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.
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; Casper, Wyo.; and Billings, Mont., all made history Tuesday, soaring to 107, 101, and 108 degrees, respectively.
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 US 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 Livermore 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 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 US Drought Monitor. Soil moisture is at or near the lowest levels seen in over 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 conditions have created a tinderbox and fire ignitions and blazes have erupted in several states. The fire risk on Thursday is particularly worrisome in parts of the West due to the prediction for dry lightning, which is a major ignition source.
Excessive heat warnings Thursday blanketed much of California outside coastal and mountainous areas, southern Nevada, western and southern Arizona, and southern Utah. In many of these areas, high temperatures were forecast to exceed 100 and even 120 degrees in desert regions of Southern California.
In some areas, exceptionally warm 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 its earliest 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.