California residents are hunkering down as the powerful Pineapple Express storm - expected to bring 4in of rain and 70mph winds - hits the state.

The storm has started battering LA and the authorities have been preparing for the worst - with road closures, evacuation orders and emergency shelters.

The torrential rain and winds are expected to cause power cuts and send debris into flooded roads, while waves could reach 12ft on beaches - where lifeguard towers have been brought in to protect them from damage.

Los Angeles County Fire Department Chief Daryl Osby said: "Do not underestimate moving water, do not underestimate mud flows, and do not underestimate downed power lines."

In Glendora, a town in the foothills of the Sierra Madre mountain range, residents have been urged to evacuate their homes because of a risk of mudslides following a forest fire earlier this year.

An evacuation shelter has been set up and 15,000 sandbags have been made available to protect homes and businesses.

Police Chief Tim Staab said: "It doesn't mean we're going into their residences or [forcing] them to leave, but we're going to strongly advise for their safety, they should leave their residences."

The National Weather Service has also warned of a risk of mudslides and flash floods in parts of Orange County, San Diego County and Riverside County, which have all been affected by fire recently.

In San Jacinto, the Riverside County Sheriff's Department ordered mandatory evacuations at an apartment complex and a synagogue in anticipation of mud flows from the Soboba foothills.

No power for 250,000 people

The Pineapple Express is a concentrated flow of tropical moisture that originates in Hawaii and runs to the Pacific coast.

The Pacific Jet Stream - a fast ribbon of air high up in the atmosphere - has moved southwards across California in recent days and will pull tropical moisture from the Pineapple Express.

The storm has already caused chaos in northern California - knocking out power to as many as 250,000 people, flooding freeways and toppling trees.

Throughout the San Francisco Bay Area on Thursday, waves slammed onto waterfronts, ferries were bound to their docks, some flights were grounded and many schools cancelled classes.

Gusting winds had motorists tightly gripping their steering wheels on the Golden Gate Bridge, where managers created a buffer zone to prevent head-on collisions by swerving cars.

The iconic suspension bridge is engineered to swing in cross winds, and engineers were standing by, but "the concern we have right now is more about vehicles", spokeswoman Priya David Clemens said.

A schoolchild was trapped for about 15 minutes when an 80ft tree fell on him in Santa Cruz. Rescuers with chain saws freed his arm and he was taken to a hospital in good condition.

A homeless man (40) in neighbouring Oregon who was sleeping on a trail was killed by a falling tree on Thursday.

The storm has already blasted parts of the Sierra Nevada with 140mph winds and it is expected to dump up to 2ft of snow on areas over 7,000ft.

The rain will be welcomed by some, particularly farmers who have been struggling to keep crops alive in parched fields during successive years of below-normal rainfall.