Utility crews across the Seattle region scrambled to clear fallen trees and restore power to nearly 300,000 homes, schools and businesses that remained without electricity on Thursday. The storm also deluged Northern California, where firefighters made water rescues as creeks overflowed from torrential rain.

Officials said the outages in Seattle could last into the weekend, even as another round of heavy wind and rain is expected, stretching even farther south this time.

The city of Issaquah, about 20 miles east of Seattle, was among the hardest hit. The storm left roads covered in fallen limbs and downed power lines. Scores of people sought warmth and electricity at the Issaquah Senior Center, said Tina Riehl, who works at the front desk. “The place is filled,” she said. “No power is a big thing.”

Fed by an atmospheric river funneling water from the Pacific Ocean, the storm killed at least two people as it tore through Western Washington this week. It moved south and parked over Northern California, flooding roadways and disrupting transit and air travel. Nearly seven inches of rain fell Wednesday at the airport in Santa Rosa, in California’s wine country north of San Francisco, breaking a one-day record.

Rescuers in Santa Rosa helped evacuate about 150 people on Thursday afternoon after they were surrounded by rising floodwaters. Paul Lowenthal, a division chief with the city’s fire department, said a creek overflowed, spilling water into the parking lot of a Hampton Inn and a health clinic next door, trapping guests, patients and employees.

Water began to pool under more than 100 cars in the parking lot, some of which started to float away, Mr. Lowenthal said. With more heavy rain expected, fire officials were working on a plan to move more people out of harm’s way using rescue vehicles.