'Bomb cyclone' threatens northern California and Pacific Northwest

Northern California and the Pacific Northwest are bracing for what is expected to be a powerful storm, with heavy rain and winds set to pummel the region and potentially cause power outages and flash floods.

The Weather Prediction Center issued excessive rainfall risks beginning on Tuesday and lasting through Friday as the strongest atmospheric river -- long plumes of moisture stretching far over the Pacific Ocean -- that California and the Pacific Northwest has seen this season bears down on the region. The storm system has intensified so quickly that it is considered a "bomb cyclone", explained Richard Bann, a meteorologist with the National Weather Service Weather Prediction Center.

The areas that could see particularly severe rainfall as the large plume of moisture heads toward land will likely stretch from the south of Portland, Oregon, to the north of the San Francisco area, he explained.

"Be aware of the risk of flash flooding at lower elevations and winter storms at higher elevations. This is going to be an impactful event," he said.

In northern California, flood and high wind watches go into effect on Tuesday, with up to eight inches of rain predicted for parts of the San Francisco Bay Area, North Coast and Sacramento Valley.

A winter storm watch was issued for the northern Sierra Nevada above 3,500 feet, where 15 inches of snow was possible over two days. Wind gusts could top 120 kilometres per hour in the mountain areas, forecasters said.

"Numerous flash floods, hazardous travel, power outages and tree damage can be expected as the storm reaches max intensity" on Wednesday, the Weather Prediction Center warned.

In northern California's Yolo County, crews spent Monday clearing culverts, sewers and drainage ditches to avoid clogs that could lead to street flooding. Mesena Pimentel said she hopes the efforts prevent a repeat of floods last February that inundated her property near Woodland.

"We had about ten inches of water in our garage, had a couple gophers swimming around," Pimentel told KCRA-TV. Woodland city officials have set up two locations where residents can pick up free sandbags. Authorities urged people to stock up on food and charge phones and electronics in case power goes out and roads become unpassable.

Meanwhile, southern California this week will see dry conditions amid gusty Santa Ana winds that could raise the risk of wildfires in areas where crews are still mopping up a major blaze that destroyed 240 structures. The Mountain Fire, which erupted November 6 in Ventura County northwest of Los Angeles, was about 98 per cent contained on Monday.

Winds will calm by the end of the week, when rain is possible for the greater Los Angeles area.

In southwestern Oregon near the coast, four to seven inches of rain is predicted -- with as much as 10 inches possible in some areas -- through late Thursday night and early Friday morning, Bann said.

A high wind warning has been issued for the north and central Oregon coast beginning at 4 p.m. Tuesday with south winds from 40 kilometres per hour to 64 kilometres per hour, with gusts to 97 kilometres per hour expected, according to the weather service in Portland. Gusts up to 113 kilometres per hour are possible on beaches and headlands. Widespread power outages are expected with winds capable of bringing down trees and power lines, the weather service said. Travel is also expected to be difficult.

Washington could also see strong rainfall, but likely not as bad as Oregon and California. From Monday evening through Tuesday, some of its coastal ranges could get as much as 1.5 inches of rain, Bann said.

The weather service warned of high winds from Tuesday afternoon until early Wednesday for coastal parts of Pacific County, in southwest Washington. With gusts potentially topping 46 kilometres per hour -- and likely faster near beaches and headlands -- trees and power lines are at risk of being knocked down, the Pacific County Emergency Management Agency warned.

Washington State Patrol Trooper John Dattilo, a patrol spokesperson based in Tacoma, posted on social media Monday afternoon that people should be prepared for "some bad weather" on Tuesday night. "Stay off the roads if you can!"

A blizzard warning was issued for the majority of the Cascades in Washington, including Mount Rainier National Park, starting Tuesday afternoon, with up to a foot of snow and wind gusts up to 97 kilometres per hour, according to the weather service in Seattle. Travel across passes could be difficult if not impossible.

Outside of this region, the central and eastern Gulf Coast, including the Florida Panhandle, is at risk for flooding on Tuesday, with two to three inches of rainfall are in the forecast, according to the weather service. Low-lying and urban regions could see flash floods.