2013.
  • Fresno and Bakersfield, Calif., are predicted to have highs of 107 to 110 degrees Thursday through Saturday, near record levels.
  • Sacramento is predicted to see highs near 110 on Thursday and 107 on Friday, near record levels.

In some areas, exceptionally high nighttime low temperatures represent even a greater danger than the sizzling afternoon highs, particularly for vulnerable groups, such as the homeless, who lack air conditioning. Thursday morning’s low temperature in Las Vegas was just 91 degrees, which would be the earliest that a low temperature that warm has been recorded.

Some relief will gradually arrive. By Sunday and into next week, the heat is predicted to transition from record-challenging to simply above-normal.

Heat and drought combine to create volatile fire conditions

In the meantime, the sweltering heat, low humidity and moisture-starved land surface make for dangerous background conditions for the spread of fires.

Already parched vegetation — by many measures at peak-season dryness and breaking records for June — is going to be even more flammable both during and after this heat wave.

Since Sunday, wildfires have erupted in Arizona, California, Montana, Utah and Wyoming, and conditions are ripe for more. Red-flag warnings for high fire danger are still in effect in Montana, Utah and Wyoming.

On Thursday, the Weather Service’s Storm Prediction Center is predicting possible dry lightning over broad region of the West, which could spur fast-moving blazes. And this will become an even greater concern deeper into fire season when winds increase.

“My expectation is that we are going to have a dramatic fire season this year,” Sadegh said.

In vulnerable parts of California, the ground is as dry and vegetation flammable as it was last year ahead of its worst wildfire season on record. “Live fuel moistures remain at or near historic lows at all sites for June 15 ... meaning live fuels in the unit are current as dry as they were during July 15-Aug 1 window last year,” Cal Fire’s San Mateo-Santa Cruz unit tweeted Wednesday.

The climate change connection

The heat wave and drought are happening against a backdrop of rising temperatures caused by the burning of fossil fuels. Simultaneous heat waves and droughts are on the rise, with climate change not only intensifying them, but increasing their extent.

In a study published in September, Sadegh and his colleagues show that extreme heat and extreme dryness are overlapping with increasing frequency, and that rare, high-end events have had an “alarming” jump since 1993. For example, dry-hot extremes that should happen only once every 75 years occurred three or four times in the past 25 years in some regions. The most widespread and significant changes are found in the western United States.

Sadegh said that as droughts worsen heat waves and heat waves worsen droughts, people should not expect this cycle of warming and drying to break until there is a significant shift in weather patterns. “We’re still in June, so expect a lot more heat waves coming this summer,” he said.