SAN FRANCISCO—Northern California residents relieved that this week’s rain helped contain stubborn wildfires and soaked dry gardens were cleaning up Friday and preparing for a massive storm this weekend that could bring flash flooding to vast areas scorched by fire. The National Weather Service for the San Francisco Bay Area issued a high surf advisory through Friday for a portion of the coast and a flash flood watch Sunday for parts of the region, especially in areas burned by last year’s wildfires. Strong winds are also expected Sunday, with gusts of up to 60 mph at the windiest spots. The weather service said elevations above 9,000 feet in the Sierra Nevada could get 18 inches of snow or more from Sunday until Monday morning and warned of possible power outages and road closures. Mike Pierre, owner of Mission Ace Hardware and Lumber in Santa Rosa in Sonoma County, said they …