The National Weather Service extended a flood watch into Saturday for areas north of San Francisco as the strongest atmospheric river — a large plume of moisture flowing onshore — that California and the Pacific Northwest has seen this season inundated the region. The storm system unleashed winds the night before that left two people dead and hundreds of thousands without power in Washington state.

Up to 16 inches of rain (about 41 centimeters) was forecast in Northern California and southwestern Oregon through Friday. By Wednesday evening, some areas in Northern California had experienced heavy rain, including Santa Rosa, which had seen about 5 inches (about 13 centimeters) within 24 hours, according to Marc Chenard, a meteorologist with the National Weather Service.

Dangerous flash flooding, rockslides and debris flows were possible, officials warned. About a dozen small landslides had struck in northern California in the last 24 hours, including one on Highway 281 on Wednesday morning that caused a vehicle crash, said Chenard.

The National Weather Service in the Bay Area warned people that the atmospheric river was focused on the North Bay and to “expect heavy rain to continue tonight, Thursday into Friday. This will result in mudslides, road closures.”

The storm system, which first hit Tuesday, is considered a “bomb cyclone,” which occurs when a cyclone intensifies rapidly.

A winter storm watch was in place for the northern Sierra Nevada above 3,500 feet (1,066 meters), where 15 inches (38 centimeters) of snow was possible over two days. Wind gusts could top 75 mph (121 kph) in mountain areas, forecasters said.

The storm had already dumped more than a foot of snow along the Cascades by Wednesday evening, according to the National Weather Service. Forecasters warned of blizzard and whiteout conditions and near impossible travel at pass level.

In Washington, there were nearly 376,000 power outage reports Wednesday evening, resulting from strong winds and rain the night before, according to poweroutage.us. Falling trees struck homes and littered roads across western Washington, killing at least two people. One woman in Lynnwood was killed when a large tree fell on a homeless encampment, while another woman in Bellevue was killed when a tree fell on a home.

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

In California, there were reports of nearly 21,000 power outages as of Wednesday evening.

Southbound Interstate 5 was closed for an 11-mile (18-kilometer) stretch from Ashland, Oregon, to the California border on Wednesday morning due to extreme winter weather conditions in northern California, according to the Oregon Department of Transportation. It was expected to be a long-term closure, the department said.

Hundreds of flights were delayed and dozens were canceled at the San Francisco International Airport, according to Flight Aware.

The weather service issued a flood watch for parts of southwestern Oregon through Friday evening, while rough winds and seas temporarily halted a ferry route in northwestern Washington between Port Townsend and Coupeville.