Extreme Heat Warnings Cover Parts of Arizona and California Through Thursday

The National Weather Service has placed millions of residents in western states under Extreme Heat Warnings as a heat dome intensifies over the region. Warnings cover parts of Arizona and California and remain in effect through at least Thursday, with some locations facing dangerous conditions into the weekend.

Afternoon highs in the Phoenix metropolitan area and surrounding south-central Arizona are forecast to reach 109 to 116 degrees through Thursday evening.

Parts of southwestern Arizona and southeastern California could climb as high as 118 degrees until Thursday night. The Tucson area is expected to hit 107 to 111 degrees through Wednesday, while California's Coachella Valley, including Palm Springs, and the San Diego County deserts may reach 117 degrees. Overnight lows are forecast to stay in the 80s, limiting relief after sunset.

Lower elevations of the Grand Canyon at Phantom Ranch are projected to reach 111 to 114 degrees through Saturday evening. Temperatures across much of the interior West are expected to run 5 to 10 degrees above historical averages. The National Weather Service said heat-related illnesses increase during prolonged periods of extreme heat.

Several warnings note that the conditions pose a major heat risk. The agency urged residents to drink water, stay in air-conditioned spaces, and check on vulnerable neighbors. AccuWeather said the heat is produced by a strong area of high pressure that forces air to sink, compress, and warm near the surface.

The system is currently building over the western United States before shifting farther north and east. The pattern may also signal the start of the North American monsoon season, with moisture from Mexico expected to increase humidity and thunderstorm chances later in the week.