The National Weather Service issued a high surf advisory for the Bay Area coast on Friday — the first in nearly two years — and warned that swells could grow to 25 to 35 feet along much of the Pacific coast from Friday evening through Sunday morning. At favored breaks such as Mavericks, waves could reach 40 to 50 feet, and later forecasts warned that some sets could exceed 60 feet.

“These waves bring tremendous power that will overpower anyone in the water and cause coastal erosion,” the weather service said in its forecast discussion. “The risk of drowning is very high this weekend. Do not go in the water.”

The dangerous conditions stem from a hurricane-strength low-pressure system churning hundreds of miles offshore in the eastern Pacific. Meteorologist Roger Gass said winds associated with the system are generating the swell locally, and as it travels toward the West Coast it builds and gains strength.

The swells are also unusually long-period, with 18 to 19 seconds between wave crests, a signal of extraordinary power. Forecasters said sneaker waves and rip currents will pose a serious threat to anyone who ventures near the water, and coastal flooding is possible as high tides combine with elevated sea levels.

Sea level off the Bay Area coast is currently running about 6 inches to a foot above average due to thermal expansion and a Kelvin planetary wave, the weather service noted. “This higher water elevation sets the stage for large waves to crash over sea walls and run up beaches onto adjacent infrastructure, particularly during high tide,” the agency warned.

The threat follows a deadly reminder of the ocean’s power. Last weekend, a 24-year-old woman disappeared at Panther Beach in Santa Cruz after being swept away by a high tide she could not escape.

By Saturday, the impacts were already visible. Videos posted on social media and broadcast by local news showed massive waves crashing along the coast in Pacifica and Half Moon Bay, sending water surging onto city streets. Witnesses reported people being knocked down by the water and running from particularly large sets. A buoy off Point Reyes recorded a wave height of 31.2 feet Saturday morning — the highest since 2015, according to meteorologist Rachel Kennedy.

In Monterey County, several roads in the unincorporated Carmel Point area just south of Carmel-by-the-Sea were closed Saturday morning after large rocks and sand were deposited on streets by crashing waves. Scenic Road was shut at Isabella Way, Martin Way, Ocean View Avenue, Santa Lucia Avenue, Stewart Way, Valley View Avenue, and Carmelo Street at 17th Avenue. Capitola officials urged visitors to stay away from the Capitola Wharf until noon Saturday.

Kennedy said the initial swell was on the downtrend but warned that more northwesterly swells are expected in the coming week because the Pacific is so active. “We are expecting that there might be subsequent rounds of damaging waves this week,” she said.

Forecasters also said a cold front is expected to move across the North Bay on Sunday, bringing rain totals of half an inch to an inch across most of the Bay Area, with wind gusts of 20 to 30 mph and up to 45 mph along the coast. Another system could arrive Tuesday afternoon or evening.

In response to the severe weather, San Mateo County announced it will open additional temporary shelter for homeless people and families starting Sunday under its Inclement Weather Program, which runs through April 15. Cots will be provided in a large space divided by curtains, and people referred through the county’s Coordinated Entry System will have access to meals, showers, restrooms, washers and dryers.

Officials stressed that the safest way to view the spectacle is from solid ground far from the surf. Gass advised people to stay on “solid ground and dry land” and to keep a wide distance from wave breaks.

The weather service said the arrival of the massive waves is not directly connected to a brewing historic El Niño in the Pacific, but that the climate pattern could worsen the effects. With the ocean already running high, the agency warned that this weekend’s surf could cause significant coastal flooding and erosion.