Dangerous heat is gripping parts of the United States, with more than 39 million people under heat alerts across much of the South as temperatures are expected to rise over the weekend. Temperatures in some cities have climbed above 100°F. …The “heat dome” that brought extreme heat to much of the central and eastern U.S. has shifted west and is expected to intensify before expanding into the intermountain West and northern High Plains this weekend. Parts of the Southwest are forecast to reach 110-120°F from late this week into early next week, with some locations expected to experience one of their 10 hottest days on record. Extreme heat warnings are in effect on Friday across parts of six states: Arizona, California, Nevada, Utah, Oklahoma and western Arkansas. …Such levels are “dangerous for anyone without adequate cooling or hydration,” the weather service warned.