FORESTVILLE, Calif.—A major storm moving through Northern California on Thursday dropped heavy snow and record rain, flooding some areas, after killing two people and knocking out power to hundreds of thousands in the Pacific Northwest.

Forecasters warned the risk of flash flooding and rockslides would continue, and scores of flights were canceled at San Francisco’s airport.

In Washington, more than 204,000 people—mostly in the Seattle area—remained without power as crews worked to clear streets of electrical lines, fallen branches and debris. Utility officials said the outages, which began Tuesday, could last into Saturday.

Meanwhile on the East Coast, where rare wildfires have raged, New York and New Jersey welcomed much-needed rain that could ease the fire danger for the rest of the year.

The National Weather Service extended a flood watch into Saturday for areas north of San Francisco as the region was inundated by this season’s strongest atmospheric river—a long plume of moisture that forms over an ocean and flows through the sky over land.

The system roared ashore Tuesday as a “ bomb cyclone,” which occurs when a cyclone intensifies rapidly. It unleashed fierce winds that toppled trees onto roads, vehicles and homes, killing at least two people in Washington.

Communities in Washington opened warming centers offering free internet and device charging. Some medical clinics closed because of power outages.

“I’ve been here since the mid-’80s. I haven’t seen anything like this,” Trish Bloor, a city of Issaquah official, said while surveying damaged homes.

Up to 16 inches of rain was forecast in southwestern Oregon and California’s northern counties through Friday.

Santa Rosa saw 6.5 inches of rain in the last 24 hours, marking the wettest day on record since 1998.

The Sonoma County Airport, in the wine country north of San Francisco, got more than 11 inches within the last 48 hours and the unincorporated town of Venado had about 12.7 inches in the same period.

Meghan Nelson, her fiancé and their corgi had to escape to a hotel Thursday after their home’s basement and the street it’s on in Fulton, California, flooded. She said they moved their belongings upstairs and put their furniture on bricks, but they don’t know if they’ll be able to get back tomorrow if there’s more rain.

“Luckily we’re safe for right now. So that’s a good thing,” she said.

In nearby Forestville, one person was hurt when a tree fell on a house. Small landslides were reported across the North Bay, including one on State Route 281 on Wednesday that caused a car crash.

Daniela Alvarado said calls to her and her father’s Sonoma County-based tree business have nearly tripled in recent days, with people reaching out about trimming or removing trees.

“We feel sad, scared, but also ready for action,” Alvarado said.

Rain slowed somewhat, but “persistent heavy rain will enter the picture again by Friday morning,” the weather service said. “We are not done!”

Flash flooding, rockslides and debris flows were possible, especially where hillsides were loosened by recent wildfires, officials warned. Scott Rowe, a hydrologist with the weather service in Sacramento, said so far the ground has been able to absorb the rain in areas where the Park Fire burned this summer.

“It’s not necessarily how much rain falls; it’s how fast the rain falls,” Rowe said.

Santa Rosa Division Chief Fire Marshal Paul Lowenthal said 100 vehicles were stuck for hours in the parking lot of a hotel and medical center after being swamped by thigh-high waters from a flooded creek.

A winter storm watch was in place for the northern Sierra Nevada above 3,500 feet, with 15 inches of snow possible over two days. Wind gusts could top 75 mph in mountain areas, forecasters said.

Sugar Bowl Resort, north of Lake Tahoe near Donner Summit, picked up a foot of snow overnight, marketing manager Maggie Eshbaugh said Thursday. She said the resort will welcome skiers and boarders on Friday, the earliest opening date in 20 years, “and then we’re going to get another whopping of another foot or so on Saturday, so this is fantastic.”

Another popular resort, Palisades Tahoe, said it is also opening Friday, five days ahead of schedule.

The storm already dumped more than a foot of snow along the Cascades in Oregon by Wednesday night, according to the weather service.

More than a dozen schools closed in the Seattle area Wednesday, and some opted to extend the closures through Thursday.

Covington Medical Center southeast of Seattle postponed elective surgeries and diverted ambulances after losing power and having to rely on generators Tuesday night into Wednesday, according to Scott Thompson, spokesperson for MultiCare Health System. Nearby MultiCare clinics closed Wednesday and Thursday after losing power.

Ben Gibbard, lead singer of the indie rock bands Death Cab for Cutie and Postal Service, drove from his Seattle neighborhood Thursday morning to the woods of Tiger Mountain for his regular weekday run, but trees were blocking the trail.

“We didn’t get hit that hard in the city,” he said. “I just didn’t assume it would be this kind of situation out here. Obviously you feel the most for people who had their homes partially destroyed by this.”

Washington Gov. Jay Inslee thanked utility crews for toiling around the clock. It could take weeks to assess the scope of the damage and put a dollar figure on it, he said in a statement, and after that “we’ll know whether we will be able to seek federal assistance.”

In California, there were reports of more than 8,000 power outages.

Authorities limited vehicle traffic on part of northbound Interstate 5 between Redding and Yreka due to snow, according to California’s Department of Transportation. Officials also shut down a 2-mile stretch of the scenic Avenue of the Giants, named for its towering coast redwoods, due to flooding.

About 550 flights were delayed and dozens were canceled Thursday at San Francisco International Airport, according to tracking service FlightAware.

The Northeast, meanwhile, got a much-needed shot of precipitation, providing a bit of respite in a region plagued by wildfires and dwindling water supplies. More than 2 inches was expected by Saturday morning north of New York City, with snow mixed in at higher elevations.

Weather service meteorologist Brian Ciemnecki in New York City, which this week saw its first drought warning in 22 years, said “any rainfall is going to be significant” but the storm won’t be enough to end the drought.

By Godofredo A. Vasquez, Janie Har, and Christoper Weber