SACRAMENTO, CALIF. -- Landslides of rock and mud closed roadways Friday across California as heavy rains kicked off what will be series of storms poised to usher in the new year, likely bringing downpours and flooding.

Officials have reported landslides that closed routes in the San Francisco Bay Area, between Fremont and Sunol, as well as in Mendocino County near the unincorporated community of Piercy and in the Mendocino National Forest, as crews clear debris.

An atmospheric river storm, a long and wide plume of moisture pulled in from the Pacific, began sweeping across the northern part of the state Friday and was expected to bring more rain through Saturday, according to the National Weather Service in Sacramento.

Officials warned that rivers and streams could overflow and urged residents to get sandbags ready. A flood watch was in effect in much of Northern California through New Year's Eve.

It was the first of several storms expected to roll across California over the next week. The current system is expected to be warmer and wetter, while next week's storms will be colder, lowering snow levels in the mountains, said Hannah Chandler-Cooley, a meteorologist at the National Weather Service in Sacramento.

Sacramento's fire officials planned to broadcast evacuation announcements from a helicopter and a boat along the American River -- a spot where many unhoused people live in encampments -- to warn of flooding.

The Sacramento region could receive a total of 10 to 13 centimetres of rain over the span of the week, Chandler-Cooley said.

Humboldt County, where a 6.4 magnitude earthquake struck on Dec. 20, also saw roadways begin to flood, according to the National Weather Service's Eureka office. A bridge that was temporarily closed last week due to earthquake damage may be closed again if the Eel River, which it crosses, gets too high, officials said.

On the Sierra's eastern front, flood watches and warnings were issued north and south of Reno, Nevada, where minor to moderate flooding was forecast along some rivers and streams into the weekend.

At Susanville, California about 137 kilometres north of Reno, the Susan River was forecast to rise from about 1.5 metres Friday to 30 centimetres above the flood stage of 3.6 metres by Saturday morning, causing moderate flooding that could affect some homes, roads and bridges, the National Weather Service said.

In Southern California, moderate-to-heavy rain was forecast for Saturday. The region will begin drying out on New Year's Day and the Jan. 2 Rose Parade in Pasadena should avoid rainfall.

Heavy showers are forecast for Tuesday or Wednesday, the National Weather Service in Oxnard said.

The rain was welcomed in drought-parched California, but much more precipitation is needed to make a significant difference. The past three years have been California's driest on record.