Orange County officials have ordered mandatory evacuations, beginning tonight at 7 p.m., of four canyon communities hit hard by the October fires -- a move they said is necessary in advance of expected heavy downpours.

The evacuation orders for residents of Modjeska, Silverado, Williams and Harding canyons come as forecasters said heavy rains are expected to start falling in Southern California during the evening commute.

The storm hitting the region tonight wreaked havoc earlier in the day in the San Francisco Bay Area, where wind gusts topped 100 mph in mountain areas, forcing the closure of bridges to car and truck traffic and shutting down U.S. 101 in San Rafael. Weather up north was so poor that California Highway Patrol officials closed I-80 at 5 p.m. from the town of Applegate to the Nevada border, citing blizzard and whiteout conditions.

Locally, rain and the possibility of mudslides in canyon communities remained the top concerns. "It looks like the main threat is going to be at 6 p.m. this evening through Saturday morning," said Stuart Seto, a weather specialist at the National Weather Service office in Oxnard.

Seto said rains could fall at rates as high as half an inch to an inch per hour with the storm, with forecasters expecting 2 to 4 inches along the coasts and in the valleys, with up to 10 inches in the mountains.

By midafternoon, the front -- the second in a series of expected storms -- was dropping a steady drizzle on downtown Los Angeles.

Officials urged motorists to avoid driving through flooded roadways.

"Do not underestimate the power of floodwaters. Only a few inches of rapidly flowing water can quickly carry away your vehicle," the National Weather Service said in a bulletin.

The heaviest rains are expected to begin around 10 p.m. in L.A. County, and about midnight in Orange and San Diego counties.

Residents of Orange County under evacuation orders can go to a center set up at Villa Park High School in Villa Park, officials said. Residents of those affected canyon communities were ordered to evacuate large animals to the Orange County Fairgrounds in Costa Mesa earlier in the day. At noon, Orange County activated its Emergency Operations Center and said the public can call (714) 628-7085 for information about storm-related matters.

Earlier, the first of three anticipated storms passed through the region with little impact after stalling overnight along the Central Coast, forecasters said.

The second storm will hit Southern California hard, but probably not with as much force as it struck Northern California, said Philip Gonsalves, a forecaster with the National Weather Service in San Diego. Bay Area residents grappled with high winds and flooding that disrupted traffic. Officials closed the Richmond-San Rafael Bridge in the morning and barred truck traffic on the Golden Gate Bridge after wind speeds on the landmark bridge registered as high as 70 mph. The greatest concern, officials said, were gusts of winds above 40 to 45 mph gusting from the east and west over the bridge.

On Kregor Peak in Contra Costa County, gusts of 107 mph were recorded, and the San Francisco Airport saw wind speeds as high was 66 mph.

Meanwhile, preparations were underway for expected heavy rainfall, expected to be the most precipitation to fall in the region in two years. Gov. Arnold Schwarzenegger met with emergency response officials in Los Alamitos to get briefed on efforts.

"It's very dangerous. We want people to be prepared. We want people to be ready and work ahead of time rather than be behind," Schwarzenegger said under dry skies this afternoon. "I love to overreact. We want to be ready for any kind of scenario."

Steve Sellers, the regional administrator for the state's Office of Emergency Services, warned residents of areas considered at high risk of flooding and mudslides to take evacuation orders and recommendations seriously.

"It's too late once the rain starts happening ... you're risking your life and your family's life."

A flash-flood watch was issued for fire-hit areas for this afternoon through Saturday morning.

Meanwhile, winds and flooding wreaked havoc in San Francisco, forcing bus service to shut down and knocking out power in several areas, authorities said. About 750,000 customers from Humboldt County to Bakersfield were affected by outages, said Pacific Gas & Electric representative Katie Romans.