Seattle was forecast to get 4 to 6 inches of snow during its second bout of winter weather in the past week, a storm that was likely to stretch until late afternoon Saturday, according to the National Weather Service. Meteorologists predict even more snow throughout the next week.

The storm that hit the area earlier this week dumped 2 1/2 inches on Seattle-Tacoma International Airport and up to 12 inches on smaller cities outside Seattle.

This pattern of winter storms is rare for Seattle, said Cliff Mass, a professor of atmospheric sciences at the University of Washington. Heavy snow in Seattle requires an unlikely mixture of moisture and cold air, Mass said, but the moisture from the Pacific Ocean tends to be relatively warm.

“What we need is a very specific weather situation that allows us to be isolated from the ocean’s warmth but also draw some of the moisture off the ocean and use that for snow,” he said.

There is also a chance for a modest snowstorm Sunday night and a heavier one Monday night, Mass said. Another storm is predicted for Thursday.

“The cold air is in place, and it’s not going anywhere,” he said.

At Seattle-Tacoma International Airport, the average yearly snowfall is less than 7 inches, said Kirby Cook, a meteorologist with the National Weather Service. As of about 7:30 Pacific time, that location had already received more than 4 inches, with plenty more to come, Cook said.

More than 200 flights at the airport were canceled Friday, according to FlightAware, a flight-tracking website.

Washington’s Department of Transportation warned of packed roads and, in some cases, extremely long travel times.

And because Seattle residents are not accustomed to this sort of weather, “people are going out of their minds,” Mass said. “The supermarkets are getting stripped of all food.”

In Portland, Oregon, which has an average snowfall similar to Seattle’s, a winter storm warning stretched from Friday evening to Saturday afternoon, and the National Weather Service was forecasting from 1 to 4 inches. Up to 6 inches was expected in areas of higher elevation east of Portland.

Another storm that is predicted for Sunday is likely to keep snowfall totals climbing.

Farther south along the West Coast, heavy snow was expected from California’s northern coast to the Sierra Nevada, said Richard Bann, a meteorologist with the National Weather Service. About 1 to 3 feet of snow was forecast for the mountainous areas, with 4 feet likely to fall in parts of Yosemite National Park.

Unlike Seattle’s low-lying terrain, the high elevation of the Sierra Nevada makes heavy snowfall common during the winter, Bann said. After a snowstorm last week, more than 120 visitors and staff members were stranded for five days at Montecito Sequoia Lodge in Kings Canyon National Park, where up to 7 feet of snow fell, The Associated Press reported.

Bann said the heavy snow and strong winds likely to hit the mountain range this weekend would make traveling in certain areas “difficult to impossible.”

The National Weather Service also warned that high winds could cause fallen trees, and that wind chills could reach 30 degrees below zero.