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Much of the western U.S. is experiencing extreme heat this week, with temperatures blazing past 100 degrees. A phenomenon known as a heat dome is to blame. NPR's Bill Chappell has more.
BILL CHAPPELL, BYLINE: If you want to visualize how a heat dome can trap a region in intensely hot weather, picture yourself making a sandwich.
ALEX LAMERS: If you've made grilled cheese in a pan and you put a lid on there, it melts the cheese faster because the lid helps trap the heat. It's a similar concept here. You get a big high-pressure system in the upper parts of the atmosphere, and it allows that heat to build underneath.
CHAPPELL: That's Alex Lamers of the National Weather Service. He's the operations branch chief at the Weather Prediction Center. Lamers says a heat dome can cook an area for days, sometimes weeks. The heat dome that's currently putting a hot lid on the southwest U.S. will bring highs that are 20 to 30 degrees hotter than normal for early June. Thursday's forecast has both Phoenix and Las Vegas hitting 112 degrees, and the heat will stick around at night, falling only to the low 80s. As of late Wednesday, more than 20 million Americans from California to Texas were living under a federal excessive heat advisory.
JOELLEN RUSSELL: We are bracing already here in Tucson.
CHAPPELL: That's Joellen Russell, a climate scientist and distinguished professor at the University of Arizona. Her city is forecast to hit 109 degrees. Russell says this is a classic heat dome. So what's causing it? The jet stream. It's moving farther north than normal, leaving stagnant high-pressure systems that heat up the southwest. Both Russell and Lamers warn that heat waves are killers and that they're increasingly common.
RUSSELL: They are more frequent and they last longer. It's not the first day of the heat wave that kills you, it's the third or the fourth.
CHAPPELL: And while the daytime highs make headlines, Lamers says to beware if nights stay too warm to let you cool off.
LAMERS: And we find this a lot is that actually the minimum temperatures have a pretty high correlation to fatality rates in these types of events.
CHAPPELL: It's important, he says, to find ways to stay hydrated and give your body a break from the heat, because until the heat dome lifts, it can feel relentless.
Bill Chappell, NPR News.
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