Hundreds of thousands of homes in California were left without power early Sunday after torrential downpours and high winds battered the northern part of the state, as forecasters warned of a "relentless parade of cyclones," over the coming days.
"Round after round of heavy rain on saturated soils will produce considerable flood potential with rapid river rises, mudslides, and burn scar flash floods or debris flows," the National Weather Service said in a bulletin, adding that "gusty winds may down trees and power lines."
More than 540,000 homes and businesses were already without power in California as of 1 a.m. local time (4 a.m. ET) on Sunday morning, according to data from PowerOutage.us.
Read more on NBC News
The West Coast "remains under the target of a relentless parade of cyclones," which will intensify over the Pacific Ocean while moving towards land the NWS said.
"With terrain already saturated from previous rainfall, additional bursts of heavy rain will lead to a heightened threat of flash flooding and rapid rise of river levels," it added.
The first of the heavier storms was due to arrive Monday, and the agency issued a flood watch for a large swath of Northern and Central California with 6 to 12 inches of rain expected through Wednesday in the Sacramento-area foothills.
State climatologist Michael Anderson told a news briefing late Saturday that officials were closely monitoring Monday's incoming storm and another behind it and were keeping an eye on three other systems farther out in the Pacific, according to the Associated Press.
The NWS' Sacramento office tweeted early Sunday that parts of the Sacramento Valley were experiencing "strong winds with gusts up to 60 mph," as well as "numerous trees and power lines down with power outages."
At least six people have died in the severe weather since New Year's weekend, including a toddler killed by a fallen redwood tree crushing a mobile home in northern California.
Gov. Gavin Newsom declared a state of emergency Wednesday as California was engulfed by rain and snow, producing flooding across the state. The was measure designed to support local jurisdictions and state agencies to respond quickly to the changing weather, as strong winds down power lines and infrastructure creating hazardous conditions.
The San Francisco Fire Department tweeted images of downed trees and flooded buildings on Saturday, but said there was no threat to life. On Friday, San Francisco Public Works announced they were able to supply ten sandbags per household and business in preparation for the weekend's wet weather.
California soil has long been weakened by drought and summer wildfires, which cause trees to brittle or burn down. This reduces the amount of interference for rainfall, which quickly forms streams on parched ground and leads to greater flood risk.
Climate change has already made extreme precipitation in California twice as likely, with extreme weather predicted to generate between 200-400% of surface runoff — rainwater that cannot be absorbed by soil — by the end of the century, according to research by the UCLA department of the environment and sustainability.
Across the rest of the U.S., showers and thunderstorms will move across the deep south Sunday morning.
Cold rain moving across the midwest will reach the central Appalachians Sunday night and Monday morning.