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

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

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, was forecast to hit 117 degrees (47 Celsius) Thursday and 116 degrees 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?