According to the governor's office, the declaration will bolster emergency response efforts and authorizes the mobilization of the California National Guard for disaster response.
It also directs Caltrans to request immediate assistance from the Federal Highway Administration to expedite road repairs due to the storms.
The governor also activated the State Operations Center to its highest level.
"This state of emergency will allow the state to respond quickly as the storm develops and support local officials in their ongoing response," Newsom said.
A powerful double whammy of an atmospheric river and bomb cyclone drenched California Wednesday, dropping several inches of rain on a region.
The heavy rain brought with it the potential risk of flooding, mudslides and power outages in what some forecasters are suggesting will become one of the most impactful storms to strike the state in years.
According to the National Weather Service, the heaviest of part of the storm is expected Wednesday evening into Thursday morning in Northern California, extending into Thursday night in Southern California. Forecasters said 2 to 4 inches of rain could fall across most of the Los Angeles area, with some mountain areas receiving 4 to 8 inches before the storm moves out.
Additional storms are expected next week as well.