At least 42 million people are under heat watches and warnings in the Southwest as a massive heat dome will send temperatures into triple-digit territory across some parts of the Southwest.

Much of Texas and New Mexico, with parts of Arizona, Nevada, and California, could record temperatures well above 100 degrees Fahrenheit today. Phoenix, Arizona, could be the hottest metro area, with expected highs around 115 degrees Fahrenheit.

Extreme heat will boost cooling demand in the Southwest, increasing power usage that could strain grids. The North American Electric Reliability Corporation, a regulatory body that manages grid stability, warned last month widespread rolling electricity blackouts were possible in the Southwest because of declining energy production due to decommissioning of fossil fuel power plants.