Forecasters warn that Friday will be a washout across much of the region, with the North Bay getting hit first between 7 and 10 a.m. Heavy rain will then sweep south, reaching San Francisco and Oakland around 10 or 11 a.m. and the South Bay about an hour later. Some bursts could fall at 1 to 2 inches per hour for 15 to 30 minutes, enough to overwhelm storm drains and cause street flooding.

Strong southerly winds will precede the front, with gusts of 35 to 45 mph around the bay by late morning and up to 50 mph over San Pablo Bay and along the coast. Combined with saturated soil, the winds could topple trees and branches, leading to scattered power outages.

By early afternoon, the heaviest rain should exit, leaving half an inch to an inch across most of the region, with higher amounts in the North Bay, Marin hills, western San Francisco, the Peninsula hills, and the Santa Cruz Mountains. Inland East Bay and South Bay may see half an inch or less. Scattered showers will linger into the evening, especially near the coast.

Coastal flooding is a concern, particularly in low-lying areas of Marin County, where southerly winds will push water ashore as heavy rain coincides with rising tides. Storm drains clogged with seawater could leave runoff with nowhere to go, creating large puddles along the Embarcadero, Eastshore Freeway, Highway 37, and Highway 101 in San Mateo and Marin counties.

On Saturday, the flood risk shifts to the Pacific Coast, where a high surf warning is in effect for waves of 25 to 35 feet at most beaches and up to 50 feet at steep breakpoints like Mavericks near Half Moon Bay. The National Weather Service warns of "life-threatening" beach conditions, with large breaking waves spilling over seawalls and onto adjacent infrastructure. Peak waves are expected Saturday morning, but violent seas will persist through the weekend.

In the Sierra, heavy snow and fierce winds will make travel dangerous from Friday afternoon into Saturday morning, with the worst conditions during a burst of intense snowfall Friday evening. The highest peaks around Lake Tahoe could receive 18 to 20 inches, with about a foot above 7,000 feet and up to 6 inches in Truckee and South Lake Tahoe by Saturday morning. Snow levels will start around 6,500 to 7,500 feet but could drop to 5,500 feet as the cold front passes, turning rain to snow in mountain towns and allowing rapid accumulation on highways.

The Eastern Sierra could see some of the storm's heaviest snowfall, with 1 to 2 feet above 7,000 feet in Mono County. Mammoth Lakes may get 3 to 6 inches, while higher elevations on Mammoth Mountain could see 6 to 12 inches. Wind gusts of 55 to 70 mph are forecast along Highway 395 near Mammoth Lakes and Lee Vining, with 60 to 80 mph along exposed ridges.

Farther south, Yosemite, Kings Canyon, and Sequoia national parks could receive up to 18 inches of snow at higher elevations, with gusts to 50 mph. Yosemite officials will close Tioga Road and Glacier Point Road at 5 p.m. Thursday ahead of the storm.

Residents are advised to avoid travel Friday morning if possible, clear storm drains, and prepare for potential power outages. The storm marks the beginning of what forecasters say could be a historically wet few months for California.