US President Joe Biden traveled to California on Thursday to survey areas hit by a series of deadly storms.

The "atmospheric river" storms caused widespread flooding and mudslides and knocked over trees in the US' most populous state. Atmospheric rivers are narrow bands of vapor that initially form over the ocean.

Biden has approved a major disaster declaration for the state and raised the level of federal assistance.

Where will Biden go?

Biden was traveling alongside Federal Emergency Management Agency (FEMA) chief Deanne Criswell to Santa Clara County in the San Francisco Bay Area, then he was set to head southwards by helicopter.

There are two scheduled stops along the Santa Cruz coast, where thousands of residents were ordered to evacuate amid flash floods and extreme tides.

Biden also planned to meet with business owners, residents, local leaders and emergency teams in the town of Capitola and Seacliff State Beach.

"Over 500 FEMA and other federal personnel have already deployed to California to support response and recovery operations and are working side by side with the state to ensure all needs are indeed met on the ground," White House press secretary Karine Jean-Pierre said on Wednesday.

What do we know about the storms?

At least 20 deaths have been attributed to the storms raging over the past three weeks.

The storms also caused outages leaving tens of thousands of residents without power.

From December 26 to January 17, California averaged 11.47 inches (29.13 cm) of rain and snow, according to the National Weather Service's Weather Prediction Center.

The influx of water has helped ease a four-year drought in the state, with badly depleted reservoirs and the Sierra Nevada snowpack being replenished. However, experts warn that most of California remains under moderate or severe drought conditions.

sdi/rt (Reuters, AP)