The southwestern US state of California is facing what the National Weather Service calls a "relentless parade of atmospheric rivers" over the next week after several days of heavy rain have already wrought havoc across the region.

"Atmospheric rivers" are the name given to elongated, narrow moist weather systems that can produce huge amounts of rain or snow. 

Stormy weather in the state in the past days has flooded roads, knocked out power to more than 300,000 customers in and around the capital, Sacramento, as of Sunday morning, and killed at least six people.

The coming storms are likely to compound such problems, with the soil already saturated after the heavy rains brought by weather systems coming in from the Pacific.

What is the forecast in California?

The National Weather Service's Sacramento office tweeted early Sunday that parts of the Sacramento Valley were experiencing "strong winds with gusts up to 60 mph (97 kmh),'' as well as "numerous trees and power lines down with power outages.''

It said heavier storms were expected in the course of Monday and warned of extensive potential flooding in the north and center of the state with 6 to 12 inches (15 to 30 cm) of rain expected to fall in the period through Wednesday in the Sacramento-area foothills.

The Los Angeles area was potentially to receive up to 8 inches of rain in the foothills.

Three other systems are further out to sea and being monitored by weather officials, according to state climatologist Michael Anderson

The rains come as the state is in a drought but will not suffice to officially end it.

tj/jcg (AFP, AP)