A wind-whipped storm tore through Northern California on Wednesday, knocking down trees, snarling highways and transit, and leaving tens of thousands of people without power. Days’ worth of rain fell on some places in just a few hours.
The damaging deluge, which drenched the Pacific Northwest overnight, killing at least two people, was expected to stretch into the weekend. It was the season’s first major atmospheric river, a type of storm that can deliver prodigious amounts of water in a narrow band from the Pacific Ocean.
Some of the worst damage was in the Seattle area, where heavy wind gusts tore down power lines and knocked out substations late Tuesday, leaving half a million customers without electricity. “This is a major storm the likes of which we haven’t seen in over a decade,” said Melanie Coon, a spokeswoman for Puget Sound Energy.
The heaviest rain on Wednesday soaked a strip of the California coast that starts at the Oregon border and stretches hundreds of miles south to the North Bay region, just across the Golden Gate Bridge from San Francisco. Wind gusts in some places exceeded 90 miles per hour — equivalent to the winds of a hurricane.
Amtrak canceled trains across the Pacific Northwest, and the storm slowed service on Bay Area Rapid Transit, the San Francisco region’s major transportation system. Gusty winds forced Northern California drivers off roadways, where they crashed into other vehicles and struck power poles.
A truck tipped over on U.S. 101 in Marin County, just north of San Francisco, and temporarily blocked traffic, according to the California Highway Patrol. And a large S.U.V. overturned on a highway near the Napa Valley winery town of Calistoga.
Before surging into California from the north, the storm felled trees that killed at least two people in Washington. A tree crashed through the roof of a home in Bellevue, on the outskirts of Seattle, killing a woman who lived there, officials said. Another woman died after a tree fell on a homeless encampment in Lynnwood, Wash.
The storm also dumped snow at higher elevations, and blizzard conditions were a possibility in the Cascade mountain range, as well as other parts of the Pacific Northwest. Snow forced the authorities to close Interstate 5 just south of the Oregon-California border for a stretch of more than 50 miles.
Electricity remained out for about 470,000 customers in Washington on Wednesday afternoon, according to poweroutage.us, which tracks utilities. Officials warned that some of the outages could last for days because of the extent of the damage.
Utilities in the Seattle area flew helicopters to locate damaged lines, and crews worked to clear trees and debris. The storm caused the most outages in almost two decades, said Jenn Strang, a spokeswoman for Seattle City Light, a local utility.
The atmospheric river will continue to batter the region into Friday before easing slightly. Forecasters have issued a rare “high risk” warning for excessive rainfall in parts of northwestern California, where more than 16 inches of rain could fall.
California’s ski resorts are in for a bounty of snow. The Sierra Nevada peaks near Lake Tahoe could receive as much as five feet over the next week, said Matthew Chyba, a meteorologist with the National Weather Service office in Reno, Nev. “Our first good snow, I’d say.”
The impact from an atmospheric river can be most extreme when it stretches on for several days, or is immediately followed by another, as has happened during recent winters. Heavy rainfall saturates the ground so much that hillsides and highways can be swept away.
On Wednesday, officials in Santa Rosa, Humboldt County and other Northern California communities said they were watching for flooding in rivers and creeks, a risk even early in the season because of how long the rain was expected to persist.
Climate change can make storms of all types more intense, because warmer air can hold more moisture. When it comes to atmospheric rivers in particular, scientists are studying whether global warming may be affecting the number that sweep through California each year and how long they last, though they don’t have clear answers yet.
The atmospheric river drenching the region this week was also made more destructive by its connection to another storm system off the coasts of Oregon and Washington. Called a “bomb cyclone” — a storm whose atmospheric pressure drops quickly over a short period — that system reached the lowest pressure reading ever recorded in the northeastern Pacific Ocean.
Trees in the Pacific Northwest aren’t accustomed to the weather pattern generated by these combined storms, said Ms. Coon with Puget Sound Energy. Their roots are oriented toward stabilizing them for a different type of wind pattern.
“Instead of winds coming from the west, from the ocean, they’re suddenly coming from the east,” she said, “and they’re in a churning, circular motion.” The result: Roots can’t hold, and trees topple over.
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