© Mammoth Times/Wendilyn GrasseschiExtreme conditions: A powerful storm has pelted California with heavy rain and snow - an incredible 13ft of snow has accumulated at Mammoth Mountain ski resort
California residents should brace themselves for the most severe of an ongoing series of storms to hit the state, forecasters have warned.

More than 12 inches of rain have fallen in parts of the Santa Monica Mountains in the south and up to 15.5ft of snow has accumulated at Mammoth Mountain ski resort over the last four days.

And the conditions are expected to worsen in the next few days, the National Weather Service has warned, with the possibility of thunderstorms, hail and even tornadoes and flash floods.

'The ground will be permeated with a lot of rain, and it was a very, slow consistent rain for the past five days,' said Stuart Seto, a specialist for the weather service in upstate Oxnard. 'This is going to be more of a thunderstorm-type rain. This thunderstorm activity is very dynamic and intense.'

Researchers have coined the ongoing influx of tropical moisture into the state as an 'atmospheric river'.

'The atmospheric river brings in the moisture,' said Lucy Jones of the U-S Geological Survey office in Pasadena. 'How much rain gets dropped out of it has a distribution, just like earthquakes.'

She added: 'We actually made a model of this type of storm, just like we made a model of a Southern San Andreas Earthquake. We used the same techniques to try to assess what the damage would be.

'And our conclusion is that the storm would cost about four times as much as the shakeout earthquake.'

Downtown Los Angeles has already received around six inches of rainfall since last Thursday, around 40 per cent of the city's average annual precipitation.

Several rock and mud slides were reported in the Palomar Mountain region but no injuries have been reported.

In the Wrightwood area, about 15 people were evacuated Monday night after the Sheep Creek Wash overflowed and threatened homes, the San Bernardino County Fire Department told the Los Angeles Times. At least three homes sustained water damage.

© Mammoth Times/Susan MorningStuck: Cars lie buried at Mammoth Mountain after just four days' snowfall
Also in San Bernardino County, a woman was rescued from her pickup truck Monday night after being swept away in rain-swollen Lytle Creek in the San Bernardino National Forest.

Flooding also was reported in Trabuco Canyon in Orange County, with four men rescued by helicopter after becoming stranded inside their vehicle.

The California Highway Patrol confirmed there had been two rain-related deaths since the onset of the storm, with a three-year-old boy killed in the Fresno area and a 22-year-old man thrown from a vehicle that hydroplaned and crashed in the Bakersfield area.

The severe storms have also caused some 282,000 companies to experience electricity problems, with nearly 21,000 Southern California Edison customers without power on Monday, according to Edison spokeswoman Vanessa McGrady.

The hardest-hit area was the city of Torrance, south of Los Angeles, with more than 4,600 outages, she said.

Elsewhere, a small twin-engine airplane was reported missing on a 65-mile flight from Palm Springs to Chino.

Federal Aviation Administration spokesman Ian Gregor says a wreckage was found near Lake Perris but investigators won't be certain that it's the missing plane until they can get to the scene on Tuesday, if weather permits.

Residents of La Canada Flintridge were among those keeping a wary eye on the rain after a 250-square-mile wildfire last year denuded a large swath of the San Gabriel Mountains.

More than 40 homes in the foothills just north of Los Angeles were damaged or destroyed by a mudslide in February.

© AP PhotoRescue mission: Firefighters Jay Hausman, left, and Ryan Beckers, right, pull a motorist from a car caught in swift water at Hughes and Avalon Road in Victorville
© The Daily MailView from above: This satellite image shows the altered jet stream funneling vast quantities of precipitation over the U.S. west coast. Thanks BP!
© AP PhotoHeavy snow fall: There were also problems with weather conditions in Minnesota which led to a multiple vehicle accident on the eastbound Interstate 94 near St Joseph
© Mammoth MountainSilver lining: One snowboarder at Mammoth Mountain takes advantage of the weather, whizzing past the tops of trees buried under the snow