A Pacific storm pummeled drought-parched northern and central California on Thursday with heavy rain and high winds, knocking out power to tens of thousands of homes, disrupting flights, washing out roads and prompting school closures in the Bay Area.

Some 240 departing and incoming commercial flights were canceled at San Francisco International Airport by late morning, and others were delayed for more than two hours, airport managers said.

A downtown San Francisco subway station serving the financial district was shut down through the morning commuter rush because of a power outage and flooding, and the city's electrified bus system was halted in many areas, transit officials said.

The Embarcadero, the city's popular waterfront walkway, was closed due to flooding and some ferries were also canceled, stranding commuters.

Some streets and major intersections were flooded in the San Francisco area, including the westbound lanes of Interstate 280 in the East Bay suburb of El Cerrito, according to the California Highway Patrol.

Winds howled through Sacramento, the state capital, rattling buildings and whipping through trees before dawn, followed by heavy downpours as the morning commute was getting started.

Parts of Oregon and Washington were also lashed by rain and high winds.

"In certain parts of the West Coast this could be the most significant storm in 10 years, "National Weather Service meteorologist Eric Boldt said.

The Weather Service issued flash-flood, heavy-surf and high-wind advisories, warning that torrential rains could lead to mudslides in foothill areas scarred by wildfires earlier this year.

The storm was expected to provide only a small measure of relief from California's record, multi-year drought that has forced water managers to sharply reduce irrigation supplies to farmers and prompted drastic conservation measures statewide, weather officials said.

As much as 3 feet (1 metre) of snow is predicted this week for the higher elevations of the Sierra Nevada mountain range. But meteorologists said many months of rainfall would be needed to pull the state out of the drought.

The Shasta Lake area of Northern California received 5 inches (13 cm) of rain overnight, and up to 4 inches (10 cm) were expected in California's Central Valley, the state's agricultural heartland, as well as in Sacramento, the weather service said.

Pacific Gas and Electric Co reported that nearly 227,000 customers lost power during the storm on Thursday morning. Cities in the peninsula area south of San Francisco were hardest hit by outages.

Several Bay Area school districts, including San Francisco, Oakland and Berkeley, canceled classes due to the storm.

The storm was expected to move into Southern California in time for the Friday morning commute, in what would be the area's second major storm in a week. A third storm system has been forecast for this weekend.

Municipalities handed out sandbags to help residents ward off flooding and reminded them to stock emergency supplies, routines that had become unfamiliar amid the drought.