A powerful winter storm sweeping the West Coast walloped areas of the region unaccustomed to snow and wintry mix, paralyzing parts of Portland, Ore., before shifting south toward California.
Forecasters in Oregon said the storm was continuing to “overperform” in terms of snowfall in areas of the state Thursday morning. On Wednesday, the storm produced 10.8 inches of snow in Portland, giving the city its second snowiest day ever recorded, forecasters there tweeted.
The snow trapped motorists, including a school bus, on freeways for hours, while other drivers abandoned their vehicles on the side of the road. Many schools and universities were closed. As of Thursday afternoon, the Oregon Department of Transportation said “multiple roads remained closed” because of “hundreds of downed trees,” and advised delaying travel if possible.
The West Coast can be one of the hardest places in the country to predict weather because of the lack of observations over the Pacific Ocean, and the intensity of the storm caught many people — including forecasters — by surprise.
Colby Neuman, a meteorologist for the National Weather Service in Portland, said “there was a lot of uncertainty” in developing a forecast in part because of the low probability that a snow of this magnitude would occur. He said that, with one exception, the forecast models did not pick up the large and widespread snow amounts.
The system also moved away from the area more slowly than predicted. By Thursday, the storm was expected to hug the West Coast as it drifted toward Southern California, delivering hazardous conditions.
As California braced for its unusual weather, a heavy storm played out in a region far more used to snow: the Upper Midwest, where hundreds of thousands of customers were without power early Thursday, hundreds of schools were closed and air travel was disrupted.
Blizzards are not typically associated with Southern California, but residents were nevertheless preparing on Thursday for a potent winter storm that forecasters warned could make mountain roads unpassable.
Wind gusts of 55 to 75 miles per hour and up to seven feet of snow were expected starting Friday, forecasters at the National Weather Service Office in Los Angeles said Thursday morning.
The service has also issued a winter storm warning for the mountains of Los Angeles, Ventura and Santa Barbara Counties until early Friday, warning of “extremely dangerous mountain conditions.” A rare blizzard warning, for the same areas, will take over from Friday through Saturday.
“This blizzard warning is something that our community is not used to hearing about,” said Jackie Ruiz, the public information officer for Santa Barbara County Office of Emergency Management. She said the county’s message was simple: “If you don’t have to go out, stay home.”
This isn’t the first blizzard warning from the Weather Service office in Los Angeles, but it has been decades since the last one. The office’s records go back only to 2006, so its forecasters, more accustomed to issuing high surf advisories and flash flood warnings, were unsure when their last blizzard warning was. After some sleuthing, they found one issued on Feb. 4, 1989.
On Thursday afternoon, the San Diego office continued the blizzard warnings along the San Bernardino mountains into San Bernardino County, making it the first ever blizzard warning issued by that office.
Forecasters in Los Angeles described the storm affecting that region as dangerous, and they predicted up to seven feet of snow in areas more than 4,500 feet above sea level. Lesser amounts, between one and six inches, were expected in elevations of less than 2,500 feet.
The lower-elevation snow means this could be the largest amount of 24- to 48-hour snowfall seen in decades — probably since a 1989 snowstorm that snarled traffic — for the Ventura County and Los Angeles County mountains, forecasters said.
“Everything is adding up for a major snow event,” said Andrew Rorke, a senior forecaster for the Weather Service office in Los Angeles.
A person standing in downtown Los Angeles can see a 10,600-foot peak that will typically have snow on it, Mr. Rorke said. By Saturday, that snow will extend much farther down the mountain, showing more snow than a typical winter storm.
But don’t expect the Hollywood sign to be lost in a snow-covered hillside.
“The Hollywood Hills are saved from snow, but the San Gabriels behind the Hollywood sign are certainly not,” Mr. Rorke said.
Los Angeles is surrounded by mountains, and when they get heavy snow, passage through them shuts down.
Emily Montanez, a spokesperson for the Los Angeles County’s office of emergency management, said the biggest concern with the extreme weather is traffic. According to KTLA, icy conditions contributed to a 20-car pileup overnight on Interstate 10 in Yucaipa.
Snow could continue to affect Interstate 10 through Southern California, which Samantha Connelly, a meteorologist with the Weather Service office in San Diego, said was “something we don’t see very often at all.”
By Friday, the snow line will rise as warmer air from a weak atmospheric river flows into Southern California, lifting the snow to 4,500 feet.
That snow was greeted with caution at Big Bear Mountain Resort, about 100 miles east of Los Angeles. Justin Kanton, a spokesman for the resort, said the mountain had received about two feet of snow in 24 hours, with more on its way.
“We are strongly advising anybody who does not have experience driving in these extreme conditions to consider postponing their travel to and from the mountain,” he said.
Areas along the coast and valleys could see rare rainfall totals of up to five inches through Saturday night, with higher totals in the foothills, where the heaviest band of precipitation may stall, forecasters said. When a storm stalls, it can produce excessive rain over the same area for long periods, increasing the risk of flash flooding and avalanches.
In the Santa Cruz Mountains to the north, there were reports of snow extending down to elevations of 500 feet on Thursday.
The Weather Service office in Hanford, Calif., about 30 miles south of Fresno, warned that “heavy snow in the mountains and snow in the foothills will likely cripple travel through Saturday.”
In the Sierra Nevada, the snowpack is already above average for this time of the year, and more snow is in the forecast. Even though snow is nothing new for this region, forecasters in Reno, Nev., were calling it a major storm that will disrupt travel.
“While we cannot definitively say there will be road closures, nor do we have any control over this, past events have certainly brought similar impacts,” the forecasters said.
There is also a high risk for avalanches across the Sierra Nevada on Friday, according to the Avalanche Forecast Center.
This will not be the last winter storm to hit the area; forecasters from all Weather Service offices in California are predicting another storm at the beginning of next week.
Derrick Bryson Taylor contributed reporting.