Temperatures are expected to reach potentially lethal levels this weekend in parts of the Southwest and the Plains. Forecasters say major cities including Phoenix, Las Vegas and Tucson, as well as parts of Kansas and Oklahoma, will reach highs above 110 degrees Fahrenheit.

The National Weather Service has issued widespread excessive heat warnings, and says such high temperatures are "rare, dangerous, and deadly." The agency is urging people in the affected areas to cancel outdoor activities between Saturday and Tuesday. The heat wave is expected to peak Sunday