A widespread and dangerous heat wave is building across the U.S., with triple-digit highs expected in the Southwest and Great Plains this weekend. The heat will spread eastward under a dome of high pressure that meteorologists say could trap oppressive temperatures for a week or more in as much as two-thirds of the continental United States. Temperatures will be 15 to 25 degrees warmer than normal in many areas, including at night. That's especially bad for people’s health because their bodies won’t have a chance to recover.