Strong winds and heavy showers were accompanied by the arrival of an atmospheric river, a plume of moisture from the Pacific Ocean. The National Weather Service's Sacramento office warned about "potentially historical rain."
In the San Francisco Bay Area, flooding was reported. It closed streets in Berkeley and inundated Oakland's Bay Bridge Toll Plaza. There were also overflowing rivers throughout Napa and Sonoma counties. Tens of thousands of North Bay residents were left without power after the poles fell.
According to the weather service, Mount Tamalpais, north of San Francisco, had received a quarter of a foot (15 cm) of rain in the past 12 hours.
Sean Miller, a weather service meteorologist, said that some of the higher elevations could receive 6, 7, or 8 inches of rain.
California Highway Patrol closed State Route 70 in Plumas and Butte counties about 150 miles (23 km) north due to multiple landslides caused by the Dixie Fire burn scar.
On Sunday, the Oroville highway patrol office tweeted that there had been "a few collisions this morning for vehicles with hydroplaning, many trees falling and numerous roadways that are flooding." Please stay off the roads today if you are able. Please use extreme caution if you're out on the roads.
The same storm system also hit Washington and Oregon, causing power outages that affected tens to thousands. Two people died when a tree fell onto a vehicle in Seattle. Eastside Fire & Rescue was called to the scene in Preston, Washington. This is approximately 20 miles (32 km) east Seattle.
According to the California Department of Transportation, state highways 16- and 20 were closed for several miles in California's Yolo and Colusa counties due to mudslides.
The threat of fire in areas that aren't under the control of vegetation is a serious concern. Land without vegetation won't absorb heavy rainfall as quickly and can lead to flash flooding.
The Sacramento weather service tweeted, "If you're in the vicinity of a burn scar and have not already prepared for debris flows, do so now." If you are asked to evacuate by authorities or feel threatened, don't hesitate to do so. You can always get to higher ground if it is too late.
Evacuation orders were issued in San Francisco's Santa Cruz Mountains due to concerns that rain could cause debris flows in the CZU Lighting Complex Fire burn scar. To the south, evacuation warnings were raised for parts of Santa Barbara County due to the Alisal Fire.
Strong winds were expected with gusts up to 60 mph (97 km/h) in the windiest areas of Northern California. From Sunday morning to Monday morning, elevations above 9,000 feet in Sierra Nevada could receive 18 inches (46 cm) or more of snow.
Recent storms helped to contain some of this year's most destructive wildfires. It remains to be seen whether the rainy weather will help alleviate the drought in California and the west. California's climate has become hotter and dryer, which means that any rain or snow that falls is likely to evaporate into the soil.
California's 2021 water-year, which ended Sept. 30, saw the state's second-driest year on record. Last year was the fifth dryest on record. Record low levels are being recorded at some of the state's most valuable reservoirs.