A scorching heat wave continues across the central and southern United States, leaving at least 40 million people under extreme heat warnings. Heat indexes exceeding 105 degrees Fahrenheit are expected from Memphis to Montana to Las Vegas. The National Weather Service says parts of the southwest could see temperatures as high as 120 degrees Fahrenheit. The sweltering conditions are the result of a heat dome that traps heat and makes temperatures more extreme. Authorities are warning that children and older adults, along with people with medical conditions, are especially at risk of serious conditions including heat stroke.