West Coast Rattled by Hurricane-Strength Winds and Blizzard Warnings

A wind-whipped storm tore through Northern California on Wednesday, knocking down trees, snarling travel 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 and was expected to stretch into the weekend, is the season’s first major atmospheric river, a type of storm that flows in a narrow band from the Pacific Ocean and can deliver prodigious amounts of water.

The heaviest rainfall on Wednesday soaked a strip of the California coast that starts at the state’s border with Oregon and stretches hundreds of miles south to the North Bay region, just across the Golden Gate Bridge from San Francisco.

By late Wednesday morning, the storm had flooded roads, forced Amtrak to cancel trains across the Pacific Northwest and slowed service on Bay Area Rapid Transit, the San Francisco region’s major transportation system.

Officials in Santa Rosa, Humboldt County and other communities in the region watched for flooding in rivers and creeks and warned drivers to stay off roads to avoid falling trees. Wind gusts in some places along California’s North Coast exceeded 90 miles per hour, the National Weather Service reported — equivalent to the winds of a hurricane.

Inland, snow fell at some higher elevations, and gusts of up to 70 m.p.h. in Northern California and parts of Oregon and Washington could lead to blizzard conditions. Seattle forecasters issued a rare blizzard warning for the Cascade mountain range.

Before moving into California from the north, the storm battered Washington, downing power lines in the Seattle area. A woman in her 50s was killed after a large tree fell on a homeless encampment in Lynnwood, Wash., the Fire Department said.

As of late Wednesday morning, electricity was out for about 485,000 customers in Washington, according to poweroutage.us, which tracks utilities. Another 41,000 customers in California were without power.

Over the past decade, some of the deadliest and most destructive floods have occurred in places that forecasters said were at this level of risk.

The atmospheric river drenching the region is connected 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, forecasters said.