SAN FRANCISCO — Officials are asking visitors to California's beaches to be extra-careful in and near the water this weekend, when the weather is expected to bring relatively warm temperatures and powerful waves.

The National Weather Service has issued high surf advisories for much of the coast through Monday night. The Coast Guard and National Park Service are warning swimmers and boaters to be on the lookout for rip tides and sneaker waves that could reach heights of 20 feet.

Meteorologist Austin Cross says that the swells arriving during that period will have traveled from the western Aleutian Islands and picked up enough energy in the process that they will pack a formidable punch when they break.

The forecast is good news for spectators headed to the Mavericks International big wave surf contest near Half Moon Bay on Sunday.