The governor of California has declared a state of emergency as residents prepare for a second major storm that drenched the state last week.
The storm is expected to develop into a bomb cyclone, causing rain and snowstorms that can cause flooding and landslides in already wet areas.
Northern California, which saw a deadly levee breach over the weekend, is expected to be hardest hit.
The storm is also bringing strong winds to areas up and down the US west coast.
The National Weather Service (NWS) has issued a flood monitoring and high wind warning for the entire Bay Area in northern California, saying high winds are expected to topple trees and cause power outages.
“Simply put, this will likely be one of the most effective systems on a widespread scale this meteorologist has seen in a long time,” the NWS forecaster for the region told a weather advisory.
“Effects will include widespread flooding, washing of roads, collapse of slopes, felling of trees (potentially full groves), widespread power outages, sudden interruption of trade, and worst of all, possibly loss of human life,” he added.
“This is a brutal system that we’re really looking at and that needs to be taken seriously.”
It is expected to develop into a bomb cyclone, a type of rapidly intensifying storm more common on the east coasts of the United States and Canada.
State of emergency
In a statement declaring a state of emergency on Wednesday, Governor Gavin Newsom said his order would “allow the state to respond quickly as the storm develops and support local officials in their continued response”.
An atmospheric river – a long, narrow stream of warm air that brings moisture from the tropics – is expected to drop into the already rain-saturated ground.
It is expected to hit hardest Wednesday through Thursday morning, before spreading to southern California on Thursday night.
Just one year after California recorded its driest years, San Francisco on Saturday saw its second wettest day in more than 170 years.
San Francisco Mayor London Breed said the city was “preparing for war” and spreading sandbags all over the city to prevent flooding.
Rivers along the coast are expected to see extensive flooding due to tidal waves. The storm is expected to rain 6 inches (15 cm) in coastal areas and reach speeds of 80mph (128kmph) on coastal hills and mountains.
In Santa Cruz County, south of San Francisco, evacuation orders have been ordered because of the “high probability” that some neighborhoods will become inaccessible due to flooding.
According to the NWS, more than 105 million people across the United States are currently at risk of severe weather conditions.
Further east, about 30 million people face major storms that cause hurricanes in many states.