LOS ANGELES (AP) — California prepared Thursday for a scorching weekend that could see temperatures hit 120 in the deserts as a massive heat wave rolled through the West, authorities said.

The National Weather Service issued excessive-heat warnings through Sunday night for areas from Santa Barbara County south to the Mexican border. Forecasters expected triple-digit highs in many areas — even above 5,000 feet in the mountains.

Excessive-heat watches starting during the weekend were also posted for the 450-mile length of the Central Valley from Bakersfield north to Redding, and advisories were issued for interior valleys of the San Francisco Bay and Monterey Bay region, where highs are expected to range from the 90s to 105.

Experts warned people to stay out of the sun, wear broad-brimmed hats outdoors, drink lots of water and avoid putting pets and children in parked cars where the temperature can quickly soar. Drivers who might find themselves stuck in a hot car should carry extra sunglasses, water, snacks and any prescription medications they may need, said Ken Kondo, a program specialist with the Office of Emergency Management of Los Angeles County's Chief Executive Office.

"You never know what could happen," he said.

The county designated dozens of air-conditioned libraries and community centers as cooling centers and planned to decide, based on temperature, whether to extend operating hours at some locations.

Los Angeles city officials also planned to decide whether to keep some buildings open until sunset to provide relief, especially for children, the elderly and people with medical conditions that might make them more susceptible to heat illnesses

Tips for dealing with heat were being added to the website of Laguna Woods, an Orange County retirement city of 18,000.

The city, which is registered with the county as a cooling center location, also had several hundred people signed up for its free Sunday afternoon movies.

The hottest temperatures were expected on Saturday and Sunday before beginning to ease, but it still will be above-average through Tuesday, said NWS specialist Stuart Seto.

The high pressure system was centered over New Mexico but was moving westward.

"It's a huge one," Seto said. "We haven't seen one like this for several years, probably the mid- to late 2000s."

The pressure causes air to sink and warm, drawing down humidity.

"As the air warms, it can hold more moisture, and so what that does is take out the clouds," Seto said.

The California Independent System Operator, which runs the state's power grid, expected to cope with any surge in demand from the heat.

Demand usually drops on weekends as office buildings and some manufacturers shut down, and if there's a surge, the system can bring more generators online and call a Flex Alert, where some customers voluntarily reduce their usage, spokesman Steven Greenlee said.

"We've been preparing all year for the summer," he said. "At least for the short term ... it looks like we're going to be OK."

However, Greenlee said the long-range forecast is for above-average temperatures through August. The grid also could be stressed in case of a disaster.

"All we need is another big wildfire or two that's going to take transmissions lines out," he said. "That's the risk that all of a sudden can turn not very good pretty fast."