EXPLAINER: What’s behind the heat wave in the American West?

PHOENIX – Much of the American West has been blasted with sweltering heat this week as a high pressure dome combines with the worst drought in modern history to launch temperatures into the triple digits, toppling records even before the official start of summer.

Record daily highs were seen this week in parts of Arizona, California, New Mexico, Montana, Wyoming and Utah. Phoenix, which is baking in some of the U.S. West’s hottest weather, hit a record-breaking 118 degrees (48 Celsius) Thursday and was expected to reach 116 degrees (46 Celsius) Friday and Saturday.

“Very dangerous record breaking heat should continue today across the deserts with well above normal highs,” the National Weather Service’s Phoenix staff wrote on Facebook. “A very good day to stay indoors.”

WHY IS THE AMERICAN WEST SO HOT THIS WEEK?

The heat comes from a high pressure system over the West, a buckle in the jet stream winds that move across the U.S. and vast swaths of soil sucked dry by a historic drought, said Marvin Percha, a senior meteorologist for the agency in Phoenix.

He and other scientists say the heat wave is unusual because it arrived earlier and is staying longer than in most years.

“June last year, things seemed pretty normal,” noted Park Williams, a University of California, Los Angeles, climate and fire scientist. “The record-breaking heat waves came in August and September.”

But with such an early heat wave this year, “this could be the tip of the iceberg,” Williams said.

WHAT ROLES DO DROUGHT AND CLIMATE CHANGE PLAY?

A two-decade-long dry spell that some scientists refer to as a “megadrought” has sucked the moisture

out of the soil through much of the Western United States. Researchers said in a study published last year in the journal Science that man-made climate change tied to the emission of greenhouse gases can be blamed for about half of the historic drought.

Scientists studying the dry period that began in 2000 looked at a nine-state area from Oregon and Wyoming down through California and New Mexico and found only one other that was slightly larger. That drought started in 1575, a decade after St. Augustine, Florida, was founded and before the Pilgrims landed on Plymouth Rock in 1620.

The hot weather can be tied to the drought drying out the landscape. Normally, some of the sun’s heat evaporates moisture in the