US northwest avoids ‘bomb cyclone’ but is deluged by ‘atmospheric river’

The northwestern US avoided being hit by a second “bomb cyclone” on Thursday when the storm's fury came up short of forecasts, but an ongoing “atmospheric river” deluged northern California with wind and rain.

The first bomb cyclone, in which rapidly dropping pressure produces hurricane force winds and precipitation, pounded the region on Monday and Tuesday, killing two people and knocking out power to hundreds of thousands of people from Washington state to northern California.

Images of cars and houses crushed by fallen trees in Washington filled social media.

A second bomb cyclone had been forecast to hit on Thursday, but the storm's pressure failed to drop fast enough over 24 hours to earn the provocative designation, said Joe Wegman, a meteorologist with the national weather service's weather prediction centre.

Even so, the low-pressure system produced more wind and rain.

Northern California continued to be drenched by an atmospheric river on Thursday, creating hazards on roads and flood and wind alerts.

Wind gusts of 105km/h were expected on Thursday night, and risks of life-threatening flooding and mud and rock slides remained throughout the region, the weather service warned.

About 272,000 homes and businesses remained without power in Washington and northern California on Thursday night, down from 600,000 on Wednesday.

The wind storm and heavy rain also damaged the power system in Canada's Pacific coast province of British Columbia and cut power to about 225,000 customers on Tuesday night, according to provincial electricity provider BC Hydro. That was down to about 20,000 by Thursday night.

The storm did bring much-needed rain as much of northern California has been experiencing abnormally dry conditions, according to the US drought monitor.

“It's never ideal to have so much rain in such a short time, but it certainly helps to fill the reservoirs and moisten the soil,” Wegman said.

About 280mm of rain had fallen on Wednesday and overnight into Thursday in northern California, bringing several metres of snow in higher elevations, said Rich Otto, a forecaster with the weather prediction centre.

Otto said the storm was likely to bring another 130mm to 250mm.

Reuters