Per National Weather Service Las Vegas:

• Extreme Heat Watch in effect on Tuesday for northwestern San Bernardino County, far southern Nye County, Death Valley National Park, lower elevations of Clark County, and the Colorado River Valley including the Lake Mead National Recreation Area.
• Major Heat Risk is expected on Tuesday across these locations, with temperatures soaring to 8 to 12 degrees above seasonal normals. This level of heat affects anyone without effective cooling and/or adequate hydration.

NWS Las Vegas